Amino group, being o-, p- directing, activates the benzene nucleus and hence further substitution in aniline is very easy.
(i) Bromination : Since -NH2 group is activating group, reaction with aqueous bromine gives tribromoaniline. For preparing monobromo derivative amino group is first protected by acetylation.
Thus in such cases if the monosubstituted product is to be prepared, the activation influence of -NH2 group must be moderated which is usually done by acetylation the (-NH2 → - NHCOCH3). The acetylamino group is an o-, p-directing but less activating than the 41 -NH2 group, and can be easily hydrolysed to regenerate the - NH2 group.
The lesser activating effect of the acetylamino group is due to the fact that the lone pair of electrons on the nitrogen atom is involved in resonance with the C = O group and hence it is comparatively lesser available for resonance with the benzene ring.
Resonance in NHCOCH3 reduces electron density of nitrogen atom making - NHCOCH, moderate activating group.
This also explains why acetanilide is a very weak base.
(ii) Iodination : The relatively unreactive iodine also substitutes aniline directly; the HI formed in the reaction is fixed up as anilinium iodide.
(iii) Sulphonation (baking) :
Sulphanilic acid has acidic (-SO3H) as well as basic (-NH2) group in the same molecule. The two groups may interact to form a sort of an 'inner salt' commonly known as Zwitterion or dipolar ion which is amphoteric in nature. The dipolar ion structure is proved by the high m.p. of sulphanilic acid and its other properties.
(iv) Nitration : Direct nitration of aniline leads its oxidation. Moreover, acidic medium of nitration converts aniline to anilinium ion, C6H5NH3+ which deactivates the benzene ring towards electrophilic substitution. Hence for preparing nitro derivatives, the amino group is protected by acetylation to give o- and p- nitroanilines.
