Nitrogen and deuterium kinetic isotope effects on the Menshutkin reaction

Nitrogen and deuterium kinetic isotope effects were measured in the Menshutkin reaction between methyl iodide and a series of para-substituted N,N-dimethylanilines in ethanol, The nitrogen kinetic isotope effect increases for the more electron-donating substituents [0.9989, 1.0032, and 1.0036 for 4-C(O)Me, H and 4-Me, respectively], in agreement with the Hammond postulate, The secondary deuterium isotope effect, however, exhibits the reverse trend (1.045, 0.989, 0.975 per deuterium, for the respective substituents). This discrepancy is rationalized in terms of solvent molecule participation in the transition state.