Deuterium solvent isotope effects on reactions involving the aqueous hydroxide ion

The effect of solvent change from H2O to D2O on the ionic product of water and on rates of proton transfer from carbon acids to hydroxide ion can be quantitatively explained on the basis of the formula OH–a(HbOHc)3 for the aqueous hydroxide ion, in which a, b, c denote non-equivalent positions and where it is assumed that different deuterium fractionation factors apply to the three hydrogen sites (ϕa≃ 1·2–1·5 ϕb≃ 0·65–0·70; ϕc≃ 1). It is thus possible to reconcile existing experimental results for deuterium fractionation in the aqueous hydroxide system and in the analogous methanolic methoxide system.The formulation of the aqueous hydroxide ion as a trihydrate makes it improbable that proton transfer from a substrate to hydroxide ion is a simple process for which ‘the degree of proton transfer in the transition state’ is a readily identifiable concept. Such considerations help to rationalise an apparent inconsistency in solvent and substrate isotope effects for the ionisation of 2-nitropropane. The primary substrate deuterium isotope effect [k(Me2CHNO2)/k(Me2CDNO2)] for methanolic methoxide is 7·6 ± 0·2 and 7·4 ± 0·1 for aqueous hydroxide, the agreement suggesting a close similarity in mechanism, whereas the solvent isotope effects in the two solvents do not correspond.Experimental results are reported for the ionisation of 2-nitropropane and of 2-nitro[2-2H]propane in aqueous sodium hydroxide (H2O and D2O as solvents, and mixtures for 2-nitropropane). There is a small interaction of solvent and substrate isotope effects.