Question

Why does acetylation of $-N{H}_2$ group of aniline reduce its activating effect?

Answer 1

Acetylation of $-N{H}_2$ group of aniline reduces its activating effect because the lone pair on the nitrogen atom is in conjugation with the carbonyl group in acetylated aniline. As a result, the activating effect of $-N{H}_2$ group is reduced i.e., the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance.