Acylation Reaction (Protection of Amino Group)

$$\text{Ar--NH}_2 + (\text{CH}_3\text{CO})_2\text{O} \xrightarrow{\text{Pyridine}} \text{Ar--NHCOCH}_3 \text{ (Acetanilide)} + \text{CH}_3\text{COOH}$$

Concept: Acylation reduces the activating power of the $-\text{NH}_2$ group by delocalizing the nitrogen lone pair onto the carbonyl oxygen via resonance, allowing controlled monosubstitution on the aromatic ring.