Nucleophilic substitution at bivalent sulphur. Part IV. A mechanistic study of the base-catalysed hydrolysis of derivatives of 2-nitro-4-trifluoromethylbenzenesulphenic acid using 19F nuclear magnetic resonance spectroscopy

Abstract

The various intermediates involved in the base-catalysed hydrolysis of bis-2-nitro-4-trifluoromethylphenyl disulphide were detected using the ¹⁹F n.m.r. chemical shift of the singlet from the 4-trifluoromethyl group. The reaction involves an initial nucleophilic substitution at sulphur to give the sulphenate anion and a thiolate ion. A second intermediate, presumably the thiolsulphinate, in which the aryl groups have different environments is formed by condensation of the sulphenate anion. The second intermediate gives the corresponding sulphinate and thence sulphonate ions. Surprisingly 2-amino-4-trifluoromethylbenzenesulphonate ion was also detected and isolated. A mechanism for its formation is proposed. The corresponding ethyl arenesulphenate gave similar results but with a higher percentage of the second intermediate. Hydrolysis of the sulphenyl chloride and bromide was much faster at all steps of the reaction and gave disulphide and sulphinate anion as the initial products. The sulphenyl thiocyanate reacted in a similar manner to the sulphenyl halides but gave a substantially higher yield of thiolate ion which is attributed to the simultaneous reaction of hydroxide ion at the thiocyanate sulphur. The aryl thiocyanate did not undergo substitution at sulphur and gave only the thiolate ion.