Preparation of azo compounds by oxidation dimerisation of 1 or 2 aromatic amines, and use thereof to prepare 3,3'-azo-bis(6-hydroxybenzoic acid) & its esters

A process for preparing an azo compound comprises subjecting at least one aromatic amino compound to an oxidative dimerisation reaction. An asymmetric azo compound is obtained by reacting two different aromatic amino compounds. The preferred reagents for the oxidative dimerisation reaction are(i) acetic acid and hydrogen peroxide followed by (ii) concentrated sulphuric acid. In an embodiment, the process comprises the preparation of a 3,3'-azo--bis(6-hydroxybenzoic acid) derivative 3 [where R is an ester forming group, eg alkyl (especially methyl), aryl or arylalkyl (especially benzyl)] by oxidative dimerisation of the corresponding 3-amino-6-hydroxybenzoic acid derivative 2 using the preferred reagents. 3,3'-Azo-bis(6-hydroxybenzoic acid) 1 and salts thereof (eg the disodium salt) may be obtained by saponification of the ester 3 prepared from the oxidative dimerisation process.