In 1884, Zincke and Bindewald 1 synthesized the same product (1, Scheme 1) from coupling benzenediazonium chloride with 1-naphthol and from condensing phenylhydrazine with 1,4naphthoquinone, discovering one of the first examples for mobile equilibrium associated with proton exchange and called later tautomerism. Scheme 1. Synthesis and tautomers of 1. Dominating tautomers of other azonaphthol dyes 2 and 3.