electrophillic addition ----1,4 and 1,2 addition reactions

• In conjugated dienes, the π bonds are separated by exactly one single bond.

• In isolated dienes, the π bonds are separated by two or more single bonds.

Conjugated dienes represent a special category, because they contain one continuous system of overlapping p orbitals—that is, the two π bonds together comprise one functional group

composed of four overlapping p orbitals. 

There fore , conjugated dienes exhibit special properties

and reactivity.

and 1,3-Butadiene is a conjugated diene

Electrophilic Addition

Addition of HX Across 1,3-Butadiene .

When butadiene is treated with HBr, electrophillic addition takes place,instead of one

two products are formed .

These two products can be explained with a  two-step mechanism:

protonation to form a carbocation followed by nucleophilic attack. which is :

first  electrophile attacks leading to formation of allylic carbocation that 

undergoes rearrangement .These two carbocations attract the nucleophile that leeds to formation of the final product .

Thermodynamic Control vs. Kinetic Control

The expected addition product from reactions of this kind is the result of 1,2-addition, i.e. bonding to the adjacent 

carbons of a double bond  and   the unexpected product 

comes from 1,4-addition

 Product compositions are  temperature dependent, as the 

addition of HBr to 1,3-butadiene demonstrates.

CH2=CH-CH=CH2 + HBr   reaction temperature	CH3CHBr-CH=CH2 + 1,2 addition yield	CH3CH=CHCH2Br  1,4 addition yield
0 ºC 40 ºC	70% 15%	30% 85%

Bonding of an electrophilic atom or group to one of the end 

carbon atoms of a conjugated diene (#1) generates an allyl 

cation intermediate.

Such cations are stabilized by charge delocalization, and it is

 this delocalization that accounts for the 1,4-addition product

 produced in such addition reactions.

Nucleophillic attack to first acrbocation leads to 1,2 product.

Nucleophillic attack to second  acrbocation leads to 1,4 product.

An explanation for the temperature influence is shown in the

 following energy diagram for the addition of HBr to 1,3-

butadiene. 1,2 product. is termed kinetic product.. At higher 

temperatures, equilibrium is established between the 

products, and the thermodynamically favored 1,4 product. 

dominates.1,4 product  is termed  thermodynamic product.

All said and done 1,2 product ,  termed kinetic product , requires lower activation energy and is formed at loewr temperatures as major product.1,4 product is the minor product.

At higher temperatures ,as per the requirements of
reaction, 1,4 product domminates ,since energy requirements in the form of higher activation energy is achieved