Benzenediazonium chloride couples with aniline to give 4-aminoazobenzene in a medium that is

In mildly acidic solution there is enough free amine to be an activated nucleophile while the diazonium ion is retained. In strong acid the amine is converted to the unreactive anilinium ion, and in alkaline solution the diazonium ion is converted to non-electrophilic species.