Catalysis of N-nitrosation and diazotisation by thiourea and thiocyanate ion

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Abstract
Thiourea is shown to be a very efficient catalyst in the nitrosation of morpholine in acid solution. Compared with other known catalysts, the order of efficiency is SC(NH2)2 > SCN– > Br– in the ratio 4 200 : 240 : 1. The kinetic data are interpreted in terms of equilibrium formation of the NO–S ion from thiourea which acts directly as a nitrosating agent. The overall efficiency of thiourea as a catalyst arises from the large equilibrium constant for the formation of this ion rather than from the rate constant (k2) for attack by the ion on morpholine. Values of k2 for the series SC(NH2)2, SCN–, and Br– at 31° increase from 7 × 106 to 2.9 × 107 and 4.8 × 107 l mol–1 s–1, respectively. Similarly the diazotisation of aniline proceeds via NO–S in the presence of added thiourea. Again the k2 values were determined at 0° for the series SC(NH2)2, SCN–, Br– as 3.7 × 105, 1.1 × 107, and 1.6 × 109 l mol–1 s–1. For both SC(NH2)2 and SCN– reactions, the rate constant is well below that of the diffusion-controlled limit (cf. Br–); the measured activation energies also support this conclusion. It is suggested that thiourea and possibly other sulphur-containing species act as excellent general catalysts for nitrosation and diazotisation reactions generally. Cysteine showed negligible catalytic activity whilst S-methylcysteine gave some indication of catalysis in the diazotisation of aniline, but the effect was small.

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