The main problem encountered during electrophilic substitution
reactions of aromatic amines is that of their very high reactivity Substitution tends to occur at ortho- and para-positions If we
have to prepare monosubstituted  aniline derivative, how can
the activating effect of –NH2 group be controlled This can be
done by protecting the -NH2 group by acetylation with acetic
anhydride, then carrying out the desired substitution followed
by hydrolysis of the substituted amide to the substituted amine