S-nitrosation of thiourea and thiocyanate ion. Nitrosyl thiocyanate and the S-nitroso-adduct of thiourea as nitrosating agents

Abstract

Thiourea reacts with N-methyl-N-nitrosoaniline in aqueous acid solution to give N-methylaniline and a S-nitrosointermediate, which decomposes to form the CC′-dithiodiformamidinium ion. The products are believed to be the same as those from the reaction of thiourea with nitrous acid. The nitrosoamine reaction is first order in both thiourea and the nitrosoamine, and is also acid-catalysed. Thiourea is shown to be almost as reactive towards the nitrosoamine in this reaction as is iodide ion. Added N-methylaniline reduces the rate of the reaction, showing that the formation of the S-nitroso-intermediate is reversible i.e. it is capable of acting directly as a nitrosating agent. Rate measurements with added N-methylaniline and various ‘nitrite traps’[X = HN₃, NH₂[graphic omitted]H₃, NH₂SO₃H, [graphic omitted]H₃OH, and CO(NH₂)₂] have enabled the reactivity (relative to N-methylaniline) of the various X species towards NO-[graphic omitted] to be established. The same order of reactivity of X is found as was recently observed for other more well known nitrosating agents NOCl, NOBr, and H₂[graphic omitted]O₂, but the discrimination by NO–[graphic omitted] is significantly greater than for these, implying that NO–[graphic omitted] is a less reactive species. Rate constant ratios obtained similarly for nitrosyl thiocyanate are remarkably similar to those found for NO–[graphic omitted] suggesting that they have much the same reactivity as nitrosating agents.