$\mathrm{Activation} \mathrm{of} \mathrm{benzene} \mathrm{by}-\mathrm{NH}2 \mathrm{group} \mathrm{can} \mathrm{be} \mathrm{reduced} \mathrm{by} \mathrm{treating} \mathrm{the} \mathrm{compound} \mathrm{with}$

In aniline, the amino group is an electron releasing group. It activates the benzene ring towards an electrophilic aromatic substitution reaction.

When aniline is acetylated with acetyl chloride, acetanilide is formed.
${\mathrm{CH}}_3\mathrm{CONH}-$group is less activating group than amino group.

Hence, activation of benzene by$-{\mathrm{NH}}_2$​ group can be reduced by treating the compound with acetyl chloride.

$\mathrm{Acetyl} \mathrm{chloride} \mathrm{converts} -\mathrm{NH}2 \mathrm{to} -\mathrm{NHCOCH}3 , \mathrm{which} \mathrm{reduces} \mathrm{the} \mathrm{activation} \mathrm{of} \mathrm{benzene} \mathrm{ring}.$