AromaticsAromatics

Frost Circles…a Great TrickFrost Circles…a Great TrickInscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ringThe relative energies of the MOs in the ring are given by where the vertices touch the circleThe MOs– below the horizontal line through the center of the ring

are bonding MOs– on the horizontal line are nonbonding MOs– above the horizontal line are antibonding MOs

ππ--MOs ofMOs of CyclobutadieneCyclobutadiene(square planar)(square planar)

AntibondingAntibonding

CycloCyclo--butadienebutadiene BondingBonding

4 4 ππ electrons; bonding orbital is filled; other 2electrons; bonding orbital is filled; other 2ππ electrons singly occupy two nonbondingelectrons singly occupy two nonbonding orbitalsorbitals

AntibondingAntibonding

BondingBonding

8 8 ππ electrons; 3 bondingelectrons; 3 bonding orbitalsorbitals are filled; 2are filled; 2nonbondingnonbonding orbitalsorbitals are each halfare each half--filledfilled

ππ--MOs ofMOs of CyclooctatetraeneCyclooctatetraene(square planar)(square planar)

CycloCyclo--octatetraeneoctatetraene

Requirements forRequirements for Aromaticity

cyclic conjugation is necessary, cyclic conjugation is necessary, but not sufficient but not sufficient

Aromaticity

notnotaromatic

notnotaromaticaromaticaromaticaromatic aromatic

Hückel'sHückel's RuleRulethe additional factor that influencesthe additional factor that influences

aromaticityaromaticity is the number of is the number of ππ electronselectrons

Hückel'sHückel's Rule for Rule for AromaticityAromaticity

To be Aromatic …a compound must :To be Aromatic …a compound must :

1.1. be Cyclicbe Cyclic

2.2. have one P orbital on each atom in the ringhave one P orbital on each atom in the ring

3.3. be planar or nearly so to give orbital overlapbe planar or nearly so to give orbital overlap

4.4. have a closed loop of 4n+2 pi electrons in the have a closed loop of 4n+2 pi electrons in the

cyclic arrangement of p cyclic arrangement of p orbitalsorbitals

ππ--MOs of BenzeneMOs of Benzene

AntibondingAntibonding

BenzeneBenzene

BondingBonding6 6 ππ electrons fill all of the bondingelectrons fill all of the bonding orbitalsorbitalsall all ππ antibonding orbitalsantibonding orbitals are emptyare empty

ππ--MOs ofMOs of CyclobutadieneCyclobutadiene(square planar)(square planar)

AntibondingAntibonding

CycloCyclo--butadienebutadiene BondingBonding

4 4 ππ electrons; bonding orbital is filled; other 2electrons; bonding orbital is filled; other 2ππ electrons singly occupy two nonbondingelectrons singly occupy two nonbonding orbitalsorbitals

ππ--MOs ofMOs of CyclooctatetraeneCyclooctatetraene(square planar)(square planar)

CycloCyclo--octatetraeneoctatetraene

AntibondingAntibonding

BondingBonding

8 8 ππ electrons; 3 bondingelectrons; 3 bonding orbitalsorbitals are filled; 2are filled; 2nonbondingnonbonding orbitalsorbitals are each halfare each half--filledfilled

ππ--Electron Requirement forElectron Requirement for AromaticityAromaticity

notnotaromaticaromatic aromaticaromatic notnot

aromaticaromatic

4 4 ππ electronselectrons 6 π electrons6 π electrons 8 8 ππ electronselectrons

[18][18]AnnuleneAnnuleneThe protons resonate at-3.0 and at 9.3δ!!!

18 18 ππ electrons satisfieselectrons satisfies Hückel'sHückel's rulerule

resonance energy = 418 kJ/molresonance energy = 418 kJ/mol

bond distances range between 137bond distances range between 137--143 pm143 pm

HH HHHH

HHHHHH

What about??What about??

HeterocyclicHeterocyclic Aromatic CompoundsAromatic Compounds

HeterocyclicHeterocyclic Aromatic CompoundsAromatic Compounds

PyridinePyridine

SS••••••••

NN

HH

•••• OO••••••••

NN••••

PyrrolePyrrole FuranFuran ThiopheneThiophene

HeterocyclicHeterocyclic Aromatic CompoundsAromatic Compoundsandand

Hückel'sHückel's Rule

6 6 ππ electrons in ringelectrons in ringlone pair on nitrogen is in anlone pair on nitrogen is in anspsp2 2 hybridized orbital;hybridized orbital;notnot part of part of π π system of ring

Rule

NN••••

system of ringPyridinePyridine

HuckelHuckel and Pyridineand Pyridine

lone pair on nitrogen must be part lone pair on nitrogen must be part of ring of ring ππ system if ring is to havesystem if ring is to have6 6 ππ electronselectronslone pair lone pair must be inmust be in a a pp orbitalorbitalin order to overlap with ring in order to overlap with ring ππsystem

NN

HH

••••

PyrrolePyrrolesystem

Huckel Huckel and and PyrrolePyrrole

FuranFuran

two lone pairs on oxygentwo lone pairs on oxygenone pair is in a one pair is in a pp orbital and is partorbital and is partof ring of ring ππ system; other is in an system; other is in an spsp22 hybridized orbital and is nothybridized orbital and is notpart of ring part of ring ππ system

OO••••••••

system

Huckel Huckel and Furanand Furan

Homework Supplement…..Please name the heterocycle and showWhich electrons on the heteroatoms are included in the pi system.Where are the unshared pairs and how are they oriented relative to the plane of the ring??

N

SN

S

Aromatic IonsAromatic Ions

H

H

H

H

H

Radical

Anion

Cation

H

H

H

Radical ___

Cation ___

Anion ___

π e’s

CyclopentadieneCyclopentadiene

Hπ e’s

Radical ___

Cation ___

Anion ___

Let’s fill these for each case, radical, anion and cation

CyclopentadienideCyclopentadienide AnionAnion

6 6 ππ electronselectrons delocalizeddelocalizedover 5 carbonsover 5 carbonsnegative charge dispersednegative charge dispersedover 5 carbonsover 5 carbonsstabilized anion

HH HH

••••––HH HHstabilized anion

HH

Acidity ofAcidity of CyclopentadieneCyclopentadiene

HH HH

HH

HH HH

HH

HH

••••––HH++ ++

ppKKaa = 16= 16

KKaa = 10= 10--1616

CyclopentadieneCyclopentadiene is unusually is unusually acidic for a hydrocarbon.acidic for a hydrocarbon.Increased acidity is due to Increased acidity is due to stability ofstability of cyclopentadienidecyclopentadienideanion.anion.

HH HH

HH HH

CyclopentadienideCyclopentadienide AnionAnion

HH HH

••••––

HH HH

HH

––HH HH

HH

HH HH

Let’s Move Electrons once

HH HH

HH HH

HH

••••––

HH HH

HH HH

HH

HH HH

HH HH

HH

HH HH

HH HH

HH

HH HH

HH HH

HH

HHHHHH

HH

HH HH

HH HH

π e’sRadical ___

Cation ___

Anion ___

Which is stable??

Compare Acidities ofCompare Acidities ofCyclopentadieneCyclopentadiene andand CycloheptatrieneCycloheptatriene

HH HH

HH HH

HH HH

HH HH

HHHHHH

HH HHHH HH

ppKKaa = 16= 16

KKaa = 10= 10--16

ppKKaa = 36= 36

KKaa = 10= 10--3616 36

nn = 0= 044n n +2 = 2 +2 = 2 ππ electrons !!electrons !!

CyclopropenylCyclopropenyl CationCation

HH HH

HH

++++

HH HH

HH

also written asalso written as

n = 0 (4n+2 = 2) fills a bonding MOn = 0 (4n+2 = 2) fills a bonding MO

Cyclooctatetraene DianionCyclooctatetraene Dianion

HH HH

HHHH

HH HH

HHHH

••••

••••

––

––

HH HH

HHHH

HH HH

HHHH

22––alsoalsowritten aswritten as

nn = 2= 244nn +2 = 10+2 = 10 ππ electronselectrons

ElectrophilicElectrophilic Aromatic Aromatic SubstitutionSubstitution

Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophileWe study– several common types of electrophiles, – how they are generated, and – the mechanism by which they replace hydrogen is the same for all

ElectrophilicElectrophilic Aromatic Aromatic SubstiutionSubstiution

Electrophilic Electrophilic Aromatic SubstitutionAromatic Substitution

EH

H+

E

H

H

H

EH

+

+

E+ H+E+

Please Please be sure be sure that you can do that you can do this and that it this and that it makes sense to makes sense to you!!you!!

H H

HH

H H E+

The The EnergeticsEnergetics

HH +

H H

H E+H

EH

H H

HH

+ H+

NitrationNitrationNO2H2SO4

O3HN

+••

••Nitric acid

H O NO2 O SH O

+•• •••• ••

••

••

Nitronium ionO2

HO=N=O

O3 H••

+H

+H NO2 HSO4

+••OH

H OH N +

ChlorinationChlorination

Chlorination requires requires a Lewis acid Chlorination requires requires a Lewis acid catalyst, such as AlClcatalyst, such as AlCl33 or FeClor FeCl33

Step 1: formation of a chloronium ion

+ - +•• ••

••

•• ••

••

••

••

+••

••

••

•••• ••Cl

Cl

Cl

Cl

ClCl

ClFeCl

Cl Fe

Cl Cl FeCl 4-

chloronium ion

SulfonationSulfonation

H2 SO4+H SSO3 O3 H

Benzenesulfonic acid

S OO

O SO

O O-

H

+ H

Sulfonation Sulfonation can be reversed by Heating in Hcan be reversed by Heating in H22OO

TheThe FriedelFriedel--Crafts Reaction.. Circa 1877Crafts Reaction.. Circa 1877

CharlesCharles FriedelFriedel James CraftJames Craft

Making CMaking C--C bonds is….C bonds is….

FriedelFriedel--CraftsCrafts AlkylationAlkylation

CH2 CH3CH3CH2Cl

AlCl3

An ion pair containing

a carbocation

+

••••

••

••

•• -+

Cl Al

Cl

Cl

Cl

Cl

AlCl + AlCl 4-

R Cl

ClR R

A word about theA word about the FriedelFriedel--Crafts Crafts AlkylationAlkylation

1.1. Carbocation rearrangements are commonCarbocation rearrangements are common

AlCl 3CH3 CHCH2 Cl

CH3

+ + HClC(CH 3 ) 3

tert-ButylbenzeneIsobutyl chlorideB enzene

Role ofRole of AlClAlCl33Acts as a Lewis acid to promote ionizationActs as a Lewis acid to promote ionizationof the alkyl of the alkyl halideBhalideB

++ClCl••••

••••AlClAlCl33––

(CH(CH33))33CC ClCl ••••

••••

••••++ AlClAlCl33 (CH(CH33))33CC

++(CH(CH33))33CC ClCl

••••AlClAlCl33––

••••++

FriedelFriedel--CraftsCraftsAlkylationAlkylation

2. They are tough to stop!2. They are tough to stop!

Product is more reactive than the starting material

CH3CH+2Cl + etc.

AlCl3

FriedelFriedel--CraftsCrafts AlkylationAlkylation

2. 2. alkylationalkylation fails fails on benzene rings bearing one or on benzene rings bearing one or more strongly electronmore strongly electron--withdrawing groupswithdrawing groups

CH

O O

COR

O OO

CR CNH2COH

NR3+C NSO3 H NO2

CF3 CCl 3

FriedelFriedel--CraftsCrafts AcylationAcylation

OCR

ORCX AlX 3+ +H HX

An acylbenzene

An ion pair containing

an acylium ion

O

AlCl 4-R-C +

+••••Cl

O

R-C-

Al

Cl

Cl••

•• •• +ClR-C

O ClCl

Cl

Al-Cl

FriedelFriedel--CraftsCrafts AcylationAcylationAnAn acyliumacylium ion is a resonance hybrid of two major ion is a resonance hybrid of two major contributing structurescontributing structures

– F-C acylations are free of ONE major limitation of F-Calkylations; acylium ions do not rearrange

– They still do not work on deactivated Rings– They stop after one substitution

RC+

O O+

RC

HydrogenolysisHydrogenolysis

OH2CH3COCl

AlCl3 Pd/C

There are two nice tricks hidden hereThere are two nice tricks hidden here

Please be sure to remember this reaction!!!Please be sure to remember this reaction!!!

DiDi-- andand PolysubstitutionPolysubstitution

Existing groups on a benzene ring influence further substitution in both orientationorientation and rate

Orientation:– certain substituents direct preferentially to ortho & para

positions; others direct preferentially to meta positions– substituents are classified as either

orthoortho--parapara directing ordirecting ormeta directingmeta directing

DiDi-- andand PolysubstitutionPolysubstitution

Rate:Rate:–– certain substituents cause the rate of a second certain substituents cause the rate of a second

substitution to be greater than that for benzene substitution to be greater than that for benzene itself; others cause the rate to be loweritself; others cause the rate to be lower

–– substituents are classified assubstituents are classified as•• activatingactivating toward further substitution, ortoward further substitution, or•• deactivatingdeactivating

DiDi-- andand PolysubstitutionPolysubstitution

-OCH3 is ortho-para directing and activating

p-Bromo-anisole (96%)

o-Bromo-anisole

(4%)

Anisole

++

OCH 3 OCH 3 OCH 3Br

Br

Br 2CH 3 CO 2 H

HBr

DiDi-- andand PolysubstitutionPolysubstitution-NO2 is meta directing and deactivating!

m-Dinitro-benzene (93%)

Nitro-benzene

+

++

o-Dinitro-benzene

p-Dinitro-benzene

Less than 7% combined

NO 2

NO 2

NO 2NO 2

NO 2

NO 2NO 2HNO 3

H 2 SO 4

Methyl GroupMethyl Group

Toluene undergoes nitration 1000 times faster than benzene.A methyl group is an activatingactivating substituent.

CHCH33

Trifluoromethyl GroupTrifluoromethyl Group

Trifluoromethylbenzene undergoes nitration 40,000 times more slowly than benzene .The trifluoromethyl group is adeactivatingdeactivating substituent.

CFCF33

.

Relative rates of Relative rates of NitrationNitration

ClOH H NO2

1000 1.0 0.033 6x10-8

ReactivityReactivity

Real FastReal Fast

Pretty fastPretty fast

KindaKinda slowslow

Pretty slowPretty slow

SlowSlow

Real SlowReal Slow

Effect onEffect on RegioselectivityRegioselectivity

•• OrthoOrtho--parapara directorsdirectors direct an incomingelectrophile to positions ortho and/or para to themselves.

• Meta directorsMeta directors direct an incoming electrophile to positions meta to themselves.

• All meta directorsAll meta directors are deactivating

•• AllAll orthoortho--parapara directors are activating except except halogen

Theory of Directing EffectsTheory of Directing Effects

• So…what’s going on here????

• The rate of EAS is limited by the slowest step in the mechanism…duh

• For EAS, the rate-limiting step is attack of E+ on the aromatic ring to form a resonance-stabilized cation intermediate

• The more stable this cation intermediate, the faster the rate-limiting step and the faster the overall reaction

DiDi-- andand PolysubstitutionPolysubstitution

Alkyl groups, phenyl groups, and all groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-paradirecting. All other groups are meta directing.

All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating

DiDi-- andand PolysubstitutionPolysubstitutionalkyl groups, phenyl groups, and all groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other groups are meta directing.All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating

Adding a Second Adding a Second SubstiuentSubstiuent

Methoxy Methoxy is is therefore an “ois is therefore an “o--p director”p director”

Adding a Second Adding a Second SubstiuentSubstiuent

Nitro is therefore a “meta director”Nitro is therefore a “meta director”

DiDi-- andand PolysubstitutionPolysubstitutionCH3 CO2 H

HNO3H2 SO4

K2 Cr 2 O7

H2 SO4

K2 Cr 2 O7

H2 SO4CH3 NO2

p-Nitrobenzoic acid

NO2

m-Nitrobenzoic acid

CO2 H

NO2

CO2 HHNO3

H2 SO4