Acylation or Acetylation.

Explanation

The replacement of an active hydrogen atom of alcohols, phenols or amines with acyl (RCO or ArCO) group to form the corresponding esters or amides is called acylation. Acylation is usually carried out by using acid chlorides or anhydrides in presence of a base like pyridine or dimethylaniline, stc. Although in principle, any acid chloride or anhydride can be used for acylation however, acetyl chloride and acetic anhydride are the common reagents.
(i) In alcohols. It is usually carried out in presence of a base such as pyridine, dimethylaniline, etc. For example,

(ii) In phenols. Acetylation of phenols with acetyl chloride is usually carried out in presence of a base anhydride may be catalysed both by acids (a few drops of conc. `H_(2)SO_(4)`) and bases (sodium acetate, pyridine, etc.). For example,

(iii) In amines. Since aliphatic amines themselves are bases, the acylation of aliphatic amines is usually carried out in absence of a base. For example,

Tertiary amines `(3^(@))` do not react due to the absence of replaceable hydrogen on nitrogen.
Since aromatic amines are less basic and hence less nucleophilic, their acetylation is usually carried out in presence of a catalyst. Whwrwas acetylation with `CH_(3)COCl` is carried out in presence of pyridine, acetylation with acetic anhydride is carried out in presence of `CH_(3)COOH` or a few drops of conc. `H_(2)SO_(4)`.