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THEMECHANISMOFPERMANGANATE OXIDATION OFALKANES, ARENES AND RELATED COMPOUNDS

BY

UDO ANTHONYSPITZERB.Ed. , University of Saskatchewan, 1968B.A., University of Saskatchewan, 1968M. Sc , University of Saskatchewan, 1970

ATHESIS SUBMITTED IN PARTIAL FULFILMENTOFTHE REQUIREMENTS FOR THEDEGREE OF

DOCTOROFPHILOSOPHYin the Department

ofCHEMISTRY

We accept th is thes is as conforming to therequired standard

THE UNIVERSITY OFBRITISHCOLUMBIANovember, 1972

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I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a 1 f u 1 f i l m e n t o f t h e r e q u i r e m e n t s f o r

a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t

t h e L i b r a r y s h a l l m a k e i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y .

I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s

f o r s c h o l a r l y p u r p o s e s m a y b e g r a n t e d b y t h e H e a d o f m y D e p a r t m e n t o r

b y h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n

o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t m y

w r i t t e n p e r m i s s i o n .

D e p a r t m e n t o f Chemistry

T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a

V a n c o u v e r 8, C a n a d a

Dec. 18, 1972

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ABSTRACT

Supervisor: Professor Ross StewartThe permanganate oxidation of alkanes and arenes has been studied

in trifluoroacetic acid (TFA)-water. This solvent system has theunique advantage of being virtually inert to oxidative degradation andyet providing adequate so lu bi lit ie s for the reactants.

The mechanistic inv estig atio n involved kine tic studies todetermine the orders with respect to oxidant, substrate,and acid.Complementary techniques such as product studies, substituent effects,activation parameters, and isotope effects completed the mechanisticinve sti ga tio n. The oxidation of several alco ho ls, aldehydes, andketones was also investigated to aid in the interpretation of theresults .

Because of the poor fit of the kinetic data with the previouslydetermined H function for TFA-water the H_ function was determinedo Rusing the Hammett approach of overlapping indic at or s. The id en ti ty ofthe oxidants in the aci d ic medium was established by cryoscopic andspectrophotometric means. It was found that the most vigorousoxidant was permanganyl ion (MnO-+), with some contributing oxidationby both permanganic acid (HMnO )and permanganate ion (MnO^ ) in thecase of easily oxidized compounds such as alcohols, aldehydes, or enols.

The mechanism of the ac id ic permanganate ox idat ion of alkanes(ethane to n-tridecane) was found to proceed via rate-determininghomolytic carbon-hydrogen bond scission as depicted below.

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, slow, fast

, fast

, fast

acids

The -mechanism of arene ox idat ion was shown to proceed v ia rate-determining electrophil ic attack by permanganyl ion on the aromaticring to yield ring degradation products. Phenols are believed tobe intermediates in this process as depicted below.

fast

- i i i

RR1CH2 + Mn03+ * [RR^CH + HMnC>3+][RR^CH + HMn03 ] [RR^H-O-MnC^H]

(I) (IDI or (II) ? R RCHOH+ MnV

VTT VI or (II) + Mn > R R ^ O + 2MnM n V I 1

RR-CHOH or RR C=0 > carboxyl ic

The mechanisms of the oxidation of al co hol s, ketones, aldehydes,and formic acid were determined and shown to be consistent withmechanisms previously established under other conditions.

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- iv -The behaviour of two electrophiles, nitrosonium ion (N0+) and

nitronium ion (N0- +), generated res pectiv ely from sodium n i t r i t e andsodium nitrate, was examined in the TFA-water medium. It was foundthat the nitronium ions thus generated could be successfu lly used tocarry out el ec tro ph ilic aromatic nitra tions in excellent y ields butthat the nitrosonium ions were inert.

It was also determined that the tetra(n-hexyl)ammonium permanganatesal t could be prepared in good yield andused as an oxidant for avariety of substrates in benzene solution.

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1

ACKNOWLEDGEMENTS

The author wishes to express his sincere appreciation toDr. Ross Stewart for his guidance during the course of this researchand for his constructive cr it ic is m and suggestions during thepreparation of this the sis.

Grateful acknowledgement i s also made to the Department ofChemistry of the University of Bri ti sh Columbia for providing f a c i l it i e sto conduct the research and providing supplementary financialassistance. Sincere appreciation is expressed to the National ResearchCouncil for providing a scholarship for the en tire duration of th isin vest ig at io n from September 1970 to November, 1972.

Finally I wish to thank my wife, Le i l a , for encouragement andpatience.

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TABLEOFCONTENTS

Page

1. INTRODUCTION 1

1.1 Motivesfor theInvestigation L

1.2 Propertiesof theTrifluoroaceticacid-WaterMedium 2

1.3 AcidityFunctions . 4

1.4 BehaviourofPermanganateinAcidicMedium 7

1.5 PermanganateOxidationofOrganic Substrates 8

1.5.1 OxidationofHydrocarbons 8

1.5.2 OxidationofArenes 10

1.5.3 OxidationofAlcohols 12

1.5.4 OxidationofKetonesandAldehydes 14

1.5.5 OxidationofFormicAcid 15

1.6 PermanganateOxidationsinOrganic Solvents 16

1.7 ElectrophilicAromaticNitration 17

1.8 Applicationof theZucker-HammettHypothesis 18

2. SCOPE OF THEINVESTIGATION 21

3. EXPERIMENTAL 22

3.1 ReagentsandIndicators 22

3.2 KineticProcedureinAcid-WaterMedium 31

3.3 KineticAnalysis 34

383.4 Inorganic Product Study

3.5 Organic Product Studies 39

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Page

3.5.1 Permanganate Oxidations in TFA-Water 393.5.2 Nitr at io n and Ni trosatio n Products in

TFA-Water 433.6 Determination of Stoich iometrics 453.7 Freezing Point Depression Procedure 453.8 Determination of the H_ Function for TFA-Water 50

R

3.9 Vapour-Liquid Phase Equilibrium Study 563.10 Determination of pK for Permanganic Acid in TFA-

H-0 and TFA-D-0 613.11 Permanganate in Benzene 69

4. RESULTS ANDDISCUSSION 754.1 Solvent System 75

4.1.1 B_ Function for the TFA-Water Solvent System 754.1.2 Id en ti fi ca tion of the Manganese VII Species

Generated in TFA-Water Mixtures 874.1.3 Nitr at ion and Ni trosatio n in TFA 97

4.2 Products and Sto ichiom etrics 984.3 Oxidation of Alkanes 1034.4 Oxidation of Arenes 1264.5 Oxidation of Alcohols 1414.6 Oxidation of Aldehydes 1524.7 Oxidation of Ketones 1554.8 Oxidation of Formic Acid 1634.9 Oxidation by Permanganate in Benzene 165

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Page

5. CONCLUSION 170

6. SUGGESTIONS FOR FURTHERWORK 174

APPENDIX A 175

APPENDIX B 186

APPENDIX C 189

APPENDIX D 216

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-ix-

LISTOFTABLES

Table

1 SpecificationsofCompounds

2 Function Indicatorsfor TFA-Water

3 Organic ProductsfromtheKMnO Oxidationsin TFA-

WaterMedium

4 NitrationandNitrosationProducts

5 ExperimentalData forStoichiometric Determinations

forKMnO.OxidationsinTFA-Water4

6 DeterminationandVerificationof forTrifluoro-

acetic acid

7 CryoscopicData forSubstratesinTrifluoroacetic

acid

8 MolarityofWeight%SolutionsofTFA-Water

9 Vapour-Liquid EquilibriumData fortheTFA-WaterSystem

10 ProtonationData forMnO ~inMediumsofTFA-Ho0or4 2

TFA-D20 '

11 Spectral PropertiesofTetra(n-hexyl)ammonium

PermanganateinBenzene

12 PropertiesofTetra(n-hexyl)ammonium Permanganate...

13 PermanganateinBenzene,Oxidation Products

14 P KR+ V a l u e s f rArylCarbinols

15 H Function forTFA-H_0

16 Temperature Dependenceof pK +

17 Thermodynamic Parametersfor theIndicatorsusedto

DetermineH

R18 ActivityofWater inVariousMedia

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-x -

Table Page

19 pKfortheIonizationofPermanganateIon to

PermanganylIon 96

20 ManganeseReduction Products fromKMnO Oxidations.. 102

21 OrderofReactantsInvolvedin thePermanganateOxidationofAlkanesin TFA-Water 105

22 KineticIsotopeEffectsin thePermanganateOxidation

ofCyclohexane . 108

23 ComparativeReactivitiesofMethylene GroupsinCyclo-alkanesRelativetothosein theCorresponding n-Alkane 110

24 ComparativeReactivitiesofCycloalkanes 110

25 Activity CoefficientsinSulfuricAcid-Water Mixtures 116

26 ActivationParametersfor PermanganateOxidations

inTFA-Water 118

*27 Calculated-a ValuesofIndividualCarbonAtomsin

somen-Alkanes 121

28 RelativeRatiosofMethylenes Presentinn-Alkanes.. 123

29 ExperimentalRatiosforn-AlkaneOxidationRates.... 124

30 TheoreticalandExperimental DatafortheChromicAcidOxidationofn-Alkanes 125

31 OrderofReactantsInvolvedin thePermanganate

OxidationofArenes 127

32 KineticIsotopeEffectsforthePermanganate

OxidationofArenes 132

33 Comparative RateRatiosforthePermanganateOxidation

ofToluene, Ethylbenzene,and Cumene 130

34 OrderofReactantsin thePermanganateOxidationof

AlcoholsinTFA-Water 143

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- x i -Table Page35 Isotope Effec ts in the Permanganate Oxidation of Alcohols14736 Kinetic Isotope Effects in the Oxidation of Benzyl

Alcohol 15137 Order of Reactants in the Permanganate Oxidation of

Ketones in TFA-Water 159

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- x i i -LIST OFFIGURES

Figure Page1 Freezing point apparatus 492 Indicator ra ti o plot s for the function 533 Vapour-liquid equilibrium study apparatus 584 Phase diagram of TFA-water mixtures 605 LogQvs. IL for the ioni za tion of MnO ~ in TFA-water 656 LogQvs.H q for the ion iza tion of MnO in TFA-water 667 LogQvs.H O for the io niza tion of MnO^ in tr i f luoro-

acet ic acid-d^-deuterium oxide 678 LogQvs.H for the ion izat ion of MnO. in tr i f luoro-o 4

acet ic acid-deuterium oxide 689 Spectra of MnO with diffe ren t cations and so lve nt .. 70

10 Acidity functions in TFA-water 7911 a vs . pK^ for substituted triph eny lcarbino ls 8312 Chromic acid oxidation of 2-propanol in TFA-water.

Log k 2 vs. H q and 8813 Pa rti cu lat ion of potassium permanganate and sodium

acetate in TFA 9114 Pa rti cu lat ion of sodium ni tr ate and sodium n i t r i t ein TFA 9215 Spectra of side products (from the nit ra ti on ofbenzene) in acidic and basic media 99

16 Spectra of 1:1 mixture of ortho and para-nitrophenolin acidic and basic media 100

17 Log vs . loglalkane] for the oxidation of a varie tyof alkanes 106

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- x i i i -

Figure Page18 Log k- vs . H_ for the oxidation of a va riet y of alkanes 10719 Substituent effects in the permanganate oxidation of

ethane I l l20 Salt effects on the permanganate ox idation of

cycloheptane 11221 Oxidation rate vs . number of carbon atoms in n-alkanes 12222 Log k- vs . log[arene] for the oxidation of toluene and

benzene 12823 Log k. vs. IL. for the oxidation of toluene and benzene 12924 Salt effects on the permanganate ox idation of benzene 13125 Substituent effects on the permanganate ox idation of

benzene 13426 Substituent effects on the permanganate ox idat ion of

toluene 13527 Revised p + vs . log k- / I ^ H P ^ o t ^ o r t o l u e t i e s - Toluenes

considered as substituted benzenes 13628 Log k- vs . for toluene, benzyl alcohol, and

benzaldehyde 14029 Decrease of oxidation rate with age of solution.

Methanol in TFA-water 14230 Log k- vs . log[alcohol] for the oxidation of avariety of alcohols 14431 Variation of oxidation rate with acidity for the

oxidation of alcohols in TFA-water 14532 Salt effects on the permanganate ox idation of methanol 14833 Log k- vs. log[aldehyde] for the oxidation of form

aldehyde and benzaldehyde 153

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- xiv -Figure Page34 Log vs. for the oxidation of formaldehyde

and benzaldehyde 15435 Typical pseudo first-order plot for the oxidation

of cyclohexanone 15736 Typical zero-order plot for the oxidation of

acetophenone 15337 Log k^ vs . loglketone] for the oxida tion of a va ri et y

of ketones 16038 Log k^ vs . log[formic acid] for the oxidation of

formic ac id 16439 Log k 2 vs . for theoxidation of formic acid 16640 Substituent effects on the permanganate ox idation of

tra ns -sti lbenes in a medium of benzene 168

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- X V -

ABBREVIATIONSANDSYMBOLS

USEDIN TEXT

a

ia c t i v i t y of i t h component

c constantofintegration

AG changei nfreeenergy

AH changei nenthalpy

AS changei nentropy

DPC diphenylcarbinol

f.1

activity coefficientofithcomponent

F

imolefractionof i t h component

g grams

h Planck's constant

hr hour

Ho acidity function defined byprotonation ofprimaryani lines

HRacidity function defined by ionizationof carbinols

k Boltzmann constant

kn

rate constant,n =0,1,2,3

k,k' proportionality constants

kg kilogram

K degrees Kelvin

In natural logarithm

log base10 logarithm

m molality,molesper 1000 g of solvent

M molarity, molesper 1000 ml ofsoluti on

microliter

ml m i l l i l i t e r

min minutes

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- xvi -n number of part ic le s generatedn^ number of par ti cle s predicted by theoryN normality, number of equivalent weights per l i t e r of solutionNMR nuclear magnetic resonancep^ pressure of ith components secondsTFA trifluor oace tic acidTPC triphenylcarbinolTPMC1 Triphenylmethyl chloridevpc vapour-liquid phase chromatographyr reaction vel ocit yt denotes transition state or activated complex[ ] denotes concentration in moles per l i t r e

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1. INTRODUCTION1.1 Purposes of the Investigation

In the past decade considerable attention has been focused onenvironmental abuse by in du st ri al po lluta nts. The polluta nt consideredas one of the most serious is o i l not only because of the largequantities usually involved but also because of the immediate andlong term to xicit y to the environment. As a re su lt of the economicloss and public pressure considerable e ffor t i s being expended tounderstand how the ecosystem attempts to cope with sudden additions oflarge quantities of hydrocarbons.

Researchers have found that certain micro-organisms can degrade1-3

hydrocarbons by u t i l i z ing them as a food source. The i n i t i a lapproach of reseachers in this area was to utilize yeasts to profitablyup-grade crude o i l s by the removal of waxy alkanes to yield some

2marketable prote ins for animal feed. Recently such micro-organismshave been recognized as potential o i l - s p i l l combatants which act by3degrading the o i l instead of dispersing i t . Pi lot batches haveshown promise, with the added advantage of not being toxic to shellfish,which is a serious drawback of dispersants .

Presently, considerable interest is being focused on establishingthe mechanism of this degradation process in order to maximize the

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efficiency of these organisms. The precise route of the degradationis at present only speculative but some aspects of the process arenow understood. It is known that somemicro-organisms u t i l i z e onlya specific series of hydrocarbons''" and that the attack occurs at theterminal carbon, sometimes involving incorporation of oxygen fromthe atmosphere, as ^ 0 studies have shown.^'^

Although the main purpose of th is investiga tion was to es tabl ishthe mechanism of the acid ic permanganate ox idation of arenes andalkanes, we were aware of the possibility that this study might providesome fundamental information about the ways in which carbon-hydrogenbonds are cleaved in saturated hydrocarbons. This information might helpelucidate the mechanism of microb iologic al oxidat ion of hydrocarbons.

Permanganate has been recognized as a versa til e oxidant that isactive in both acidic and basic media and is capable of ox id iz ing a

5-7 cwide variety of substrates. The mechanisms by which theseoxidations are performed are quite well understood with the exceptionof the oxidation of alkanes and arenes. Very little research hasbeen directed towards this problem, probably because of low solubilitiesof these substrates i n media capable of disso lv ing permanganate.This study attempts to estab lish the mechanism of the oxidation ofalkanes and arenes by acidic permanganate in a homogeneous medium.

1. 2 Properties of the Tr ifluoro ace tic Acid-Water MediumAlkanes are known to have some solubility in water but the alkane

content even in saturated solutions is very low, in the order of- 4 - 5 810 to 10 molar. These concentrations are in su ffi cien t for ki ne tic

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inv es tigati on by conventional means. A co-solvent was required whichcould dis so lve both permanganate and the alkanes and yet be resistantto ox ida tion. Tri flu or oa ce tic acid (TFA) was found to be sat isfa cto ryfor th is purpose. TFA had the further advantage of allowingvariation of the ac id it y of the medium, since i t i s a moderately strong

9acid with pK = -0.26 .r aTrifluoroacetic acid has been successfully used for a variety

of reactions ranging from oxidations"^ to e le ct ro p h ili c aromaticsubstitutions."''"'" Some of the prop ertie s of th is solvent system arebrief ly out lined below.

Trifluoroacetic acid-water mixtures are not very strong protonating12media as shown by the work of Randies and Tedder. A maximum value13of H = -3.03 is reached in neat TFA (46% wt. su lf uri c a cid haso

14 s-H = -3.03 ). The d ie le ctr ic constant of neat TFA is _ - c = 8.3 +o 25 O.I,"''"' which indicates that TFA is not a very strong ly ionizing solvent.Conductivity studies confirm this but also show that salts containingammonium cations are more dissociated than expected in th is solvent,an effect which is thought to be due to hydrogen bonding."'""' Brownand Wirkkala"'""'" found TFA to be an exceptionally good medium foraromatic el ec tr op hi lic substitut ions such as n itr ati on s and brominations.The bromination k in eti cs were in vestigated and found to be simplesecond order without any of the complications which were encounteredwhen acet ic acid was used.

Several groups have observed that TFA can have strong solventinteractions with compounds'^'^ and intermediates. i t hasbeen reported that neat TFA can so lu b il iz e salts by hydrogen bonding

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- 4 -to the a n io n . ^ Aromatic systems such as nitrobenzene and anisole

13

showed considerable Cshifts by the carbon para to the substituent ,indicating strong solvent-solute in tera ctio ns ."^ Mechanisticinvestigation s on de silyla tio ns and de tri tia tio ns of aromaticcompounds i n various mixtures of TFA-water indicated considerable

18

solvation of the cationic transition state. Rapid deuterium incorporation was observed in the alkyl group of te rt -a lk yl triflu oroaceta teesters when the ester was disso lved in TFA enriched with deuterium.This is believed to be due to the unusual stability of the alkyl

19carbonium ion generated from the alkyl group of the ester.1.3 Acidity Functions

In thi s inv esti gation the ac id ity function is determined forthe TFA-water medium and ac idity functions are used to explain therate acce lerations observed with increased a c id it y . In order to makethe discussion more meaningful a descr iption of the acid ity-fu nctionconcept w i l l be given.

20TheH q function originally derived by Hammett and Deyrup measures

the tendency of the solution to donate a proton to a neutral base. Thisfunction is defined by a series of substituted an iline s by the followingderivation.

+ K B H + +ArNH^ v ArNH2 + H , Ar5= phenyl group(BH +) (B)

KBH+ = aB aH+ /aBH+ ' 3 = a c t i v i t l e s

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- 5 -K B R + = [B] f_ aH +/[BH +]f_ H 4: , f = ac tiv ity coeficient

The definition is then made that hQ =f^a^+/f^^_. This then septhose terms hard to determine from those easily accessible byexperimental measurements.

p K B R + = -log = -log[B]/lBB+] - log hQ

p K B H + = Ho " l 0 g tB1/1BH"*"]

or H q = pKgj-f + log [ B ] / I B H + ]

Another function often used is which measures the tendency of21

the medium to ionize carbinols. The function is derived in thefollowing manner.

H-0 + (Ar)-C (Ar)-COH + H(R +) (ROH)

V = aROHaH+/aH_OaR+ -r R O H ] f R O H a H + / [ R + ] f R + a H 0

hR = fROH aH+ /fR+ aH 20 a n d \ = hR

pK_+ = -log[ROH]/[R+] - l o g h R

= pK-+ + log[ROH]/[R +]

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Usually these functions are determined by using a techniquereferred to as "the method of overlapping in di ca to rs ". The methodconsists of taking an indicator which has measurable ionizat ion in amedium which has a measurable pH. Then the assumption is made thatIL = pH in thi s very dil ut e re gion . The pK for th is ind ica tor canthen be calculated. This pK value is used to then determine forstronger mixtures by measuring the new ionizat ion ra tio ([B] /[B H +]).Eventually this indicator wi l l no longer be useful, which is a rbitra rlydefined to be when 0.1 < [B]/[BH +] < 10. To then extend a weakerindicator has to be used which has measurable ioni za tio n in somesolution of known H^, i . e . i t overlaps with the previous indicator.The pK can then be calcu late d since H is known and the new indicator

xratio can be measured. This overlapping process is continued untilH i s e stablished for the en tir e medium range, or unt i l no more .suitable ind icators are availa ble to extend H .

xIt is important that only those indicators are used where the

overlap is good and, most important, that the indicators have parallelresponses to the medium changes, i . e . logf_. / f u = log c ( f / fD t l )

B^ 2 2 2c = a constant. This condition must be fu l f i l l e d otherwise H hasxno meaning because the overlap method uses from the previousindicator to determine pK of the next indicat or . This essentiallyforces H (indicator 1) to be equal to H (indicator 2), i . e .- log fB aH+/f B H = "lo g fB aH+/ fBH * T h i S c o n d i t i o n c a n b e testedby checking to see that log [3]/[BH +] vs. H x plots give line s thatare pa ra ll e l for overlapping ind ica tor s. If deviations are severethe deviant indicator should not be used.

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1.4 Behavior of Permanganate in Acidic MediumThe d ist inctive absorbance by permanganate ion has been shown to

22 23be insensitive to solvent changes and cation va ri at io n, ' but i fthe medium is made very ac id ic a di s tin ct sp ec tral change is observed

24 28 29from purple to li ght green ^ m a x ^ 8 r e e n species is 458 nm ' ).The cause of th is sp ec tral change has not yet been resolved . Symons

25et a l . have suggested from cond uc tiv ity measurements that the sp ec tralchange is due to the following process,

KMnO/ + 3H.S0. * 0oMn0S0_H + H_0+ + K + + 2HS0."4 2 4 3 3 3 426

but Royer interp reted sim ilar conductiv ity measurements and cryoscopicdata to be consistent with the reaction,

KMnO. + 3HS0. K + + MnO_+ + H o 0 + + 3HS0."4 2 4 - 3 3 427

Stewart has suggested that these interp ret ations may be reconc iledby considering the following equ ilibrium .

0_Mn0S0oH MnO.+ + HSO."3 3 3 4This sp ec tral change has been observed i n p ar t ia ll y aqueous media where

28 29the process is taken to be a pro tonation. '

H + + MnO." HMnO.4 - 4

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28Syraons et a l . has estab lish ed this pK& value to be -2.25 in perch lori c29acid usingH qbut Stewart and Mocek found a value of -4.6 insulfuric acid also using H . The difference in pK values has beeno aat tribu ted to the changes in the medium but there is considerableinconsistency in the int erpretation of the spec tra l changes. In

+ 26strongly acidic media the green color is attributed to MnO- or

25to 0-MnOSO-H but in the more aqueous region permanganic ac id i sthought to cause the green color.

In this study it w i l l be demonstrated that only three manganesespecies are present in TFA-water mixtures, namely permanganate ion(MnO ) , permanganyl ion (MnO-+) and permanganic acid (HMnO^),and that th eir proportions depend upon the amount of TFA in the medium.1.5 Permanganate Oxidation of Organic Substrates1.5.1 Oxidation of Hydrocarbons

Very l i t t l e is known about the permanganate ox idation of alkanesprobably because of difficulties encountered as a result of theirlow so lu b i li ty in aqueous media. Some researchers have avoided th is

30 31problem by introducing inert so lu bi li zi n g groups such as ca rboxy l. 'It was found that oxidations of such substrates as 4-methylhexanoicacid could be ca rr ied out in neutral or basic media. However the

30 31carboxyl group was shown to par tici pa te in the ox ida tions . ' Theseoxidations were believed to proceed via hydrogen atom abstraction togive a radical pair trapped in a solvent cage. The rad ica ls quicklyrecombine to give an ester which can decompose in severa l ways, asoutlined b e l o w . ' " ^

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RCH + MnO. > [R-C-MnO.H ] RC-0-MnOH3 4 3 4 3 3

R3C-0|Mn03H" + H 20 > R3COH + MnV

or R 3C40-Mn03H~ > R 3 C + + MnV

(*0-MnOH3or R-C-CH 2CH 2 *- R - C ~C H2C H 2

R J C=00R 0 C=0

18The major pathway is thought to be through attack by water since 0studies show that labelled oxygen from permanganate is found present

18in the hydroxy group. The amount of 0 incorporation was aminimum

31of 25%.Although there are no further reports on the permanganate

oxidation of alkanes the chromic acid oxidation of alkanes has been3336

well es tab lishe d. Some of the mechanistic features are :35(a) The ra te law was found to be r = k[alk ane ][Cr0 3]h Q .(b) Akinetic isotope effect of k /k = 2.5 was found for theri D36oxidation of 3-methylheptane.(c) Hydrogen atom abstractio n appears to occur in the rateo o 3^determining step since the ra tios of 1 :2 :3 were 1:110:7000,

similar values to those found for free-radical bromination.(d) The methylenes in n-alkanes appear to be equivalent since

the rates increase only by a s ta ti st ic al factor for the series of35

C4 H10' C 7 H 1 6 C9 H20' C11H24' C16H34' a n d C22H46*

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(e) In cycloalkanes the tr an si tio n state is thought to involve atr i-co va lent carbon rather than a penta-covalent carbon since thelogarithm of the oxidation rates parallel the logarithm of the

33solvolysis rates of corresponding tosylates.Although Rocek and Mares found that the methylenes in n-alkanes

35were id en tic al in the ir re ac tiv ity to chromic ac id , other researchershave reported that in processes such as nitration the terminal methylenes

37 38react faster than the in tern al methylenes. '

Alkanes appear to be oxidized via a radical mechanism, not onlyin the case of chromic acid oxidation but also in the photochemical

39reaction with oxygen.1.5.2. Oxidation of Arenes

From a va rie ty of reports on the use of permanganate to oxidize40-43 44 45arenes only Cu ll is and Ladbury and Lee and Singer attemptedto study the mechanism of this oxidation in acidic media. Cul l i s and

44Ladbury were the f i r s t to attempt a thorough study of the oxidationof toluenes in a medium of 54.2% w/v acetic acid-water. Unfortunatelysecond order kinetics were not maintained and i n i t i a l rates had to beused. The complicated kin et ics were thought to be due to the p ar ti ci pa tionof intermediate oxidation states of manganese. They reported thatsome ring degradation occurred but that the major products, benzoicacid and benzaldehyde, resulted from side chain oxidation. Electron -donating substituents were observed to accelerate the rate and also toincrease the degree of ring decomposition. Salt effects were onlyobserved i f salts were added that interacted with higher oxidationstates of manganese.

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45Recently Lee and Singer attempted to cl ar ify the mechanism of

arene oxidation. They used p_-toluenesulfonic acid to avoid theso lubi l i ty problems and perchloric acid to avoid the kinetic problemsthought to be caused by the acetic acid. Although the kinetics wereno longer complicated the sulfonate group caused important mechanisticchanges. They found that the order with respect to permanganatechanged from fi rs t- or de r to zero-order at higher a c id it ie s. Themechanism proposed i s the follo wing.

k lp_-toluenesulfonate anion * p_-toluenesulfonatek - l +[arene] [arene H ]

+ VII 2[arene H ] + Mn * products

Using the steady state approximation on [arene H*"] the followingrate law results .

k-k-[arene] [MnV I*]r = k_- +.k 2[M n V I 1]

It was stated that the observed order sh if t could be explained asfollows.

VTT VTTIf, k_-^> k-fMn ], r = Kk- [arene ] [Mn ]

If, k_-

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At higher ac id it ie s the protonation of the p_-toluenesulfonate ionbecame the rate-determining step.

In general the oxidation of arenes seems to have a rate law of theform, r = kfarene][MnO^ ] in the acidic region but the exact mechanismis not completely cl ea r.

The alkaline permanganate oxidation of benzylic tertiary carbon-40 42 43

hydrogen bonds to alcohols is quite well established . ' ' Theprocess is known to have a rate law of r = k[arene][MnO^ ] in thedilute a lk al ine region ([OH ] < 0.01) but has the form of r = kfarene]-- 42[MnO^ J[OH ] in more concentrated a l k a l i . There is s t i l l some disputeas to how the hydrogen is abstracted in the rate-determining step.

42Heckner et a l . have interpreted the ir data and their observedisotope effec t of = 7.8 to ind ica te hydride abs tractio n but

43Brauman and Pan del l, who observed an isotope effect of\L^/k^=11.5and some retention of con figuration, interpret their data in terms ofhydrogen atom abstraction.1.5.3. Oxidation of Alcohols

Since this inv estig ation is prim arily concerned with the permanganateoxidation of arenes and alkanes a l l subsequent compounds considered wi l lonly have the pertinent features of their oxid ation mechanisms out line d.

Alcohols have been subjected to extensive mechanistic invest igat ionin acid, neutral, and basic media. Some of the features of theoxidation mechanism are outlined below,(a) Basic media

(i) In basic media the catalysis observed is due to the generation

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46 47

of an alkoxlde ionfrom the al co ho l. ' The rate law has the

form r = k[alcohol][Mn0 4~][0H~].

( i i ) The observed salt effects are consistent with a bimolecular

transition state resultingfrom the alkoxide ion and permanganate

ion.

( i i i ) Carbon-hydrogenbondcleavage is involved in the rate-

determining step since substantialdeuterium isotope effects are

observed. Thesecanbecomeexceptionally large i n fluo rin ated

alcohols, e.g. k^/k = 16 fo r the oxidation of l-phenyl-2,2,2-

t r i fluoroetha nol.^

(b) Acidicmedia

(i) In acidicmedium the rate accelerations observed with

increased acidity are believed to be due to the generation of

48-50

permanganic acid.

( i i ) The rate-determining step involves carbon-hydrogenbond

scission. Isotopeeffects of k^/k^ -2.4-3.2are observed for

the oxidation of cyclohexanols. In the caseof 2-carboxycyclo-

hexanolseffects of ^/^-p = 7-8 were reported."''*

( i i i ) Therewas no evidencefound for the pa rt ic ip at io n of

intermediate oxidation states ofmanganesein the rate-determining

step for the oxidation of benzyl alcohol tobenzaldehyde in

52perchloricacid-water mixtures.(iv) Inmost cases the rate law follows H ^ 50,52 xri(p_-tolyl)-

48carbinolwas exceptional in that i t followed H . Simil ar

dependenceuponH was found for the chromic acid oxidation ofK53

triphenylcarbinol.

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48

(v) Stewart andBanoo found that thepermanganate oxidation

of d i - and tri-phenyl carbinols involved theionizationof the

carbinol to the carbonium ion which then formed apermanganate

ester. This ester thendecomposedto productspossiblyvia a

cyclic processsimilartothat accepted for chromic acid

54 55oxidations ofalcohols. '

1.5.4. Oxidation of Ketones and Aldehydes

Ketones are subject to f a c i l e oxidation only intheir enol form.

As such they arera pidl y oxidized inalkalineand acidic m e d i a . U n d e r

most conditions permanganateoxidizesdoublebondsby cisadditionto

57 58yield cisdiols, ' butwhenthemediumisg l a c i a l acetic acidthe

59

major products are a-diketones (yieldsup to 80%). Enolicdouble

bondsare not always degraded by cisadditi on since Wiberg and Geer^

havepresented evidence toshowthat the enolate ion generated from

acetone reactsby electron transferin thefollowingmanner.

0~ 0

CH0-C=CH. + MnO ~ s l o w> CH--C-CH + MnO.2"

3 2 4 3 2 4

The oxidationpathwayfor acetone can beshownas a attack,with

60subsequent carbon-carbonbondcleavage,

0 0 0 0II II II II

CH3-C-CH3 CH3-C-CH20H * Ct^-C-CHO + Cl^-C-CO^

CH 3-C0 2 or 2C - C 0 2

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orinamoregeneral formfora l l enolizableketonesas;

0 0 H

II OH" I MnV I 1

R-C-CHR- +'R-C=CR- RCO-H + R-C=0orH

OH

RC=0 - > etc.orH

Aldehydesarereadilyoxidizedinanymedium,yielding chiefly

the respective acidalthoughsomecarbon-carbonbondcleavagemay

alsooccur,presumably viatheenol."^ Inbasicmediathereactionis

6162believedto goviathealdehydehydrateanion. ' WibergandGeer

haveshownthat onlysuchanintermediate canexplaintheincorporation

18 62of 0fromthe medium in theoxidation,offurfurals. Inbasicand

acidicmediatheoxidativeschemeshave beenfoundto bethefollowing;

basic route: R-CHC-H + OH R-C-H R-CHCO-HZ ' I _ 2.

OHJf

R_C=C-H carbon-carbonbondcleavageproducts

f M V I 1acidic route: R-CH-C-H R-CHCO-H

Jf

R2C=C-H * carbon-carbonbondcleavage products.

1.5.5 OxidationofFormicAcid

The permanganateoxidationofformic acidtocarbondioxidehas

beenthoroughlystudiedandthemechanisminbothacidicandbasic

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16-636A

solutions is well estab lished. ' In aqueous solution more basicthan pH5the reaction cl ea rl y involves the formate ion and the6567permanganate ion. Carbon-hydrogen bond cleavage is taking placein the transition state since isotope effects near 7were ob ser ved.^ ^A substantial solvent isotope effect of k_ Q = 2.7 (pH not

66specified) was reported by Taylor and Halpern, in contrast to thevalue of kp -/k^ _= 0.92 (again pH not specified) observed by Bell and

64 2 2Onwood. There is some oxygen tran sfer from the permanganate to the

18formic acid during the oxidation, since 0 from permanganate wasfound in the product, carbon dioxid e.

As the medium becomes more acidic the rate decreases due to the63decrease in the concentration of formate ion but when the aci dit y isincreased beyond 20% sulfuric acid the rate increases, which is believed

68to be due to formation of permanganic acid . Further rate increasesare observed at acidities beyond50% sulfuric acid and these arebelieved to be due to formation of permanganyl ion (MnO-+).

The oxidation is visualized as successive one-electron transfersaccording to the following scheme.

VII - VIHCO- + Mn * CO-' + Mn , slow

T VII VI VI VCO- + Mn or Mn CO. + Mn or Mn , fast

1.6 Permanganate Oxidations in Organic SolventsRecently two methods have been described which extend the use of

69ermanganate as an oxidant into organic solven ts . Sam and Simmons

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were able to prepare crown polyether complexes of permanganate whichare soluble i i organic solvents. These complexes can be used in s i tuor can be isolated and then used in some other solvent to achieve a

70

wide variety of oxidations. Starks was able to use the method ofphase-transfer cat aly sis to rea dily oxidize 1-octene to heptanoic acid(author states hexanoic acid, possibly a printing error). Thisprocess involves the use of quaternary a lkyl ammoniumsalts to extractthe permanganate anion into the benzene laye r. In th is inve st igat ionStarks' method^ wi l l be extended by examining the oxidation of alcohols,aldehydes, and st ilb enes by permanganate in benzene.1.7 El ec tr op hi lic Aromatic Nit rat ion

One of the oxidants that might be generated in acidic media ispermanganyl ion (Mn0.j+). This species could attack the aromatic ringsof arenes in a manner similar to that of the electrophile, nitroniumion (N0 2 +) . Because of this possibility some of the features ofelectrophil ic aromatic nitrations wi l l be presented.

Electrophilic aromatic nitrations are known to follow the72 73 21 73function, ' ' more pr ec isely + log a Q , carbon-hydrogen

bond cleavage does not occur in the rate-determining step since onlyvery small (secondary) isotope effects are observed.^4 ^ Thenitration rates show some solvent dependence.^ The rate-determ iningprocess is generally agreed to be ele ctr op hi lic a tt a ck ,^ ^ which

78is followed by fast proton-loss to the solvent.Small amounts of phenolic products have been isola te d from the

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18 -

79

nitrat ion products of toluene. The mechanism of the processyielding phenols is at present unknown. It could be possible that thecomplex resulting from 0 attack by nitronium ion can be hydrolyzed toyield phenols. The following scheme accounts for the known facts andproposes a route to the observed phenols.

HN03 + H H20 + N02 , fast

N attack, + NO,+

(ArHN02+)slow

[ArHN02+] ArN02 + H , fast

0 attack,

0-N=0

R-

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- 19 -

80s.or A-2 mechanism. They proposed that i f log k- vs -H giveslinear plots with close to unit slope that an A - l mechanism i sindicated but if log k- vs log [H +] gives linear plots with closeto un it slope that an A-2 mechanism is operative. Although someknown A - l and A-2 reactions were found to be con sisten t with this

80bc r i t e r i a exceptions were reported thus l imiting the u t i l i t y of thehypothesis.

One feature of thei r hypothesis remains pot en tia lly very use ful.This i s the conclusion that when log k- vs -H gives lin ea r p lot swith unit slope the ratio of the activity terms in the rate expressionchanges in the same way as does the ratio of the activity terms ofthe indicators used to establish the acidity scale , (k- is theexperimentally determined rate constant, corrected only for substrateconcentration). This po stulate can be generalized in the followingmanner: For a general case of a bimolecular reaction

k 2A + B p roduc ts, k. is rate-determining

the measured rate law is r = k-[B][A]where

k l = k 2 f A f B / f t '

If log k 1 vs -H gives a linea r plot thi s means that h = f f R / f fand i f h x is defined as h x = aH +f_ /f_ R + then f A f f i / f + = a ^ f . /f_ R + .

X X X X

This reasoning in itself does not cl ar if y any mechanistic features buti f some of the a ct iv it y coeffi cients are known practical use can be

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- 20 -

made of th is re lat ion sh ip. This r ela tio nship can be used to evaluatea proposed mechanism by seeing i f the ac t iv it y terms pred icted do infact show p ara l le l solvent changes to those of the indica tors used insetting up the acidity scale.

In thi s investi gatio n three dif fer ent oxidants could be generated,which leads to at least three di fferen t mechanisms. The choice madebetween the three mechanisms l i s ted below pas based on how logcorrelated with ei ther H or L , The three mechanisms are:

o R(a) Permanganate ion as oxidant./

MnO + substrate -* X productsr =kV[Mn04-][S]f f/f

4

(b) Permanganic ac id as oxidant.

MnO ~ + H + HMnO,4 4K+ t k +HMnO + S X * products

r = KK+k +[Mn0 4-][S] V f M n O , - f S / f t4(c) Permanganyl ion as oxidant

MnO " + 2H + Mn0_+ + H o04 3 2/

M n 0 3 + s 7 x > products

r - KK+k f[MnOA-][S] a j + f ^ f / a H f +4 2

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2. SCOPEOF THE INVESTIGATIONThe purpose of this in ves tiga tion is to determine the mechanism

of the oxidation of alkanes and arenes by permanganate. The mediumused to achieve the necessary solubilities was trifluoroacetic acid-water. A pseudo fi rst-order approach was used to determine the orderof the oxidant. Concentration varia tio ns of substrate and acidityestablished th ei r respective orders. Complementary techniques such asproduct studies, substituent effects, activation parameters andisotope effects completed the mechanistic inve st igat ion. The oxidationmechanism of several alco ho ls , aldehydes, and ketones was alsoinvestigated to aid in interpreting the results obtained with alkanesand arenes.

As a result of the poor fit of the kinetic data with the previouslydetermined H function the H function was determined for the mediumo Rof TFA-water. The id en ti ty of the oxidants in th is ac id ic medium wasestablished by spectrophotometry and cryoscopic means.

Since the arene oxidation showed similarities to electrophilicaromatic substitu tion reactions the behavior of several ele ctrophi lessuch as nitronium and nitrosonium ions was examined in the TFA-watermedium.

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3. EXPERIMENTAL3.1 Reagents and Indicators

Most of the compounds used for this investigation were availablecommercially. The reagents, their source, method of pur if ic at io nand pur ity are presented in Table 1. The ind icators used to es tabl ishthe H acidity function along with pertinent physical data areRpresented in Table 2.

The tr if lu or oa ce ti c acid (TFA) used throughout th is inves tigationrequired special care in purification to minimize the decompositionof the oxidant, which was found to be due to minute impurities presentin the acid. This procedure, and the synthetic routes followed forthe preparation of the remaining compounds, are outlined below.

Trifluoroacetic a cid: The commercial product supplied by Eastmancould not be sufficiently purified by distillation through a 12"Vigreaux column. It was found that i f the acid was di s t i l l e d fromsmall amounts of potassium permanganate, approximately 0.5 g per kgof acid, that the fraction collected between 71.0-71.5 had no sign ifi cantpermanganate decomposition for aqueous solutions less than 5 Mand gaveonly small blank corrections, ranging from 1-10% of the observed rate,for solutions 5-12 M. This method of using small amounts of potassium

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Table 1. Specifications of Compounds.Compound Source Pur ific atio n method Puri ty Litera turen-Pentane E v.p.c. column1 >99.8%iso-Pentane E used as received >99.8%n-Hexane M spectral grade >99.8%n-Heptane E dis t . 9 7 . 5 - 9 8 . 0 >99.8%n-Octane A used as received 99.5%n-Nonane F dis t . 1 4 9 . 5 - 1 5 0 . 0 99.5%n-Decane A used as received 99.6%n-Undecane A v.p.c. column1 99.7%n-Dodecane E dis t . 215-216 >99.8%n-Tridecane A used as received >99.5%Cyclopentane A spectral grade U.A.R. >99.8%Cyclohexane E spectral grade U.A.R. >99.8%Cycloheptane A U.A.R. >99.7%Cyclooctane A U.A.R. >99.7%Methanol B spectral grade U.A.R. >99.9%Ethanol F dis t i l led >99.8%2-Pentanol A v.p.c. column1 >99.8%3-Pentanol A it II it >99.8%Formaldehyde F 37.4% standard soln. -2-Pentanone E v.p.c. column1 >99.8%3-Pentanone E II II II >99.8%Cyclohexanol E dis t . 1 5 8 . 0 - 1 5 8 . 5 99.5%Cyclohexanone E dis t . 1 5 1 . 0 - 1 5 1 . 5 99.8%Acetone E spectral grade U.A.R. >99.8%Formic acid E dis t . 1 0 1 . 0 - 1 0 1 . 5 >99.8%Propionic acid F U.A.R. >99.8%2,4-Pentanedione E dis t . 1 3 7 . 0 - 1 3 8 . 0 99.5%Propionitrile E v.p.c. column1 >99.8%Nitroethane M II tt II >99.8%1,1,1-Trichloro- F II II II >99.8%ethane

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Table 1 (Continued)

Compound Source Purification method Purity LiteratureToluene E dis t . 1 0 9 . 0 - 1 0 9 . 5 >99.8%Ethylbenzene E v.p.c. column1 >99.8%Cumene E II II ti >99.8%t- Butylb enz ene E II II II >99.8%Benzyl alcohol E dis t . 2 0 3 . 0 - 2 0 4 . 0 >99.8% Benzaldehyde E dis t . 1 7 3 . 5 - 1 7 4 . 0 >99.8% Acetophenone E v.p.c. column1 >99.8% Trifluoroaceto- K & K II tt ti 99.5% phenoneBenzene F spectral grade U.A.R. >99.8% 1 3 2 8 1 a8 5 . 2 8 1 b2 1 0 . 8 5 8 1 cChlorobenzene E dis t . 1 2 9 . 0 - 1 3 0 . 0 >99.8% 1 3 2 8 1 a8 5 . 2 8 1 b2 1 0 . 8 5 8 1 cFluorobenzene E dis t . 8 2 . 5 - 8 3 . 0 >99.8%

1 3 2 8 1 a8 5 . 2 8 1 b2 1 0 . 8 5 8 1 cNitrobenzene E dis t . 2 0 7 . 0 - 2 0 8 . 0 >99.8%1 3 2 8 1 a8 5 . 2 8 1 b2 1 0 . 8 5 8 1 cAnisole E dis t . 1 5 0 . 0 - 1 5 0 . 5 >99.8% 15581d2-Phenyl-2-propanol A U.A.R. 99.5% 2-Phenylethanol A U.A.R. 99.0% 1-Phenylethanol A U.A.R. 99.5% p-Bromotoluene E recrystallized >99.8% 1 8 4 8 1 e1 6 2 o81f-Bromotoluene F dis t . 1 8 0 . 0 - 1 8 0 . 5 >99.8% 1 8 4 8 1 e1 6 2 o81fm-Chlorotoluene K & K dis t . 1 5 8 . 0 - 1 5 8 . 5 >99.8% 1 8 4

8 1 e1 6 2 o81f

KMnO4 B Baker analyzed 99.5% Ethane M U.A.R. 99.99% Propane M U.A.R. 99.5% Butane M U.A.R. 99.5% neo-Pentane FLUKA U.A.R. 99.92% Cyclohexane-d^2 MSD U.A.R. 99% D Methanol-d^ MSD U.A.R. 99% D Deuterium oxide MSD U.A.R. 99.8% D Toluene-a-d3 MSD U.A.R. 98% D Toluene-ds MSD U.A.R. 98% D H 2S0 4 B U.A.R. 97.4%

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Table1(Continued)

Compound Source PurificationMethod Purity Literature

Benzoicacid Ep-Nitrotoluene Em-Nitrotoluene Ep-Methylbenzoic acid E

m-Methy-benzoic acid ETetra-n-hexyl-ammoniumiodide E

m-NitrobenzylalcoholEm-Methoxylbenzyl

alcohol Ap-Chlorobenz-

aldehyde Em-Methylbenz-

aldehyde K & Kp-Nitrobenz-

aldehyde K & KBenzophenone Ecis-Stilbene K & Ktrans-Stilbene FTolan A

p-Nitro-trans-stilbene A

p,p'-Dinitro-trans-stilbene A

p,m'-Dinitro-trans-stilbene A

Sodiumnitrate BSodium nitrite F

sublimedrecrystallizeddist.230.0-231.0recrystallized

U.A.R.

dist. 210.0-212.010 mm

dist.254.0-256.0

recrystallized

dist. 201.0-202.0recrystallized

II

U.A.R.U.A.R.recrystallized

mp122.0-123.0C

mp50.0-51.0

mp178-180

mp11.0-112.0

U.A.R.U.A.R.

mp47.0-48.0

mp104.0-105.0mp48.0-49.0fp4-5

mp121.0-123.0mp60.0-62.0

mp155.0-158.0

mp296-299

mp220-221analargrade

96.6%

1 2- 081g

52o81h

227181 8 1 1

lll-11381i

175-180at 3mm81J

252 o 8 1 k

4791

201 8 1 m

106 8 1 n4981o

5-681P124 81P6 2 5o81q

155 8 1 r

288 8 1 a

2]_yo81s

to

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Table 1 (Continued)

A l lcompoundsexceptthelast 31 werecheckedby v.p.c

Column 1 - 20% Carbowax 20 M onfirebrick - 20' x 3/8"Column 2 - 20%Dionylphthalate on Chromosorb - 20' x 3/8"

u.A.R. - usedasreceivedE -EastmanChemicalCo.

F - Fisher Scientific Co.K & K -K & KLaboratoriesLtd.A - Aldric hMSD -Merck, Sharp andDohmeB -J.T. Baker Co.

M -Matheson of Canada Ltd.

usingcolumn 1.

> i

ho

ON

I

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Table 2. IL, Functio n Indicato rs for TFA-H-0No. Compound Experimental Literature

mp A logemax mp max log Source

1 4,4',4"-TrimethoxyTPC

2 4,4'-Dimethoxy-4"-methylTPC3 4,4'-Dime thoxy

TPMCL4 4,4',4"-Trimethyl-

TPC5 4-Methoxy TPC6 4,4'- Dim ethyl TPC

7 4,4'-Di ethox y DPC8 Triphenylcarbinol9 4,4' ,4"-Trichloro

TPC10 4-N itro TPC

81 .0-82 .0 480 5.04 81.0-82.074.0-76.0

72.0-74.078.0-78.5

56.0-57.0

487427111.0-113.0 495

91.0-92.5410446469394

506163.0-164.0 425

91.0-93.0403460

96 .5-98 .0 436388

4.934.674.884.564.974.824.38

451 4.81

5.224.604.605024.514.47

,21 4854782182 5.024.94

2182

50021 5.47 2194.0' ,83 45221 5.03 2160.08 2 . 0 8 57 5 . 0 8 675.5-76.5

82,84 47621 4.75 21

.8780.0' 88 456

5058787

-4.44 875.10 89

1 6 4 - 1 6 5 8 1 t 4 3 1 2 1 , 425 8 2 4 . 6 0 2 1 , 4 . 6 4 8 24 0 4 2 1 , 410 8 2 4 . 6 0 2 1 , 4 .63 8 2219 3 . 5 - 9 4 . 0 9 0 465 219 5 . 5 - 9 7 . 0 9 1 454 21

5.01"

synthesized

synthesizedAidrichlab stock |

N

lab stock i

lab stocklab stocklab stocklab stocklab stock

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Table2. (Continued)

3 References21, 87, 89useda medium ofH-SO^-H-0.

Reference82useda medium of CF-CO-H-CCF-CCO-O.

Valuesfor therespectivecarbonium ion generated

A l l compoundswerecheckedby NMR forsubstituent identity

TPC = triphenylcarbinol

TPMCL = triphenylmethyl chloride

DPC = diphenylcarbinolN3oo

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- 29 -

permanganate avoided a problem also noted by Lee and Johnson. Itwas observed that a green species contaminated all of the distillatewhen large quantities of potassium permanganate were used.

Water: The water used for the determination of the pK of permanganicacid and the H_ function was r ed is t i l l ed from basic permanganatethrough a 14" Vigreaux column. For the kin eti c analysis d i s t i l l edwater was found to be satisfactory, causing no further permanganatedecomposition.

Benzyl al co hol -q -d-; This compound was prepared by the lithiumaluminum deuteride reduction of benzoic acid according to the followingprocedure. A 100 ml so lutio n of dry ether containing 0.01 moles ofbenzoic acid was carefully added over a period of one hour to 0.008 molesof lithium aluminum deuteride suspended in 100 ml dry ether . There su lt in g mixture was allowed to reflux for a period of 14 hours. Theso lu tion was then quenched by the ca reful addi tio n of 5 ml 95% ethanolfollowed by 10 mi water. The complex was destroyed by the ad ditionof 50 ml 10% sulf uric acid . The ether layer was recovered, washedonce with 50 ml 1 N sodium hydroxide, twice with 50 ml water and thendried over anhydrous magnesium sulfate. The ether layer was recoveredand fla sh evaporated. The residue was puri fi ed by v .p .c . using a 20%carbowax 20 M on f ir ebric k column 20' x 3/8". The y ie ld was 20% of thedesired alcohol with an iso to pic puri ty of 95% as determined by NMR.The i n i t i a l deuterium content of the lithium aluminum deuteride was96.8%.

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4,4',4"-Trimethoxytriphenylcarbinol: The method outlined by Baeyer92 93and V i l l iger was used with minor modifications from Vogel. To

a Grignard solution prepared from 0.10 moles of magnesium turningsand 0.10 moles of 4-bromoanisole i n 300 ml dry ether was added 0.10moles of 4,4'-dimethoxybenzophenone dissolved in 100 ml dry ether ata rate to keep the ether re fluxi ng. The mixture was refluxed afurther two hours and then poured over a s lu rry of 750 g ice and 25 mlconcentrated su lfu ric ac id . Twenty-five grams of ammonium chloridewas then added to further decompose the magnesium complex. Theether layer was recovered, washed successively with 100 ml water, 100 ml2% sodium bicarbonate, and 100 ml water. The ether was removed byflash evaporation yield ing a viscous red o i l . This o i l was steamdi s t i l l e d and then dissolved i n hot n-heptane. Recovery of thedesired product was extremely difficult and the following somewhatprimitive method was found succes sful . The hot so lu tion of n-heptanewas slowly cooled, whereupon a red substance oiled out. Upon standingfo r a day cr ys ta ls were observed growing from the red o i l . Severalseed cry stal s were choosen to impregnate a st r in g , the so lu tio n wasreheated to dissolve a l l substances, and then the seeded string wassuspended in the solution which was allowed to cool slowly.Considerable crystal growth occurred on the string and beaker walls.These pale orange crystals were further readily recrystallized fromn-heptane yie ldin g 30% of the white product mp 8 1 . 0 - 8 2 . 0 , l i teraturemp 8 1 . 0 - 8 2 . 0 . 2 1

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4,4'-Dimethoxy-4"- methyltriphenylcarbinol: This compound was preparedin a similar manner to that described for 4,4',4"-trimethoxytriphenyl-carbinol with the exception that 4-bromotoluene was used to preparethe Grignard reagent. Similar puri fi ca ti on problems were alsoencountered. The f ina l white cr ys ta ll in e product, obtained in 10%yield , had mp = 7 4 . 0 - 7 6 . 0 .

Elemental analysis: the oret ical, C - 79.02%, H - 6.63%.experimental, C - 78.96%, H - 6.51%.

3.2 Kinetic Procedure in Acid-Water MediumIn a l l of the kin etic experiments except numbers 180-184, 219-223,

240-249, 252-266 and 275-284, a pseudo first-order approach was used,the i n i t i a l substrate concentration being in a 20-fold excess or more.A l l of the rate constants were obtained by observing the disappearanceof the 526 nm permanganate absorbance. These spectrophotometricmeasurements were made using a Bausch and Lomb 505 spectrophotometerexcept for some very slow reactio ns in which a Cary 16 was used. Bothmachines were equipped with thermostated c e l l compartments. A l lkinetic experiments except where noted were performed at 2 5 . 0 + 0 . 2 .

A typ ica l kine tic run was in it ia te d by placing nine parts (2.70 ml)of substrate stock so lutio n into a cuvette, adding one part (0.30 ml)of potassium permanganate stock so lu tion , s topper ing, mixing thoroughlyand then placing the cuvette in to the c e l l compartment and observingthe 526 nm absorbance with respect to time. The permanganate stocksolution had previously been thermostated at 2 5 . 0 and was composedof a precisely weighed amount of potassium permanganate in 100 ml of

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water to give a concentration close to 4.0 x 10 M. The kin eti c-4

concentration was then 4.0 x 10 Mwhich gave an i n i t i a l absorbanceof 0.9. The substrate stock solution was also thermostated prior touse at such a temperature that the mixture reached a temperature of 25upon mixing. Usually the heat of mixing was no more than 2 . Afterthe kinetic run the 3.00 ml sample was titrated with 2.00 N sodiumhydroxide to obtain the acid molarity.

Because a wide variety of compounds were examined several methodshad to be used for the preparation of substrate stock solut ion s. TheTFA-water solutions of the desired acid strength were prepared bymaking up to volume a measured quantity of water with TFA. Stocksolutions of substates which were neither excessively volatile norunderwent reaction with the solvent (such as alcohols which esterified)were simply prepared by adding a known weight or volume of substrateto a known volume (usually 10.0 ml) of TFA-water so lutio n and mixingthoroughly. Stock solutio ns of alcohols were sometimes prepared inthis manner but had to be used within one minute because of este rific a-t ion. Some of the kinetic experiments involving larger quantities ofalcohol were initiated by adding several p of alco hol to a 9:1mixture of acid-water and permanganate-water so lu tions.

Stock solutions of the gaseous compounds investigated, namelyneopentane, n-butane, propane and ethane, were prepared in thefollowing ways. Neopentane at 0 was syringed in to a 25 ml volumetriccontaining a known quantity of TFA-water solu tio n. The solut ion wasshaken vig oro usly, opened to the atmosphere and then reweighed. Theincrease in weight was taken as the amount of neopentane per known

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- 33 -volume of TFA-water so lu tio n. Stock solutions of the other threegases were prepared by subjecting 10.0 ml of TFA-water solution in a25 ml volumetric flask containing a small magnetic s t ir r in g bar to2 lbs pressure of the desired gas by means of a rubber tube connectingthe gage and the fl ask . The tubing , but not the flask had beenflushed with the desired gas to exclude a i r . The so lutio n was thenst ir red at a moderate rate by means of the magnetic s t irrer for atime not exceeding two minutes. The s t ir r in g time depended upon theamount of dissolved gas de si red. The connecting hose was removed andthe volumetric flask was restoppered and reweighed. The weight gainwas taken as the amount of gas uptake. These so lution s were usableup to one hour after preparation after which time gas losses becameserious. The rate constants obtained from these experiments were offa i r re pr od uci bi li ty , ranging from the poorest, + 10% per dup licateand + 20% for k_ (k= k , ,/[substrate]) over a 2.5-fold concentra-

I I observedtion range for ethane, to the best , n-butane, which gave + 10% fordupl icates and + 11% forY.^ over a twenty-fold concentration range.It was concluded that these results were not biased by air displacementto an extent beyond their reproducibility.

Some experiments were performed using a medium of sulf uric acid-water. These experiments were ca rr ied out in the same manner asdescribed for those in which a medium of TFA-water was used.Unfortunately the re su lts were unsat isfactory, as w i l l be demonstratedIn Section 4.3.

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3.3 Kinetic AnalysisThe time-absorbance data were analyzed by the method of least

squares by arranging the data i n a linea r form of y = mx + b. Theleast squares^analysis was performed by a set of programs previously

94developed by the author.The basic kinetic equations used vrere derived from the general

rate equation (1) in the following manner.

[0 x]=[Mn04].

[S] = [substrate]h = applicable aci dit y functionT = temperature K

k_ = Boltzmann's constanth = Planck's constantc = constant of integrationn,m, and 1 are powers

(1) -d[o]/dt=k3[ox]n

[s]Vx

In the pseudo-order approach h, S and T are kept constant whichreduces equation 1 to equation 2.

(2) -d[ox]/dt=k-[ox]

where k- = k - t s A 1

Only two values of n were encountered in this study, namely n = 1, apseudo first-order case, and n = 0, a zero-order case. These aresolved below.

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when n = 0:-d[0 ]/dt = k

x o(3) -d[0x] = kQdt

Integration of equation 3 yiel ds equation 4.

(4) [0 ] = k t + cx oWhen [0 x] is plotted against time the slope is the desired rateconstant, k .

' owhen n = 1:

-d[o x]/dt =

(5) - d I O x 1 / I O x ] = k l d t

Integration of equation 5 yields equation 6.

(6) -ln[0 ] = k n t + cx 1When - ln[0 x] is plotted against time the slope gives the desired rateconstant k^.

In practise the actu al analy sis is performed by using absorbancevalues in equations 4 and 6 since by Beer's law = Absorbance(e = molar extinctio n co ef fi ci en t) . In the case where n = 0 the

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slope is then equal to but in the case where n = 1 the slope iss t i l l since ln[0 1= In Abs. - Inzand In e i s a constant which1 xthen only affects the intercept.

Once k, or k had been determined for a set of different conditions1 othe influence of the other parameters could be evaluated as follows.For a set of k, values (a pa ral lel situation exists for k ) where1 oonly [S] varies,

k- =k - I s A1

= k 2 [ S j m , k 2 = k 3 h 1

and i t follows that log k- = mlog[S] + logk^. When log k. is plottedagainst log [S] the slope is the order of the substrate. By varyingthe acidity one can in an analogous manner, establish which acidityfunction best f i t s the experimental data.

k 2 = k.h1

= ^/[sf

log k 2 = 1 l o g h + l o g ^ 3

or in the more familiar form

-logk_ = 1 H + c2 x

where the slope of the -log k2 vs H x plot gives the order of thereaction with respect to h .

r xThe activation parameters were obtained in the usual manner by

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- 37 -95converting the Wynne-Jones Eyring equation into a lin ea r form.

-AG +/RTk 2 = (k T/h).e

AG+ = AH1"- TAS1*AS t/R -AH +/RT

k 2 = (kT/h)-e

ln(k 2 /T ) = -(AH+/RT) +AS+/R + ln(k/h)

fWhen ln(k 2/T) was plotted against 1/T the slope gave -AH /R andt t tthe intercept was equal to AS /R - ln(k_/_h), from which AS and AHwere easily obtained.

The previously mentioned 45 trials which were not analyzed by apseudo fi rs t- or de r approach were analyzed by equation 8 which i sderived below. This equation is valid for all similar kineticswhere the substrate is not in sufficient excess. A l l that isrequired is the stoichiometry of the reaction.

12[Mn04~] + 5[n-alkane] + 5R'C02H + 5RC02H + 12Mn2+

the stoichiometry for this example is 12/5 = 2.4.The subscripts t and o denote time = t and time = 0, respective ly

-dtOx]/dt = k 2IO xJ t.IS] t

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^ t * t S ] o - TJ x ] o " l* ]t>

d [ x ] tdt - k 2 t V t ( [ s ] o - n t V o + i 4 [ x ] t )

-d[o x] t

(7) [0 Jx o 2 4 IVt> = k-dt

Equation 7 can be readily integrated since i t is of type ^ dx(a+bx)1 . a+bx 96 QIn to give equation8.

, [s ] , [0 ] .(8) \

l n (ToV -TJ WT + _ 7 ) = k 2 t + c

In actual practice absorbance was used instead of[0 1 . Thisx tconversion was re adily made by Beer's law, ^ x l t = A/e.

3.4 Inorganic Product StudyThe inorganic products were determined by iodometric ti t ra t io n .

-3A 4 x10 Msolu tion of permanganate was made up in a TFA-watermedium in which the substrate had a fast oxidation rate. Then to10.0mlof th is so lu tion was added enough neat substrate to ensure at leasta ten-fold excess. The solu tion was thoroughly mixed by means of amagnetic stirrer unt i l no further reaction was observed but in no casewere any reactions allowed to go past two hours. After th is time nopermanganate ion could be detected but in most cases manganese dioxide

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prec ip ita te could be observed. The to ta l solu tio n was ti tr at ed forremaining manganese oxidizing power (e ssen tial ly the amount of

-2manganese dioxide present) by adding 10 ml of 4 x 10 M potassiumiodide in aqueous 2% sodium bicarbonate and then ti t ra t in g with astandardized solut ion of sodium thiosulf ate to a starch end-point.The Thyodene indicator was added near the end of the t i t r a t io n . Blankcorrections for particular TFA-water solutions were made, i f necessary.3.5 Organic Product Studies3.5.1 Permanganate Oxidations in TFA-Water

Organic products were determined for se lected compounds which arelisted in Table 3 along with reaction medium and product detectionmethods. Typically the procedure was to prepare 20-25 ml of thedesired TFA-water solution in which was dissolved 0.5 g of potassiumpermanganate. When a l l of the potassium permanganate had disso lved4.00 ml of the substrate was added quickly with constant stirringby means of a magnetic s t i r r e r . The reac tio n was allowed to proceedfor one hour or unt i l a l l the permanganate was consumed. The amountof substrate added was enough to ensure at least a f ive-fold excessexcept for three substrates. n-Pentane, cyclohexane and toluene werenot completely soluble so a two phase system existed in these cases.The reac tion after completion or one hour duration was quenched by theaddition of 40 ml water, saturated with sodium chloride and thenextracted three times with 50 ml ether. The ether extracts werecombined, dried over anhydrous magnesium sulfate and flash-evaporatedto reduce the volume to 30 ml or less depending upon the volatility

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Table3. ProductsofKMnO.Oxidationsin TFA-WaterMedium

Substrate wt. a -H^ %Recovered0 Products wt. % d Method

n-Pentane 2.5 7.5 40%

2-Pentanol 3.2 7.5 89%

3-Pentanol 3.3 7.5 90%

2- Pentanone 3.3 7.5 94%

3- Pentanone 3.3 7.5 90%

Cyclohexane 3.1 7.5 83%

Cyclohexanol 3.9 7.5 95%

Cyclohexanone 4.0 7.5 89%

Toluene 3.5 4.5 95%

Toluene 3.5 7.5 82%

2-Pentanone - 66% 13-Pentanone - 34% 1Propionic acid 2

2-Pentanone - 3Propionic acid - 2

3-Pentanone - 3Propionic acid - 2

Propionic acid - - 2

Propionic acid - - 2

Cyclohexanol *y0.01 5% 4Cyclohexanone M).01 5% 4

Ad ipic acid 0.2 90% 5

Cyclohexanone ^0.03 4% 4Ad ipic acid 0.35 91% 5

Tar 0.03 5%

Adip ic acid 0.5 90% 5

Tar 0.05 10%

Benzoic acid .25 - 6

Benzoic acid .05 6

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Table 3 (Continued)Substrate ^ awt. -H b %Recovered Products wt. % d MethodBenzyl alcohol 4.2 4.5 91% Benzoic acid .3 - 6Benzaldehyde 4.2 4.5 93% Benzoic acid .4 - 6Acetophenone 4.1 7.5 92% Benzoic acid .2 - 61-Phenylethanol 4.1 5.0 95% Acetophenone .4 100% 72-Phenylethanol 4.1 5.0 93% Phenylacetaldehyde (trace) 8Phen ylacetic aci d .1 99% 9

i n i t i a l amount of substrate added in grams; b cac id ity of r eac tion medium; based on tota l materials I(products and reactants) recovered; based only on products recovered .1. Co llec ted from chromosorb 102 1/4" x 10' column, re pu ri fi ed on the 20% carbowax 20 M on firebrick60/80 column 3/8" x 20' as a mixture. Identif ied as the ketones by retention times and NMR.Ratio determined from NMR integral.2. Collecte d from underneath theCF3CO2H peak from the chromosorb 102 column. Identified by NMR.3. Iden ti fied by retent ion times on chromosorb 101 and 102 columns against known compounds.4. Iden ti fied by retention times on chromosorb 101 and Carbowax 20 M columns against knownsamples.Cyclohexanol existed as the trifluoroacetate ester.5. Isolated, then purified by recry sta lliz ati on, mp = 145-1486. Isolated, the puri fie d by sublim ation, mp = 1 2 1 - 1 2 2 , li te ra tu re mp = 122

lite rat ure mp = 149-150c81q81u

7. Ide nti fied by reten tion times against known compound on the Carbowax 20 M and the 10% S il icon GE SF-96firebrick 60/80, 1/4" x 10' columns.8. Collected from the Si li co n GE SF-96 column9. Isolated, then pu ri fi ed , mp = 70-73

Identified by NMR.81vliter atur e mp = 76 . Verified by NMR.

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of the compounds. It was observed that reaction would continueafter d il ut io n with 40 ml water for the phenylethanols, which wasprobably due to the hydrolysis of esters formed with TFA (see Section4.5) . For n-pentane a sample of the original ether extracts wasretained before flash-evaporation to determine the amount remainingafter ox ida tion. The concentrated ether extracts were then analyzedby v .p .c . for vo la t il e components on a Varian Aerograph 90-P coupledwith a Varian A-25 recorder using the series of columns mentioned inTables 3 and 4. Non -volat ile compounds, namely phenylacet ic acid ,benzoic ac id , and ad ip ic acid were recovered by evaporating the extractsto dryness and then pur ify ing the crude products. Benzoic ac id wasreadily sublimed and adipic acid could be recrystallized from etheror water. Phenylacetic ac id , which was recovered from the oxidationof 2-phenylethanol, was identified by NMRsince after severalre cr ys ta lli za tio ns from petroleum ether the melting point was s t i l llow.

Several problems were encountered in the product analy si s, theseverest of which was that large quan tit ies of TFA were extracted alongwith products. This masked the ac id ic products from the C-5 series,preventing y ie ld ca lcu lat ions from being made. Only propionic acidcould be co llect ed. Although one would expect some acet ic acid noneof the columns, even the Chromosorb 101 columnwhich i s specif ica l lydesigned for low molecular-weight acids, were capable of separatingi t from the larger amounts of TFA. It was also found that 2-pentanoneand 3-pentanone could not be separated from each other. This problemwas finally avoided by collecting the trace amounts present of this

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ketone mixture and ve rif ying th eir id ent it ie s by NMR. The 2-pentanoneto 3-pentanone ratio could be calculated from the peak integrals ofthe NMR spectrum of the mixture.

3.5.2 Nitr at ion and Nitro sation Products in TFA-WaterIt was found from the freezing-point depression experiments that

the TFA medium could be used to generate n itronium ions from sodiumni trat e and nitrosonium ions from sodium n i t r i t e . It was consideredworthwhile to see how eff ec tively ni tr at io n and nit ro sat ion could becarried out in this medium.

The general procedure was to dissolve 0.01 moles of the sa l t inneat TFA and then to add 0.01 moles of substrate while constantlyst i rr ing the mixture with a magnetic s t i r r e r . The react ion wasallowed to continue for four hours after which time i t was quenchedby the additio n of 20 ml water. The re su lt in g so lu tio n was made basicby the add ition of ei ther 6 N sodium hydroxide or sodium hydroxidepellets, then saturated with sodium chloride and extracted with threesuccessive 50 ml portions of ether. The ether extracts were combined,dried over anhydrous magnesium sulfate, and reduced to 50 ml byflash-evaporation . The re su lti ng concentrates were analyzed by v .p .c .The results are li st ed in Table 4.

It was noted that the basic aqueous layers were highly colored.This co lor could be removed by ac id if ic at io n and restored by the ad dit ionof base. Subsequent inv es tigation of the u .v . ch arac teris tic s of thebasic and ac id ic solutions ind ica ted phenolic compounds. The recordedspectra strongly resembled those of ortho and para ni troph enol . SeeSection 4.1.3 for further discussion and also reference 79.

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Table\4. Nitration and Nitrosation Products.

Reactants %Recovered %Converted Products %ofProduct DetectionMethodPhenol andNaN03 - black t a r

3 - -

TolueneNaN03 and 97 \ 9 5 p-nitrotoluene 31.7p_-nitrotoluene 67.0Tfi-nitrotoluene 1.3phenols

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3.6 Determination of Stoichiometri.esThe stoichiometry could only be determined for those compounds

where the competitive decomposition of permanganate was sm al l. As are su lt the stoichiometry for weak reductants such as ketones and somealkanes could not be determined.

The general procedure was to make up substrate solutions in TFA-water of such strength that when 20-30 \i was added to 3.00 ml of a

-4solution 4 x 10 M in potassium permanganate made from the sameTFA-water solution as the substrate the oxidant was always in excess.The absorbance of the permanganate ion was determined before ad dit ionof substrate and then again when no further decrease in absorbancewas noted. This was done for both the reacting cuvette and thecontrol cuvette. Then 2.00 ml from each was added to 10.0 ml of anaqueous solution which was 4 x 10 M in potassium iodide and 2%in sodium bicarbonate. The liberate d iodine was ti tr at ed with a

-42 x 10 M standardized sodium thiosu lfa te so lut ion to a starch endpoint. These experimental values are tabulated in Table 5.3.7 Freezing-Point Depression Procedure

It was observed that when potassium permanganate was dissolvedin neat TFA that a green species was generated similar to that

24previously reported for su lf uri c acid solu tions of this compound.It was considered important to determine the identity of this speciessince i t could very well be involved in the oxidatio ns. Previous

26attempts to identify the species in su lf ur ic acid had not been25 27conclusive. ' It was fe lt that since TFA is a mono-protic acid the

http://stoichiometri.es/http://stoichiometri.es/

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- 46 -Table 5. Experimental Data for Stoichiometric Determinations for

KMnO. Oxidations in TFA-H-O.a bCompound R_ Moles substrate MolesMn04 Difference Y/Xapplied = X x10 7x10 7in x10 7= YBlank Trial

Isopentane -8. 42 5.15 9. 49 3.95 5.54 1.08n-Pentane II 2.60 9. 37 8.36 1.01 0.39n-Hexane II 2.30 9. 67 8.09 1.58 0.68n-Heptane II 1.36 9. 43 8.15 1.28 0.94n-Heptane II 2.72 9. 37 6.45 2.92 1.08n-Octane II 2.46 9. 49 6.08 3.41 1.39n-Nonane II 1.67 9. 49 6.42 3.07 1.85n-Decane II 1.54 9. 25 6.05 3.20 2.08n-Undecane ti 1.40 9. 31 5.99 3.32 2.33n-Dodecane II 0.88 9. 31 6.57 2.74 3.13n-Tridecane II 0.82 9. 37 6.51 2.86 3.45Cyclopentane -7. 82 4.82 9. 67 6.69 2.98 0.62Cyclohexane II 4.17 9. 40 6.72 2.68 0.64Cycloheptane II 3.72 9. 12 6.69 2.43 0.65Cyclooctane II 2.23 9. 25 6.99 2.26 1.01Methanol II 5.56 10. 10 5.90 4.20 0.76Methanol -6 . 70 5.56 11. 07 9.91 1.16 0.21Ethanol -7 . 90 5.13 9. 37 6.99 2.38 0.462-Pentanol II 2.76 9. 12 8.09 1.03 0.373-Pentanol ti 5.55 9. 37 6.69 2.68 0.48Cyclohexanol -7. 82 1.92 10. 46 9.61 0.85 0.44Cyclohexanol II 5.77 9. 37 7.24 1.95 0.34Formaldehyde II 5.27 10. 28 5.23 5.05 0.96Cyclohexanone II 3.05 9. 82 8.18 1.64 0.54Benzene -6 . 70 . 3.37 11. 07 3.89 7.18 2.13Benzene -7 . 50 1.13 7. 53 4.52 3.00 2.67Phenol

II

0.611 6. 86 5.47 1.40 2.28Toluene ti 0.941 6. 86 4.98 1.88 2.00Toluene -6 . 70 2.82 11. 01 2.92 8.09 2.86Toluene -3. 65 8.47 10. 89 2.07 8.82 1.04Ethylbenzene -7. 50 0.817 7. 47 5.89 1.58 2.35Ethylbenzene -6. 70 1.63' 11. 56 6.08 5.48 3.33Cumene it 1.44 11. 19 6.45 4.74 3.33Cumene -7. 50 0.719 7. 95 6.13 1.82 2.53t-Butylbenzene -7 . 50 0.646 7. 59 6.07 1.52 2.35_t-Butylbenzene -6. 70 1.29 11. 37 7.06 4.31 3.33Benzyl alcoho l -2. 30 14.5 9. 79 2.25 7.54 0.52Benzyl alcohol -3. 65 8.67 10. 97 2.92 8.05 0.96Benzyl alcoho l -6. 70 2.89 11. 19 4.32 6.87 2.38Benzaldehyde -2. 30 14.7 9. 73 2.37 7.36 0.50

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Table 5. (Continued)

Compound Moles substrateapplied = X x 107 Molesx 10 7BlankMnO~4inTrial

Difference' 3x 107 = Y Y/X

Benzaldehyde -2.30 7.36 10.73 4.22 6.51 0.88Benzaldehyde -3.65 8.83 11.86 4.44 7.42 0.84Benzaldehyde -6.70 2.94 11.13 5.96 5.17 1.751-Phenylethanol -3.65 4.97 11.31 9.85 1.46 0.291-Phenylethanol -7.50 0.834 7.77 6.98 0.79 0.952-Phenyle thanol -7.50 0.837 7.71 5.22 2.49 2.972-Phenylethanol -3.65 7.54 11.50 3.95 7.55 1.0Acetophenone -8.42 5.13 9.12 4.44 4.68 0.91C-H C-OH 0.40C-H -> C=0 0.80C-H ->- CO-H 1.20C-OH-*-C=0 0.40C-OH -> C02H 0.80C=0 + C02H 0.40R2CH 2 > 2RC02H 2.40

acidity of the TFA-H-0 medium in which the reaction took place.the moles of MnO^ used up according to the data.

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data analy sis would be simpler than in the case of su lfu ric ac id . Weattempted to substantiate our conclusions by investigating analogousreactions in which known species are generated, such as nitronium andnitrosonium ions.

A ty pic a l freez ing-point depression experiment was car rie d out inthe fol low ing manner. Using the apparatus illu str at ed in Figure 1 a20.0 ml sample of pure TFA was supercooled to 10 below i t s freezin gpoint by means of a dry ice-acetone bath. Cryst a ll iz at io n was inducedby touching the wall with a small piece of dry ic e . The so lution wascompleted frozen to degas i t and then remelted. Next i t was cooled toone degree below the freezing point and crystallization was againinduced with a piece of dry ice . The res ult in g warming curve wasrecorded unt i l the temperature reached the equilibrium.plateau indicativeof the liq ui d -s o li d equilibrium temperatures. When the solution wassupercooled by no more than 2 this plateau extended over a timeinterval of 2 to 4 minutes, allowing preci se determination of thefreezing poin t. Once the freezing point had been ve ri fi ed for thepure compound the sample of interest was added and completely dissolved.Then using the same supercooling technique the new freezing point wasdetermined and verified at least twice. Sometimes when larger amountsof compounds were added the solution s tended to supercool too much,giving no warming plateau . Whenever this occurred the acetone-bathtemperature was slowly lowered unt i l crystallization could be maintained.In th is manner freezing points could be determined to + 0 . 0 1 . Forpure TFA the re pr od uc ib il ity was + 0 .005 . Fresh TFA was used foreach sample investigated and a l l determinations were performed under a

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nitrogen atmosphere. The re su lt s obtained are presented in Table 6and 7.

3.8 Determination of the H Function for TFA-WaterK

When the i n i t i a l kinetic oxidation data were analyzed it wasfound that the order with respect to hQ was extremely high, in excessof f ive . It became apparent that this was not the proper func tion .The function was then considered as a poss ible al te rn at iv e.

The function for TFA-water solutions was determined in thefollowing manner. Stock solut ions of the in dicators were made up inneat TFA of such concentration that when 0.50 ml was made up to 5.00 mlthe fully ionized absorbance value would be near 0.8 . Stock solutio nsi n neat TFA were stable for up to several weeks with no indicatordecomposition. The absorbance of the cat ions in the series of solutionsused -

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Table 6. Determination andVerificat ion of for TrifluoroaceticAcid.

Substrate "oles of Molality AT Kf=AT/msubstrate = m

DeterminationBenzene 0.02250 0.7329 3.94 5.384.00 5.463.96 5.403.99 5.44

4.08 5.574.00 5.46K f=5.45+0.02m=AT/Kx f n=m/mx bno

VerificationToluene 0.01882 0.6130 3.46 0.6349 1.04 1.0Sodium 0.001533 0.04993 0.835 0.1532 3.07 3.0acetate 0.001102 0.03589 0.580 0.1064 2.96 3.0

II 0.000625 0.02035 0.330 0.06052 2.98 3.0II 0.000244 0.00794 0.140 0.02796 3.42 3.0II 0.000244 0.00794 0.148 0.02569 3.23 3.0II 0.000138 0.00450 0.085 0.01559 3.46 3.00.080 0.01468 3.26 3.00.075 0.01376 3.06 3.0

is the molal freezing point depression constant,n i s thenumber of particles predicted theo retica lly.

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- 52 -Table 7. Cryoscopic Data for Substrates in Trifluoroacetic Acid

Substrate Moles added Molality AT mx=AT/Kf n=m/ iX= m n=m/ iX

Water 0.01665 0.5423 3.41 0.6257 1.15ti 0.00555 0.1808 1.28 0.2349 1.30II 0.01665 0.5423 3.65 0.6697 1.23II 0.01110 0.3619 2.37 0.6349 1.20

KMnO. 0.001179 0.03840 0.63 0.1156 3.01it4 0.0009430 0.03072 0.60 0.1101 3.58II 0.0008464 0.02757 0.56 0.1028 3.73ti 0.0006348 0.02068 0.39 0.0716 3.46it 0.0004945 0.01611 0.33 0.0606 3.76it 0.0002781 0.009059 0.22 0.0404 4.46ti 0.0003122 0.009912 0.16 0.0294 4.25it 0.0006356 0.02070 0.37 0.0679 3.28i i 0.0001948 0.006345 0.17 0.0312 4.92ti 0.0001429 0.004650 0.13 0.0239 5.14II 0.0001215 0.003958 0.11 0.0202 5.10

Triphenyl- 0.0007547 0.02458 0.46 0.08441 3.43carbinol 0.0004380 0.01427 0.26 0.04771 3.34it 0.0001984 0.006463 0.135 0.02477 3.69it it it 0.140 0.02569 3.83

Sodium 0.001271 0.04141 0.575 0.1053 2.55nitrate 0.000475 0.01548 0.28 0.05138 3.32ti 0.000377 0.01226 0.205 0.0376 3.14i i 0.000233 0.07604 0.142 0.0261 3.42it it ti 0.165 0.0303 3.98it 0.000168 0.005461 0.105 0.0193 3.53II it tt 0.110 0.0202 3.69ti 0.000113 0.003675 0.090 0.0165 4.49it tt it 0.095 0.0174 4.74Sodium 0.001401 0.04563 1.02 0.1872 4.10ni t r i te tt it 1.03 0.1890 4.140.0009384 0.03057 0.73 0.1339 4.38

ti it 0.75 0.1376 4.500.0007304 0.02379 0.575 0.1055 4.43" it it 0.60 0.1101 4.620.0003567 0.01167 0.270 0.04954 4.26

it ti 0.275 0.05046 4.340.0001471 0.004792 0.110 0.02018 4.21it it 0.120 0.02202 4.59

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12Since the H function established by Randies and Tedder waso Jin terms of weight percent we found it necessary to determine themolarity for these solu tions so that the two functions could becompared. These data are presented in Table 8.

The equation used to es tabl ish is derived in the followingmanner.

+ * * + +R + Ho0 > ROH + Hz

or

lRQH] a f f f f R 0 H

l R + ] a H 2 0 V

log - log-M- + log ROH[R ] "H 20 iR+

i . u H+ ROH _u .le t h = r T then IL = -log hR aH 2 0 fR+ 1 1 R

u . , , [ROH]IL = -lo g \ + + log1 1 * [R+]

v a . 1 [ROH]H,, = pK^+ + log* * [R +]

Experimentally [R0H]/[R+] was determined as follows;

[R ] = Absorbance of cation observed[ROH]= Absorbance of ful ly ionized cation - absorbance observed.

Since none of the in dica tors used had detectable decompositionsspectrophotometric measurements were re ad ily made. Occasional

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Table8. Molarity ofWeight%Solutions ofTFA-Water.

Molarity Weight% of TFAinTFA-H-0

0.77 8.33

1.76 18.11

2.53 24.23

3.46 34.12

3.90 37.83

4.28 40.77

4.82 44.91

5.51 50.35

6.20 55.36

6.78 59.62

7.98 67.76

8.76 71.71

9.32 75.599.85 78.78

10.58 83.32

11.05 86.32

11.45 89.07

12.20 93.67

12.95 96.49

13.13 100

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diff iculty was encountered in determining the fully ionized absorbance,presumablybecause of solvent effects on the spectra of the carboniumions. There were noA shif ts , only a small increase of absorbance

maxwith increased a c id ity . Whenever th is was the case the absorbancevalues close to the region where the sigmoid curve leveled off wereused to determine the fully ionized absorbance value. Only oneindicator, 4,4',4"-trimethoxytriphenylcarbinol, had absorbance increasesof more than 0.01 over a range of three H units beyond the approximateful lv ionized absorbance value.3.9 Vapour-LiquidPhase Equilibrium Study

In the previous section the H re latio nship was derived. One ofR

the factors that determine it s magnitude is a _ . The normal methodto determine a _ i s to measure vapour pressure under isothermal

98conditions varying substrate and then applying the equation a^ = kp^where a^ = ac ti v it y of solvent in so lut ion p^ = vapour pressure ofsolvent over solution and k = prop or tiona lity constant. But for puresolvent a = 1 .'. 1 = kpQ or k = 1/p so the equation takes thefa mil ia r form a_ =P^/P o We did not have the necessary equipment tomeasure the vapour pressure at 2 5 directly but the amount of waterpresent when vap our-liquid bo il in g equilibrium is achieved should beproportional to ac ti vi ty . Boiling-po int equilibrium studies werequite simple to perform on the TFA-water system in the following manner.One hundred ml of one of the components was brought to r eflux and theequilibrium temperature recorded; then 10 ml portions of the othercomponent were added unt i l the added component was in excess. After each

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10 ml add ition the so lutio n was brought to bo iling -ref lux eq uil ibr ium ,the equilibrium temperature was recorded and a sample of thecondensate was trapped by means of the apparatus i l lustrated inFigure 3. After cool in g, a 2.00 ml portion of the condensate andresidue was t itrated to determine the amount of TFApresent in eachsample. In th is manner by st arting with each component in turn acomplete vapour-liquid phase diagram could be constructed. The a^ _could be then approximated in . the following manner; a - = mole fr ac tionof water in the vapour which results from the following deri\_tion.As prev iou sly mentioned a^ = kp^. If one makes the assumption that thepart ia l pressure of the i th component i s prop ortiona l to the molefraction of i present in the vapour then p^ = k 'F ^ (F = mole fr ac ti on).Then a^ = kk'F^. But for a system composed of pure component i theactivity of i ( a 0 ) ^ = 1> and the mole fraction of component i in thevapour must be unity.So, a = 1 = kp' o r obut, p = k*F = k*o othen, kk' = 1therefore, a^ =

It must be remembered that these values of a^ _ are only approximatesince isothermal conditions did not ex is t . The values obtained arepresented in Table 9. Figure 4 presents the vapour-liquid equilibriumdata.

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Figure 3. Vapour-liquid equilibrium study apparatus.

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- 59 -Table 9. Vapour-Liquid Equ ilibrium Data for the TFA-Water System.

Boiling N of N of F * in F u * in Log F"2 2point l iquid vapour l iquid vapour vapour

71.0 13.0 13.0, 0 0 -79.2 12.40 12.70 0.254 0.169 -.77288.1 12.0 12.80 0.346 0.130 -.88696.0 11.65 12.80 0.410 0.130 -.88699.9 11.30 12.45 0.466 0.250 -.620103.7 10.60 11.60 0.556 0.416 -.381104.7 10.30 10.70 0.585 0.547 -.262104.9 10.00 9.60 0.614 0.652 -.186104.5 9.70 8.60 0.641 0.720 -.143103.5 9.00 7.00 0.693 0.800 -.097102.3 8.40 5.80 0.731 0.852 -.070101.8 7.80 5.15 0.765 0.875 -.058101.3 7.35 4.00 0.785 0.893 -.049101.0 7.00 3.95 0.800 0.910 -.041100.8 6.55 3.55 0.821 0.921 -.036100.65 6.20 3.50 0.837 0.922 -.035100.35 5.65 3.15 0.858 0.932 -.031100.15 5.00 2.80 0.881 0.941 -.026100.0 4.20 2.40 0.905 0.951 -.02299.4 3.30 2.00 0.928 0.960 -.01899.6 2.30 1.50 0.953 0.971 -.01399.3 1.30 0.80 0.975 0.986 -.00698.0 0 0 1.0 1.0 0.0

F designates mole fraction of water present in solution or vapour.

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- 60 -110.0

- - i 1 1 1 . 10.0 2.6 5.2 7.8 10.4 13.0MolarityFigure 4. Phase diagram of TFA-water mixtures.

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3.10 Determination of pK of Permanganic Acid InH^O and D^OIt has been previously..mentioned that permanganate exists as a

green species i n neat TFA. It was further noted that addit ion of waterregenerated the permanganate ion, MnO^ . It became apparent that anequilibrium existed between these two species which was determined inthe follow