Question

Cationic polymerization of 3-phenylpropene (${\mathbf{CH}}_{\mathbf{2}}\mathbf{=}{\mathbf{CHCH}}_{\mathbf{2}}\mathbf{PH}$)affords A as the major product rather than B. Draw a stepwise mechanism to account for this observation.

Short answer

Answer

Mechanism

Step-by-step solution

Carbocation Stability

Carbocation stability can be explained based on hyperconjugation and resonance. The stability order of carbocations is:

Benzyl>allyl>tertiary>secondary> primary.

The structure of 3-phenylpropene

The molecular structure of 3-phenylpropene is as shown:

3-phenylpropene

The possible cations formed from this compound on reaction with mild acid is

The types of carbocation intermediates formed

As structure “I” is a benzylic cation, it is more stable than II.

The cationic polymerization of the given monomer

The cationic polymerization of the given monomer forms the more stable benzylic intermediate and predominantly forms polymer A. The cationic polymerization of the given monomer forms the more stable benzylic intermediate and predominantly forms polymer A.

Cationic polymerization