Abstract The effect of pH on the diazo coupling reaction of some commonly used coupling components (J-acid, Gamma acid, H-acid and S-acid) with monosubstituted diazobenzenes has been investigated. The results demonstrate that, contrary to previous reports, selective diazo coupling ortho to an amino group of an aminohydroxynaphthalenesulfonic acid does not occur in weakly acidic (pH 5–6) media. In fact, such media were found to give nearly exclusive diazo coupling ortho or para to the hydroxyl group. The desired amine coupling reactions required a pH of 3.0–3.5 and occurred only with the more reactive diazonium salts. The structures of the dyes obtained were unambiguously determined with the aid of 1H-NMR spectroscopy.

Selective diazo coupling ortho to an amino group of aminohydroxynaphthalenesulfonic acid does not occur in weakly acidic (pH 5-6), but rather occurs with more reactive diazonium salts at a pH of 3.0–3.5.

Anomalous behavior of aminohydroxynaphthalenesulfonic acids during diazo coupling

Key Takeaway: Selective diazo coupling ortho to an amino group of aminohydroxynaphthalenesulfonic acid does not occur in weakly acidic (pH 5-6), but rather occurs with more reactive diazonium salts at a pH of 3.0–3.5.