Primary hydrogen isotope effects on the rate of ionization of nitroethane in mixtures of water and dimethyl sulphoxide

The rate of ionization of nitroethane and [1-2H2]nitroethane has been measured in solutions of sodium hydroxide in six aqueous solvents containing 0–58 mol % of dimethyl sulphoxide. The observed values of the kinetic isotope effect (kH/kD), together with previously published values for the ionization of several nitro-alkanes in aqueous solution, provide further evidence supporting the existence of an isotope effect maximum when the basicities of the substrate anion and the catalysing base are approximately equal (i.e.ΔpK=ca. 0). The results further indicate that the variation of kH/kD with ΔpK is at least qualitatively the same whether the variation in ΔpK results from the use of different substrates and bases or from the effects of solvent variation on a fixed substrate–base pair.