How does acetylation with acetic anhydride affect the reactivity of the -NH₂ group in aniline during electrophilic substitution reactions, and why is the activating effect of the -NHCOCH₂ group less than that of the amino group?

Acetylation with acetic anhydride involves the conversion of the -NH₂ group in aniline to -NHCOCH₃. This acetyl group is less activating than the amino group in electrophilic substitution reactions. The nitrogen lone pair in the -NHCOCH₃ group is partially involved in resonance with the carbonyl, reducing its availability for donation to electrophiles. The acetyl group’s electron-withdrawing nature also weakens its activating effect compared to the amino group. Consequently, during electrophilic aromatic substitution, the acetylated aniline exhibits reduced reactivity, allowing for better control over the substitution process and minimizing poly-substitution issues encountered with the unmodified amino group.