Question

Most activating groups are ortho-para directors and most deactivating groups are meta directors. What is an exception to the above statement?

Answer 1

answer : halogens (F,Cl, Br, I) are exceptional

explanation:

Halides are ortho- para- directing groups but unlike most ortho- para- directors halides tend to deactivate benzene. This unusual behavior can be explained by two properties:

1. Since the halogens are very electronegative they cause inductive withdrawal (withdrawal of electrons from the carbon atom of benzene).
2. Since the halogens have non-bonding electrons they can donate electron density through pi bonding (resonance donation).

The inductive and resonance properties compete with each other but the inductive dominates and halogens as a result become deactivating ortho para directors.