A kinetic and thermodynamic study of the addition of methoxide ion to substituted benzaldehydes

Abstract

Reaction rates have been investigated for the addition of methoxide ion to eleven substituted benzaldehydes in methanol at 25 °C. Thermodynamic parameters for the addition of both methanol and methoxide have also been determined, using a combination of n.m.r. and calorimetric data. The reaction rates are somewhat higher, and the adducts more stable than those of the corresponding reactions in water. The forward rates for the addition reaction are again considerably more sensitive to the effect of substituents than the reverse rates. The results are discussed in terms of a two step process, (1) formation or breaking of the C–O bond (sensitive to substitution) and (2) reorganization of the solvent (insensitive to substitution). It is concluded that the transition state for the reaction resembles the final products with respect to C–O bond formation, but that the state of solvation of the transition state corresponds to that of the reactants.