Question

Propose a structure for a conjugated diene that gives the same product from both 1,2 and 1,4-addition of HBr.

Short answer

Cyclopentadiene

Step-by-step solution

Electrophilic addition

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

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.

Electrophilic addition to dienes

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 Markovnikov and Anti-Markovnikov products. 1, 4-addition product is the result of the formation of stable allylic carbocation.

Prediction of structure of diene

Let us understand this with the 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:

The conjugated diene which will give same product from both 1,2 and 1,4 addition of HBr is any cyclic diene. Any cyclic diene will give the same 1,2 and 1,4 addition products because here the carbocations formed will all be same.

For example cyclopentadiene: the carbocation formed will be

The reaction and products: