6. alkenes: structure and reactivity · 13 6.4 cis-trans isomerism in alkenes carbon atoms in a...

Based on McMurry’s Organic Chemistry, 7th edition

Dr Morteza Mehrdad

University of Guilan, Department of Chemistry,

Rasht, Iranm-mehrdad@guilan.ac.ir

Alkenes: Structure and

Reactivityساختار و واکنش پذیری: آلکن ها

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2

Alkene - Hydrocarbon With

Carbon-Carbon Double Bond

Also called an olefin but alkene is better

Includes many naturally occurring materials

Flavors, fragrances, vitamins

طعم دهنده ها، خوشبوکننده ها، ویتامین ها

سقز از درخت بَنَه

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Why this Chapter?

C=C double bonds are present in most

organic and biological molecules

To examine consequences of alkene

stereoisomerism

To focus on general alkene reaction:

electrophilic addition

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6.1 Industrial Preparation and Use of

Alkenes

Ethylene and propylene are the most important organic

chemicals produced

Made by thermal cracking of light alkanes (petroleum)

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6.2 Calculating Degree of Unsaturation درجه سیرنشدگیمحاسبه

Relates molecular formula to possible structures

Degree of unsaturation درجه سیرنشدگی: number of multiple

bonds or rings

Formula for a saturated acyclic (سیرشده خطی) compound is CnH2n+2

Each ring or multiple bond replaces 2 H's

سیرنشده سیرشده

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Example: C6H10

Saturated is C6H14

Therefore 4 H's are not present

This has two degrees of unsaturation 2= سیرنشدگی درجه

Two double bonds?

or ring and double bond?

MW = 6 x 12 + 10 x 1= 82

or two rings?

or triple bond?

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Degree of Unsaturation With

Other Elements Organohalogens (X: F, Cl, Br, I)

Halogen replaces hydrogen

C4H6Br2 and C4H8 have one degree of unsaturation

Organoxygen compounds (C,H,O) - if connected by single bonds

These don't affect the total count of H's

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Organonitrogen compounds

Nitrogen has three bonds

So if it connects where H was, it adds a connection point

Subtract one H for equivalent degree of unsaturation in

hydrocarbon

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Method 1

Add the number of halogens to the number of hydrogens.

Ignore the number of oxygens.

Subtract the number of nitrogens from the number of hydrogens.

Method 2

Method 3

Summary - Degree of

Unsaturation

2

H-Hion Unsaturatof Degree

S)O, (ignore NX22CH

actsat

sat

سیرنشدگیدرجه =2 + تعداد اتم( ظرفیت-2)

2

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مثال: C20H32ClN

= 2

= 5

Diazepam, marketed as an antianxiety medication under the name

Valium, has three rings, eight double bonds, and the formula

C16H?ClN2O. How many hydrogens does diazepam have?

(Calculate the answer; don’t count hydrogens in the structure.)

= 2+16(4-2) +X(1-2)+1(1-2)+2(3-2)+1(2-2)

= 112

= -X+35

= 112

X= 13 C16H13ClN2O

سیرنشدگیدرجه =2 + تعداد اتم( ظرفیت-2)

2

2 + 20(4-2) + 32(1-2) +1(1-2)+1(3-2)

No.H + No.X = 32 + 1 = 33

- No.N = 33 -1 = 32

CnH2n+2 = C20H42

42-32 = 10

10 / 2 = 5

Name the parent hydrocarbon—change ending to “–ene”

Number carbons in chain so that double bond carbons have lowest

possible numbers

Alkenes higher priority than alkanes: even shorter chain

Multiple “C=C“ are named as “diene” “triene” “tetraene” etc…

Rings have “cyclo” prefix—double bond always C#1, C#2

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6.3 Naming of Alkenes

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Many Alkenes Are Known by

Common Names

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6.4 Cis-Trans Isomerism in

Alkenes

Carbon atoms in a double bond are sp2-hybridized

Three equivalent orbitals at 120º separation in plane

Fourth orbital is atomic p orbital

Combination of electrons in two sp2 orbitals of two atoms

forms bond between them

Additive interaction of p orbitals creates a bonding orbital

Occupied orbital prevents rotation about -bond

Rotation prevented by bond - high barrier, about 268

kJ/mole in ethylene

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Rotation of Bond Is Prohibitive

This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond).

Creates possible alternative structures

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The presence of a carbon-carbon double bond can create two possible structures

cis isomer - two similar groups on same side of the double bond

trans isomer - similar groups on opposite sides

Each carbon must have two different groups for these isomers to occur

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Cis, Trans Isomers Require That

End Groups Must Differ in Pairs

180°rotation superposes

Bottom pair cannot be superposed without breaking C=C

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6.5 Sequence Rules: The E,Z DesignationE , Z نامگذاریسیستم : قواعد ترتیب تقدم

Neither compound is clearly “cis” or “trans”

Substituents on C1 are different than those on C2

We need to define “similarity” in a precise way to

distinguish the two stereoisomers

Cis, trans nomenclature only works for disubstituted

double bonds

E/Z Nomenclature for 3 or 4 substituents

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E,Z Stereochemical Nomenclature

Priority rules

( قواعد ترتیب تقدم)

of Cahn, Ingold, and

Prelog

Compare where higher

priority groups are with

respect to bond and

designate as prefix

E -entgegen, opposite

sides

Z - zusammen,

together on the same

side

Hi(C1)-Low(C1)-Hi(C2)-Lo(C2)

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Ranking Priorities:

Cahn-Ingold-Prelog Rules

RULE 1

Must rank atoms that are connected at comparison point

Higher atomic number gets higher priority

Br > Cl > S > P > O > N > C > H

1 2

2 1

1

22

1

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RULE 2

If atomic numbers are the same, compare at next connection point at same distance

Compare until something has higher atomic number

Do not combine – always compare

Extended Comparison

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RULE 3

Substituent is drawn with connections shown and no double or

triple bonds

Dealing With

Multiple Bonds:

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6.6 Stability of Alkenesها پایداری آلکن

Cis alkenes are less stable than trans alkenes

Compare heat given off on hydrogenation: Ho

Less stable isomer is higher in energy

And gives off more heat

tetrasubstituted > trisubstituted > disubstituted > monosusbtituted

hyperconjugation ( مزدوج شدن موقت) stabilizes

ناپایدارتر از 2/8kJ/mol

ازدحام فضایی

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Comparing Stabilities of

Alkenes Evaluate heat given off when C=C is converted to C-C

More stable alkene gives off less heat

2/8 kJ/mol

(Gocis > Go

trans)

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trans-Butene generates 4 kJ less heat than cis-butene

120

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CH3CH2CH2CH3

but the numbers aren’t exactly the same

4 kJ/mol (energy difference)

2.8 kJ/mole (quilibrium data)

for two reasons:

1. There is probably some experimental

error, since heats of hydrogenation

are difficult to measure accurately.

2. Heats of reaction and equilibrium

constants don’t measure exactly the

same thing.

Heats of reaction measure H°,

equilibrium constants measure G° a

(slight difference between the two)

Alkenes become more stable with increasing substitution:

As a general rule, alkenes follow the stability order:

This order of stability is due to:•Hyperconjugation (مزدوج شدن موقت)

•bond strength ( قدرت پیوند)

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Hyperconjugation (مزدوج شدن موقت) Electrons in neighboring filled orbital stabilize vacant antibonding orbital net

positive interaction

برهم کنش پایدارکننده ای بین اوربیتال ضدپیوندی پرنشدهC=C و اوربیتال پربرروی استخالف های مجاورC-Hشده پیوند

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Alkyl groups are better than H

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Hyperconjugation involves a sigma orbital

component, usually a C-C or C-H bond.

Resonance involves pi orbitals

Alternative Explanation:

bond strength (قدرت پیوند)

sp3—sp2 bond > sp3—sp3 bond

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6.7 Electrophilic Addition of

Alkenes

General reaction

mechanism:

electrophilic addition

Attack of electrophile

(such as HBr) on

bond of alkene

Produces carbocation

and bromide ion

Carbocation is an

electrophile, reacting

with nucleophilic

bromide ion

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Two step process

First transition state is high energy point

Electrophilic Addition Energy

Path

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Electrophilic Addition for

preparations

The reaction is successful with HCl and with HI as well as

HBr

HI is generated from KI and phosphoric acid

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6.8 Orientation of Electrophilic Addition: Markovnikov’s Rule قاعده مارکونیکوف

In an unsymmetrical alkene, HX reagents can add in two different

ways, but one way may be preferred over the other

If one orientation predominates, the reaction is regiospecific

( جهت ویژه) Markovnikov observed in the 19th century that in the addition of HX

to alkene, the H attaches to the carbon with the most H’s and X

attaches to the other end (to the one with the most alkyl

substituents) This is Markovnikov’s rule.

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Addition of HCl to 2-methylpropene

Regiospecific – one product forms where two are possible

واکنشی که افزایش فقط در یکی از دوجهت ممکن صورت می گیرد–جهت ویژه If both ends have similar substitution,

Example of Markovnikov’s Rule

then not regiospecific

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Markovnikov’s Rule (restated) (به بیان دیگر) More highly substituted carbocation forms as intermediate

rather than less highly substituted one

Tertiary cations and associated transition states are more

stable than primary cations

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6.9 Carbocation Structure and

Stability Carbocations are planar and the tricoordinate carbon is

surrounded by only 6 electrons in sp2 orbitals

The fourth orbital on carbon is a vacant p-orbital

The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents (Hyperconjugation stabilizes C+)

خالی از الکترونpاوربیتال

• The stability of carbocation (R+) can be determined by measuring energy needed to form it from R-X:

شکیل آن از الزم برای تانرژی گیری اندازه توسط کربوکاتیون می تواند تعیین پایداری •R-X آیدبدست

R-X R+ + :X-

• 3º alkyl halides dissociate to give R+ more easily than 2ºand 1º

تفکیک هالید نوع سوم بسیارسریع تر نوع اول و دوم است

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Inductive stabilization of cation species پایداری القایی

•Hyperconjugation

(موقتمزدوج شدن )آلکیل واقع بر روی تعداد گروههای هرچه

مزدوج رای کربوکاتیون بیشتر باشد، امکان بیشتری بد و ایجاد کربوکاتیونی پایدارتر، وجوشدن موقت

.خواهد داشت

ان کم الکترونی هرچه تعداد گروههای آلکیل بیشتری به کربن مثبت متصل باشد، چگالی الکترونی بیشتر و میزکاسته می شود( رنگ آبی) کربن مثبت

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6.10 The Hammond Postulateپذیره هاموند

If carbocation intermediate is more stable than another, why is

the reaction through the more stable one faster?

حصول چرا پایداری یک کربوکاتیون بر سرعت تشکیل آن اثر گذاشته و ساختار منهایی را تعیین می کند؟

The relative stability of the intermediate is related to an

equilibrium constant (Gº)

حدواسط کربوکاتیون به وسیله پایداری نسبی(Gº)تعیین می شود

The relative stability of the transition state (which describes

the size of the rate constant) is the activation energy (G‡)

به وسیله انرژی فعال سازی سرعت واکنش اما(G‡)بدست می آید

The transition state is transient and cannot be examined

ی گذرایی هستند و به دلیل عمر بسیار کوتاه خود، قابل بررسحاالت گذار، حاالتنیستند

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Transition State Structures

A transition state is the highest energy species in a reaction step

عبارت از پرانرژی ترین جزء در یک مرحله واکنش استحالت گذار یک By definition, its structure is not stable enough to exist for one vibration

قابل مشاهده باشندبرطبق تعریف، ساختار آنها به اندازه کافی پایدار نیست که But the structure controls the rate of reaction

اما ساختار آنها سرعت واکنش را کنترل می نمایند

So we need to be able to guess about its properties in an informed way

We classify them in general ways and look for trends in reactivity – the conclusions are in the Hammond Postulate

“The structure of the transition state resembles the structure of the nearest stable species. T.S. for Endergonic steps resemble products. T.S. for Exergonic steps resemble reactants.”

ساختار یک حالت گذار به ساختار نزدیک ترین گونه شیمیایی پایدار شباهت داردT.S. هافرآورده از لحاظ ساختاری، به انرژی خواه، مراحل ،

شباهت داردواکنش دهندگان ، از لحاظ ساختاری به انرژی زامراحل .T.Sو

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Examination of the Hammond

Postulate

A transition state

should be similar

to an intermediate

that is close in

energy

Sequential states

on a reaction path

that are close in

energy are likely

to be close in

structure - G. S.

Hammond

ط واکنش سریع تر حدواسندپایدارتری تولید می ک

واکنش آهسته تر منجر به تولید حدواسط پایدارتری

می شود

در مرحله انرژی خواه، انرژی حالت گذار ترازهای

و فرآورده، مشابه هستند

در مرحله انرژی زا، انرژی حالت گذارترازهای

و واکنش دهنده، مشابه هستند

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Competing Reactions and the

Hammond Postulate

Normal Expectation: Faster reaction gives stable intermediate

Intermediate resembles transition state حالت گذار حدواسط شبیه به است

یک تشکیل یک کربوکاتیون با پروتون دار شدنبنابراین حالت. آلکن، مرحله ای انرژی خواه است

گذار پروتون دار شدن آلکن، از لحاظ ساختاری، باشدباید به حدواسط کربوکاتیونی شباهت داشته

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6.11 Mechanism of Electrophilic Addition:

Rearrangements of Carbocations

Carbocations undergo structural rearrangements following set patterns

1,2-H and 1,2-alkyl shifts occur

Goes to give more stable carbocation

Can go through less stable ions as intermediates

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Alkyl Shifts Can Also Occur