Why does bromination of aniline, even under very mild conditions, gives 2,4,6.- tribromo aniline instantaneously.

Explanation

Because of the electron donating effect of 'NH', group, the electron density incrẻases at-the ortho and para positions. When aniline is treated with 'Br_y' the 'Br^+' attacks the benzene ring at 0 -and p-positions to form carbocation intermediates which are stabilised not only by the normal resonance of the benzene' ring but also by the '-NH_2' group as shown:
figure
In the case of o-bromination. the carboeation is stabilised notoonly by usual resonating structure ( 'I III', IV) but is also stabilised by resonance structure II in which the lone pair of electrons on the 'N' -atom interacts with the positively charged carbon of the ring. In case of 'p' -bromination, the carbocation is stabilised not only by the usual resonating strúctures (V, VI and VIII) but is also stabilised by the resonance structure (VII) in which the lone pair of electrons on the 'N' -atom interacts with the positively chirged carbon of the ring. These additional resonating structures (II
VII) increase the stability of the carbocation to, sucti an extent that bromination occurs instantaneously at the p-and o-positions giving '2,4,6' - tribromoaniline.