Constituent analysis of the interaction of solvents with the leaving group anions for SN1–E1 reactions. Thermodynamic evidence for the nature of the conjugate base of p-nitrophenylhydrazonomalononitrile

Abstract

Enthalpies of solution in dimethyl sulphoxide and in methanol for the tetraethylammonium salf of the conjugate base of p-nitrophenylhydrazonomalononitrile were measured and compared with those of tetraethylammonium chloride. The former salt is more stable in the strongly dipolar aprotic solvent dimethyl sulphoxide than in the protic solvent methanol, wheras the latter is more stable in methanol because of hydrogen bonding of the solvent with chloride ion. Single ion enthalpies of transfer were determined in acetonitrile–methanol mixtures for the hydrazono anion as well as for tosylate ion and were dissected into constituent terms; a ‘more physical’ and a ‘specific’ interaction, solvation being due to hydrogen bonding. The hydrazono anion resembles BBuⁿ₄^– and CIO₄^– in that the contribution from the latter term is negligible. On the other hand, tosylate ion is susceptible to hydrogen bonding as is halide ion. The result is consistent with the kinetic behaviour of the leaving group anions for S_N1–E1 reactions.