Deuterium solvent isotope effects in methanol solution. Part III. Proton transfer from 2-nitropropane and from 3-methyl-1-phenylbutyl phenyl ketone to methoxide ion

Abstract

The extreme solvent isotope effects for the racemisation of D-(+)-3-methyl-1-phenylbutyl Phenyl ketone (α-phenyl-isocaprophenone)(k_D/k_H= 2·04) and the ionisation of 2-nitropropane (k_D/k_H= 2·28) have been measured for methanolic solutions of sodium methoxide at 25° C (where k_D refers to MeOD as solvent). It is shown that the existence of large solvent isotope effects in reactions of this type implies a solvation number of 3 for the methoxide ion, if larger values are ruled out on structural grounds. Measurements in mixtures of MeOH and MeOD are interpreted in terms of the appropriately modified theory of solvent isotope effects and are shown to be consistent with a multiply solvated methoxide ion. The primary (substrate) isotope effect for the ionisation of 2-nitro[2-²H₁]propane is found to have the value 7·4 in MeOH solution and ca. 6·9 in MeOD.