Kinetic and thermodynamic preferences in the reactions of thiolate ions with 1-substituted-2,4,6-trinitrobenzenes

Abstract

Kinetic and equilibrium data are reported for reaction in water of ethanethiolate, thioglycolate, and thiomalate ions with 1-X-2,4,6-trinitrobenzenes (X = H, SEt, OEt, and Cl). The thiolate ions show strong kinetic preference for attack at unsubstituted ring positions. However, the isomeric 1 : 1 adducts formed by attack at substituted and unsubstituted positions have similar thermodynamic stabilities. The latter is in marked contrast with corresponding reactions of alkoxides. Adducts of stoicheiometry thiolate:nitro compound (2 : 1) are formed by thiolate attack at two unsubstituted ring positions and are stabilised in water by the good solvation of negative charge localised on the nitro groups. A 3 : 1 adduct has been identified by reaction of ethanethiolate ions with 1,3,5-trinitrobenzene and the kinetic data recorded. Comparison of nucleophilic attack by thiolate ions and by hydroxide ions indicates that the kinetic barrier to reaction with thiolates is considerably lower than for reaction with the oxygen base; a consequence is that (5; X = OEt) is an observable intermediate during nucleophilic substitution. 1 H NMR measurements in DMSO have been used to examine the structures of intermediates and reaction products; they show the irreversible displacement of nitro groups by thiolate ions.