Secondary deuterium isotope effects in proton transfers from 2-nitropropane

Abstract

Kinetic and equilibrium secondary isotope effects have been measured for the deprotonation of 2-nitropropane by hydroxide and acetate ions. Although the Brønsted coefficients deduced lie between 0 and 1, they cannot be regarded as ‘degrees of proton transfer’ within the formalism of the Marcus theory. The isotope effects are interpreted in terms of two contributions, one of which does not change monotonically along the reaction co-ordinate. A recently reported dependence of the primary isotope effect upon isotopic solvent composition is discussed in the context of the rule of the geometric mean. A revised value of the equilibrium constant for iodination of the 2-nitropropyl anion is reported.