Solvent effects on the rates and equilibria of the proton transfer reaction of 4-nitrophenylphenylcyanomethane to tetramethylguanidine

The equilibrium constants and second-order rate constants, both at 25 °C, have been determined for the proton transfer reaction from 4-nitrophenylphenylcyanomethane to tetramethylguanidine in a number of different solvents. While the equilibrium constants show a good correlation with the dielectric constant function D − 1/2D + 1, the rate constants are randomly scattered with respect to this parameter and also the solvent parameter ET. The rate constants do show a general trend in relation to the solvent parameters AN and DN. These results are interpreted as support for the hypothesis that the transition state of the proton transfer occurs early along the reaction path.