Studies on coupling reaction of 1-naphthol. XI. Effect of additives on the dissociation equilibrium of 4-(4-nitrophenylazo)-1-naphthol in pyridine.

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The dissociation of 4-(4-nitrophenylazo)-1-naphthol (azo dye) in pyridine was found to be accelerated by the addition of small amounts of quaternary ammonium compounds, R4N+X−. The apparent equilibrium constant of the azo dye was influenced enormously by the species of X−, but was little affected by the structures of R. The effect increased as the X− became more basic, ClO4−<I−<BF4−<Br−<HSO4<Cl−, in proportion to the ability of X− to bind with protons formed by dissociated azo dye. The basicity of the X−, pKc, was correlated linearly with the variation of the equilibrium constant, owing to the addition of R4N+X−, δpKa. The dissociation of the azo dye in pyridine–water mixtures was also accelerated upon increasing the amount of water (0–10 vol%), correlating with an increase in the basicity of the solution.
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