Nucleophilic substitution at bivalent sulphur. Part III. Kinetics and mechanism of the base-catalysed hydrolysis of ethyl 2-nitro-4-trifluoromethylbenzenesulphenate, the corresponding sulphenyl chloride, and the disulphide

Abstract

In the base-catalysed hydrolysis of ethyl 2-nitro-4-trifluoromethylbenzenesulphenate in 50% v/v aqueous dioxan, the reaction leading to the disappearance of ester was of the first order in ester and in hydroxide ion, while that leading to the disappearance of the intermediate sulphenate anion was of the second order in the anion. The rate of the latter reaction was inversely proportional to the concentration of hydroxide ion. The maximum concentration of sulphenate anion (80%) was much greater than with ethyl 2-nitrobenzenesulphenate and presumably reflects the decreased nucleophilicity and base strength of the anion. The mechanism proposed involves nucleophilic substitution at sulphur by hydroxide ion followed by reversible formation of the thiosulphinate from sulphenate anion and finally reaction of the thiosulphinate with hydroxide ion. Bis-2-nitro-4-trifluoromethylphenyl disulphide hydrolysed 100 times faster than the ethyl sulphenate and consequently gave a higher maximum concentration of sulphenate anion. These two hydrolyses are considered to follow similar mechanisms. 2-Nitro-4-trifluoromethylbenzenesulphenyl chloride hydrolyses very rapidly to form the disulphide and the sulphinate anion. The increased rate of reaction of the intermediates is attributed to electrophilic assistance by the sulphenyl chloride.