C 10J Lecture 6

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Synthesis of Alkenes by Dehydrohalogenationof Alkyl Halides

When an alkyl halide is treated with a strong base such as

sodium or potassium hydroxide (NaOH, KOH).

Sodium methoxide (NaOCH3), sodium ethoxide (NaOC2H5)

or potassium tertiary butoxide, (KOC(CH3)3) an

elimination reaction takes place and one or more alkenes is

formed.

C C

X

H

CCstrong base

-HX

X = Br, Cl, I

An Elimination Reaction

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The E2 Mechanism of Dehydrohalogenation

C C

X

H

CC X

B

strong base ( B ) BH+ +

?The mechanism is concerted i.e. bond making

and bond breaking occur simultaneously.

?Rate of Reaction = k[base][alkyl halide]

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This type of elimination is referred to as a 1,2- elimination or

as a ? -elimination. The hydrogen that is lost is attached to thecarbon that is in the ? -position with respect to the halogen

atom.

C C

Br

H

?

?

12

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Stereochemistry of the E2 Mechanism

For the E2 mechanism to take place, the alkyl halide mustadopt a conformation in which the halogen atom and the ? -

hydrogen are anti to each other.

H

X

H

X

This is called the anti-periplanar or anti-coplanarconformation

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Note!

Dehydrohalogenation reactions can also take place under

conditions that favour an E1 mechanism. However, the

synthesis of alkenes is better achieved by way of an E2

mechanism.

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Dehydrobromination of 2-Bromopentane

CH3 CH2 CH CH CH2

H

Br

HH3C CH2 CH2 CH

CH2

CH3 CH2CH CH3CH

2-bromopentane +

1-pentene

2-pentene (disubstituted alkene)

B

(monosubstituted alkene)

Zaitsevs rule would dictate that the more highly

substituted 2-pentene be the preferred product and thisis so when bases such as sodium hydroxide, potassium

hydroxide, sodium methoxide and sodium ethoxide are

used.

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The Hofmann Product

When potassium tertiary butoxide which is a much bulkier

molecule than sodium or potassium hydroxide; sodium

methoxide and sodium ethoxide, is used, the less substituted

alkene, 1-pentene is the one that predominates and is referredto as the Hofmann product.

C

CH3

CH3

CH3

OK

potassium tertiary butoxide

C O

CHH

H

C

H

HH

CHH

H

K

potassium tertiary butoxide

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Hofmann Product contd

Bulky bases abstract the least hindered H+

Least substituted alkene is major product.

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Reactions of Alkenes

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Reactivity of C=C

Electrons in pi bond are loosely held.

Electrophiles are attracted to the pi electrons.

Carbocation intermediate forms (typically).

Nucleophile adds to the carbocation.

Net result is addition to the double bond.

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Addition Reactions

The typical mode of reaction of alkenes is that of

addition. In addition reactions, two molecules combine to

give a single molecule of product.

Notice the bonding changes that occur.

C C + A B C CA B

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Addition Reactions Contd

The reaction is energetically favourable. The amount of

energy released by the formation of two sigma bonds is in excess of

that needed to break one sigma bond and one pi bond. As a result,

addition reactions of alkenes are usually exothermic (heat is given

off).

C C

C CA B

B

Reaction Coordinate

G

G negative (exothermic reaction)

G H T S

Energy

A,

Energy Profile of Addition Reactions

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What is the Mechanism of AdditionReactions of Alkenes?

Remember that the term Reaction Mechanism refers to

the sequence and timing of the bond breaking and bond

making processes that bring about the conversion of

starting materials into products.

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The Reaction Mechanism tells us theorder in which the following bond

changes occur:

C C + A B C CA B

Bond Breaking

One pibond and one sigmabond are broken.

Bond Making

Two new sigmabonds are formed

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The Mechanism of AdditionReactions of Alkenes

Some addition reactions of alkenes are stepwise. They occurby way of a series of steps and involve the formation ofintermediate species.

Some addition reactions are concerted. The conversion ofstarting material to product takes place in one step . Bondsare broken and formed simultaneously with no intermediate

species being formed.

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Electrophilic Addition

An alkene is a nucleophile by virtue of itspi bond. The

two lobes of the pi bonding orbital above and below the

molecular framework contain a pair of fairly loosely held

electrons which can be donated to an electrophile. We say

that the electrons are loosely held because the pi bond isweaker than the sigma.

CC lobes of pibonding orbitalabove and below the molecularframework

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Electrophilic Addition Contd

The addition is called Electrophilic Addition when the

molecule, E Nu that adds to the alkene is highly

polarized such that an electrophile E is provided.

C C E Nu C CNu E?? ??

+

An electrophile is an electron deficient species that will accept an electron pair into an empty

orbital to form a bond.

The nucleophile is the electron rich species that will donate a pair of electrons to the

electrophile

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REMINDER!Representing Reaction Mechanisms

on PaperUse of Curly Arrows

A curly arrow represents the movement of a pair of

electrons. The arrow originates at the source of the

electrons and terminates at the place where the electronsend up.

source ofelectrons

destination ofelectrons

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A B A B

CC

A

CC

A

CC

A B

CC

A

B

+

+

Electrons usually end up either on an atom itself or

in the gap between two atoms where a new bond is

being formed.

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Step 1: Pi electrons attack the electrophile.

The Mechanism of Electrophilic

Addition is a Stepwise Mechanism.

Step 2: Nucleophile attacks the carbocation.

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Typical Highly Polarized Molecules thatadd to Alkenes

H X

H OSO3H

H

H

OH

????

?? ??

Hydrogen halides, (X= Br, Cl, I)

Sulfuric acid (H2SO4)

??

Hydronium ion

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C C

H Br

C C

H

Br

C C

H

Br

C C

H

Br

+Fast

Slow

Mechanism of Addition of HBr toAlkenes

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Electrophilic Addition Reactions

When a symmetrical alkene undergoes electrophilic

addition of an unsymmetrical molecule, HY, only one

product is possible regardless of the direction in which

HY adds.

C C

H

H H

H

C C

H

HH

Y

H

H

C C

H

HH

Y

H

H

HY

2 1 2 1 2 1

ethene, symmetrical alkene

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Regioselectivity in Electrophilic AdditionReactions Contd

If the alkene is unsymmetrical, for example propene, the

addition of HY can in theory lead to two different products

I and II which are structural or constitutional isomers

C C

H

H H

CH3

C C

CH3

HH

Y

H

H

C C

CH3

HH

Y

H

H

HY+

I II

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The Two Possible Carbocations

C C

H

H H

CH3

C C

CH3

HH

Y

H

H

C C

CH3

HH

Y

H

H

Y H

HY

+

I II

+

C C

CH3

HH

H

H

IIY

Compound

carbocation B

C C

CH3

HH

H

H

IY

Compound

carbocation A

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Markovnikovs Rule

(The Modern Statement)

When an unsymmetrical reagent adds to a double bond by

way of an ionic mechanism, the positive portion of the

reagent (the electrophile) attaches itself to a carbon atom of

the double bond so that the more stable of the two possible

carbocations is formed as the intermediate.

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Markovnikovs Rule Obeyed

C C

CH3

HH

H

H

C C

H

H H

CH3

C C

CH3

HH

H

HY

ICompound

HY

A

secondary carbocationformed preferentiallyfrom more stablecarbocation

C C

CH3

HH

H

H

IICompound

HY

B

primary carbocation

Not formed or formedin a minor amount

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The MarkovnikovProduct is formed byway of the energeticallymore favorable route.

Markovnikov

product

AntiMarkovnikov

product

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Markovnikovs Rule(The Original Statement)

In the addition of HX to an alkene, the hydrogen atom adds

to the carbon atom of the double bond that already has the

greater number of hydrogens.

C C

H

H H

CH3

C C

CH3

HH

X

H

H

C C

CH3

X

HH

H

H

HX+

I II

When a reaction such as the one above has the potential to

produce two or more constitutional isomers and yet only one is

formed or a mixture in which one predominates, the reaction is

said to be regioselective.

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Electrophilic Addition With Rearrangement

CH3 CH

CH3

CH2

CH CH3 CH

CH3

CH3

CH

CH3 C

CH3

CH3

CH2

Cl

Cl

HCl

2-chloro-3-methylbutane


3-methyl-1-butene

+

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The Expected Outcome

CH3 C

CH3

H

C C

H

H

HH

CH3 C

CH3

H

C C

H

H

HH

Cl

CH3 C

CH3

H

C C H

HH

H Cl

Cl

2-chloro-3-methylbutaneCarbocation formed in keepingwith Markovnikov's Rule

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Rearrangement

CH3 C

CH3

H

C C

H

H

HH

CH3 C

CH3

C C

H

H

H

H

H

1,2-hydride shift

Cl

CH3 C

CH3

C C

H

H

H

H

HCl

2 31 4


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Addition of HBr to alkenes

C C

H

H H

CH3

HBrC C

CH3

HH

Br

H

H

Markovnikov Product

II

C C

H

H H

CH3

HBr

Anti-Markovnikov Product

(under certaincircumstances)

C C

CH3

HH

Br

H

H

Sometimes the expected Markovnikov addition was observed,

yet at other times the regioselectivity was just the opposite.

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Anti-Markovnikov addition of HBr toalkenes

II

C C

H

H H

CH3

HBr


(under certain

circumstances)

C C

CH3

HH

Br

H

H

It was discovered that the anti-Markovnikovaddition occurred when there happened to beorganic peroxides present in the reaction

mixture.

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Anti-Markovnikov Addition of HBr toAlkenes contd.

Organic peroxides have the following structures:

O O RR

O O HR

alkyl peroxide

alkyl hydroperoxide

II

C C

H

H H

CH3

HBr


C C

CH3

HH

Br

H

HROOR

orROOH

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In the presence of organic peroxides, addition of HBr

proceeds by way of a Free Radical Mechanism.

STEP 1 (Initiation)

The weak oxygenoxygen bond in the organic peroxideundergoes homolysis to produce alkoxy radicals.

O OR R +R O O Rheat

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STEP 2 (Propogation)

An alkoxy radical abstracts a hydrogen atom

from H-Br thus producing a bromine radical.


+R O H Br R O H + Br

Electrophile

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C C

H

H

H3C

H

Br

C C

Br

H

HCH3

H

2 alkyl radical

The bromine radical then adds to the double bond of thealkene in such a way as to give the more stable secondaryalkyl radical.

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In the product forming fourth step, the secondary radicalabstracts a hydrogen atom from HBr to give an alkylbromide.


C C

H

H

Br

CH3

H

H Br

C C

H

H

Br

CH3

H

H

+ Br

Electrophile

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Anti-Markovnikov ??

Tertiary radical is more stable, so that intermediate forms

faster.

CH3 C

CH3

CH CH3 Br+

CH3 C

CH3

CH CH3

Br

CH3 C

CH3

CH CH3

Br

X

C 10J Lecture 6

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