2-naphthol derivatives coupling with aniline

Both 2- and 3-nitrothiophenes are reduced by tin and hydrochloric acid to the corresponding aminothiophenes. In contrast to anilines, the free bases are very unstable their salts and acyl derivatives, however, are stable. 2-Aminothiophene can be diazotized and the resulting diazonium salt coupled with / - naphthol. The chemical instability of aminothiophenes compared with aniline is illustrated by the ring opening of 2-amino-3-ethoxycar-bonylthiophenes (157) with ethanolic sodium ethoxide to give cyanothiolenones (158) (75JPR861). 

Pigment synthesis follows the typical route to azo pigment lakes the aniline derivative or the aniline sulfonic acid is diazotized with sodium nitrite in an acidic medium (hydrochloric acid), followed by coupling on the Naphthol AS derivative, which is initially dissolved in an alkaline solution and then precipitated by adding inorganic or acetic acid. If a Naphthol AS sulfonic acid is used as a coupling component, it must be neutralized with alkali for dissolution and coupled directly. 

Conant and Peterson123 made the first kinetic study of the coupling of diazonium ions with aromatics, and measured the rates of reaction of diazotised aniline and its 2-MeO, 4-Me, 4-Br and 4-S03H derivatives with the sodium salts of 4-hydroxybenzenesulphonic acid, 2-naphthol-3,6-disulphonic acid, 1-naphthol-3,8-disulphonic acid, and l-naphthol-4-sulphonic acid. The reaction was second-order, viz. 

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