Azo Coupling Reaction

Azo coupling is an electrophilic aromatic substitution in which the weakly electrophilic benzenediazonium cation attacks an electron-rich aromatic ring such as phenol or an aromatic amine. Because the diazonium ion is a weak electrophile, it can only attack rings activated by strong electron-donating groups, and it does so at the position of highest electron density. In phenol the –OH group is a strong ortho/para director, and the bulky diazonium electrophile preferentially attacks the less hindered para position, forming p-hydroxyazobenzene, an orange dye. The meta option is wrong because –OH is not a meta director; meta attack would contradict the activating, ortho/para-directing nature of the hydroxyl group. The ortho-only claim is incorrect because, although some ortho product can form, para dominates due to reduced steric crowding. The ipso position is already bonded to –OH and is not the site of substitution. Mildly alkaline conditions are required so phenol exists partly as the more reactive phenoxide while the diazonium ion remains stable. This is the classic NCERT and JEE Advanced coupling reaction. As a consistency check, the intense colour arises from extended conjugation across the –N=N– bridge linking two aromatic rings, confirming successful para coupling.