Question

why acetylation of-nh2 group is done in aniline before preparing its ortho and para compound

Answer 1

To get selective product only at para position otherwise we wud have got grps at both para and ortho positions.
it is used for selective reactivity at specific positions and thus decrease the lone pair donating capacity of nitrogen.

Answer 2

Explanation:

Aniline has a $-NH_2$ group bonded on aromatic ring.

Here the $-NH_2$ group has a lone pair which can show the resonance with aromatic ring. Also it can be given to electron deficient molecule that makes aniline basic in nature. Before electrophilic substitution acylation of $-NH_2$ is necessary otherwise, it will react with $AlCl_2$ that is catalyst of these reaction and also Lewis acid.

Hence acetylation of $-NH_2$ group is done in aniline before preparing its ortho and para compounds.