Above (C-N) coupling reaction take place at :

Effect of pH on the coupling reaction :
Coupling is best carried out in solutions that are neither too strongly acidic nor alkaline. Ttre reasons for this can be seen if we examine a typical case, the coupling of benzenediazonium drloride with dimethylaniline. At high pH, the diazonium ion is presenr in very low concentration, since most of it has been convefted to ArN = N - OH and ArN = N - O^-. Neither ArN = NOH (the diazohydroxide) nor ArN = N - O^- (the diazotate ion) is electrophilic, and thus does not couple with the amine. At low pH, the dimethylaniline will be largely protonated to ArN⁺Me₂H, and thus the activating effect of the -NMe₂ group is desffoyed, since ttre nitrogen atom no longer possesses an unshared pair of electrons: