Deuterium solvent isotope effects on reactions involving the aqueous hydroxide ion

Abstract

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(H_bOH_c)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 (ϕ_a≃ 1·2–1·5 ϕ_b≃ 0·65–0·70; ϕ_c≃ 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.