ELECTROPHILIC ADDITION REACTIONS

Types of alkene addition reactions

ELECTROPHILIC ADDITION

What does the term "electrophilic addition" imply ?

An electrophile, E+, is an electron poor species that will react with an electron rich species such as C=C.

An addition implies that two systems combine to give a single entity.

An electrophilic addition reaction is an

addition reaction which happens because

what we think of as the "important"

molecule is attacked by an electrophile.

The "important" molecule has a region of

high electron density which is attacked by

something carrying some degree of

positive charge.

Generalized mechanism for electrophilic addition

Addition of HX to Alkenes

+ H X H X

Mechanism for HX addition to alkenes

CH3

|

CH2 = C – CH3 > CH3CH = CHCH3 ,

CH3CH2–CH=CH2 , CH3CH=CH2 >

CH2=CH2 > CH2=CHCl

Reactivity of Alkenes towards Acidic Reagents

Evidence for the MechanismEvidence for the Mechanism

Orientation of addition as well as reactivity of

alkenes are consistent with the mechanism i.e.

formation of carbocations as an intermediate.

Rate of reaction depends upon the concentration

of both the alkene and the reagent.

Reaction requires an acidic reagent.

Products obtained are unexpected ones,

wherever structure permits rearrangements.

REGIOSELECTIVITY

WHY THIS REGIOSELECTIVITY?

MARKOVNIKOV RULE

The addition of HX and other acidic reagents to

carbon-carbon double bond is found to form

major products in which the electrophile (proton)

is found to add to that doubly bonded carbon to

which more hydrogens are bonded.

These observations were first made by Markovnikov.

Addition of HBr to Isobutene and Markovnikov Rule

Protonation of double bond yields the most stable carbocation

?CH

CH3

H3C

CH3

CH2

HCl

CH

CH3

H3C

CH3

CH2

H+CH3

H3C

CH3

Cl-

H

CH3

CH3

H3C

CH3

H

CH3

Cl

1,2-methyl shiftCH

H3C

H3C

CH3

CH3

Cl-

CH

H3C

H3C

CH3

CH3

Cl

CH

H3C

H3C

CH3

CH3

Cl

CH3

H3C

CH3

H

CH3

Cl +

3-Chloro-2,2-dimethylbutane 2-Chloro-2,3-dimethylbutane

STEREOCHEMISTRY OF THE IONIC ADDITION TO AN ALKENE

Achiral reactants produce achiral products.

Orientation of addition :Stereochemistry of Orientation of addition :Stereochemistry of HH--Br addition.Br addition.

H3C C CHCH3

H

H

+ Br

Br H

Br H

++H Br

H Br

No Control ofNo Control ofStereochemistryStereochemistryBrBr-- adds to adds to either side either side of of carbocationcarbocation..

Regiochemistry is perfect.Only the product from secondarycarbocation is formed.

But stereochemistry is not controlled

H-Br

Addition of HBr in Presence of peroxide

Anti-Markovnikov product

Free - Radical addition to alkenes Mechanism

Essence of the mechanism is that hydrogen

and bromine add to the double bond

homolytically rather than heterolytically.

The intermediate is a free radical rather than

a carbocation.

Like halogenation of alkanes, this is a chain

reaction, this time involving addition rather

than substitution.

Free - Radical addition to alkenes Mechanism

FACTS SUPPORTING THE MECHANISM

• A few molecules of peroxide can change

the orientation of addition of many

molecules of HBr.

• This strongly indicates a chain reaction.

• So, too, does the fact that a few molecules

of inhibitor (such as hydroquinone) are

able to prevent this change in orientation.

• The anti - Markovnikov addition is caused

not only by the presence of peroxides but

also by irradiation

• This is done with irradiation using light of

a wavelength known to dissociate HBr

into hydrogen and bromine free radicals.

You can now predict the product of the reactions given below:

Br

+?HBr

Br

+ HBrPeroxide

?Br

Br

Br

Br

HX addition to alkenes

The order of reactivity of the HX is

HI > HBr > HCl >HF, and unless the alkene is highly

substituted, HCl reacts so slowly that the reaction is

not useful as a preparative method.

The addition of HX to an unsymmetrical alkene can

occur in two ways.

In practice, however, one product only predominates

When a reaction, potentially can yield two or more

constitutional isomers, but yields only one of them as

the major product, then such a reaction is known as

regioselective reaction.

Markovnikov vs Anti- MarkovnikovAddition of HBr to Alkenes

In the absence of peroxides, the reagent that

attacks the double bond first is a proton.

Because proton is small, steric effects are

unimportant.

It attacks carbon atom by ionic mechanism

so as to form the more stable carbocation.

The result is Markovnikov addition.

In the presence of peroxides, the reagent that

attacks the double bond first is the larger

bromine atom.

It attaches itself to the less hindered carbon

atom by a radical mechanism, so as to form

the more stable radical intermediate.

The result is anti-Markovnikov addition.

Addition of Addition of IClICl to alkenesto alkenes