Q. 48 Under what reaction condition (acidic, basic) the coupling reaction of aryl diazonium chloride with aniline is carried out?

In strongly basic conditions, benzene diazonium chloride is converted into diazohydroxide and diazoate as both of which are not electrophilic and do not couple with aniline.
C6H5 NNCI¯+OHSO2CH5N=NOHNaOHC6H5N=NONa+Diazohydroxide 
Similarly, in highly acidic conditions, aniline gets converted into anilinium ion, From this, result aniline is no longer nucleophilic acid and hence will not couple with diazonium chloride. Hence, the reaction is carried out under mild conditions, i.e., pH—4— 5
C6H5NH2+H+AnlilineC6H5NH3       Anilinumion(couplingdonotoccur)