Diazonium salts act as weak electrophiles — they can only attack highly activated aromatic rings (phenol, aniline, N,N-dimethylaniline). With phenol (in weakly alkaline medium): C6H5N2+ + C6H5O- -> p-HO-C6H4-N=N-C6H5 (p-hydroxyazobenzene, orange). With aniline (in weakly acidic medium): C6H5N2+ + C6H5NH2 -> p-H2N-C6H4-N=N-C6H5 (p-aminoazobenzene, yellow). With N,N-dimethylaniline: C6H5N2+ + C6H5N(CH3)2 -> p-(CH3)2N-C6H4-N=N-C6H5. Coupling always occurs at the para position of the activated ring (ortho if para is blocked). The -N=N- group (azo linkage) is a chromophore responsible for colour. These azo compounds are the basis of the vast majority of synthetic dyes (Congo red, methyl orange, etc.).