29E
Question
Propose a structure for a conjugated diene that gives the same product from both 1,2 and 1,4-addition of HBr.
Step-by-Step Solution
VerifiedCyclopentadiene
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 Markovnikov and Anti-Markovnikov products. 1, 4-addition product is the result of the formation of stable allylic carbocation.
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: