Chapter 13, Problem 55P

Interpretation Introduction

Interpretation:

The structural formula of the most stable cyclohexadienyl cation intermediate formed in each of the given reactions is to be written. Whether this carbocation is more or less stable than the cyclohexadienyl intermediate formed by benzene, is to be determined.

Concept Introduction:

Compounds containing benzene ring undergo electrophilic aromatic substitution reaction with electrophilic reagents.

The mechanism of electrophilic aromatic substitution involves two steps.

In the first step, an electrophile accepts an electron pair from pi system of benzene giving an intermediate called as cyclohexadienyl cation or arenium ion intermediate.

In the second step, the cyclohexadienyl cation intermediate undergoes deprotonation to restore the aromaticity of benzene ring.

In order to form cyclohexadienyl cation intermediate, the electrophile must be reactive.

Electron donating substituents present on the benzene ring increase the reactivity towards the electrophilic aromatic substitution reactions by stabilizing the cyclohexadienyl cation intermediate.

When an electron donating substituent is attached to the benzene ring, the cyclohexadienyl cation intermediate formed will be more stable than the cyclohexadienyl intermediate formed by benzene.

When an electron withdrawing substituent is attached to the benzene ring, the cyclohexadienyl cation intermediate formed will be less stable than the cyclohexadienyl intermediate formed by benzene.

Stability of a cyclohexadienyl cation intermediate depends mainly on two factors – its proximity to an electron donating or electron withdrawing group and whether it is an allylic carbocation or not.