91Ó°ÊÓ

The electrophilic substitution at disubstituted benzenes is governed by several factors:

• If the directing effects of both groups reinforce each other, a single product is obtained.
• If the directing effects of both groups oppose each other, the most powerful activating group has a major influence and leads to a mixture of products.
• In meta-disubstituted compounds, substitution happens rarely because of steric reasons

(a)Both the $−OC{H}_3$and $−Br$groups are ortho-para directors. Methoxy group is a more powerful activating group than bromine. Hence the substitution happens at ortho para positions with respect to the methoxy group. Both the groups occupy same position with respect to each other. But the attack does not happen between the two groups because of steric reasons.

Reaction (a)

(b)Both the $−N{H}_2$and $−Br$groups are ortho-para directors. Both the groups occupy different position with respect to each other. This is because$−N{H}_2$is more powerful activator than the $−Br$group. Hence, the substitution occurs ortho and para to the$−N{H}_2$group.

Reaction (b)

(c)Here the$−N{O}_2$group is a meta director. Both the$−N{O}_2$and $−Cl$groups are deactivating groups. But chlorine has lone pair of electrons and therefore increases the electron density on the ring. Hence the electrophilic substitution happens at the position ortho and para to the chlorine atom. The orientation of substitution makes the groups in the same position under electrophilic substitution

Reaction (c)