Question

Using resonance contributors for the carbocation intermediate, explain why a phenyl group is an ortho–para director.

Short answer

The carbocation generated by putting an electrophile at the ortho-para position can be stabilized by the resonance electron donation from the phenyl substituent.

The carbocation generated by putting an electrophile at the meta position cannot be stabilized by the resonance electron donation from the phenyl substituent.

Step-by-step solution

Effect of Substituents on orientation

All activating substituents and weak deactivating halogens are ortho-para directors. Substituents that are more deactivating than halogens are meta-directors.

The stability of a carbocation intermediate helps in the understanding of the position of a substituent, which directs a particular electrophile.

Explanation of why a phenyl group is an ortho-para director

The carbocation generated by putting an electrophile at the ortho-para position can be stabilized by the resonance electron donation from the phenyl substituent.

Carbocation generated by putting an electrophile at the ortho-para position

The carbocation generated by putting an electrophile at the meta position cannot be stabilized by the resonance electron donation from the phenyl substituent.

Carbocation generated by putting an electrophile at the meta position