Solvent effects on the relative stabilities of 1:1 and 1:2 adducts formed from 1,3,5-trinitrobenzene and thiolate ions

Abstract

The equilibrium constants K₁(=[TNB·SR^–]/[TNB][SR^–]) and K₂(=[TNB(SR)₂^2–]/[TNB·SR^–][SR^–]) for the formation of covalently bound Meisenheimer-type adducts from 1,3,5-trinitrobenzene and thiolate ions have been measured by a spectrophotometric method in alcohol (methanol or ethanol)–water mixtures. The values of K₁ are smaller in water than in alcohol but go through a maximum at ca. 80%(v/v) alcohol. The values of K₂ increase monotonically as the proportion of water in the solvent is increased and are ca. 10³ larger in water than in the pure alcohols. The factors affecting the stabilities of the complexes are discussed. The enhanced stability of the 1:2 adducts in water is attributed partly to the good solvation by water of a localised negative charge on the nitro-group between the positions of addition.