91Ó°ÊÓ

When pi bond in a double or triple bond containing compounds is broken with the formation of two new sigma bonds, it is an electrophilic addition.

Dienes are the unsaturated compounds containing two double bonds between carbon atoms. Dienes are used in organic synthesis and as monomers in polymer industry. Based on the position of double bonds in the whole molecule, dienes are of three types-

• Cumulated dienes
• Conjugated dienes
• Isolated dienes

Cumulated dienes

When two double bonds are present successively on adjacent carbons, cumulated dienes are constituted. They are also called as allenes.

Conjugated Dienes

Dienes having two conjugated double bonds which are separated by a single bond, are called conjugated dienes.

Isolated Dienes

When double bonds in the molecule are separated by more than one single bond, it is isolated diene.

Dienes do undergo electrophilic addition reactions. In isolated dienes the reaction proceeds the way alkenes undergo electrophilic addition reactions. In case of conjugated dienes, a 1,4-addition product is formed in addition to the Markonikov and Anti-Markonikov products. For example- Addition of HBr to conjugated dienes generates a mixture of 1,2 and 1,4 addition products due to the formation of two different stable allylic carbocation. For example:

When 1 equivalent of HCl reacts with 1-phenyl-1,3-buta-di-ene, a mixture of six products are formed because of the possibility of the formation of different carbocations. Let us look at how these products are formed:

Out of all the six products the one which is obtained from carbocation D is the most stable, as this allylic carbocation is stablised through delocalization of pi bonds in the aromatic ring (giving extra stability) and in the side chain both. So 2-chloro-1-phenyl- butene is formed predominantly.