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)22–]/[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.