The influence of the solvent on organic reactivity. Part 3. The kinetics of the reactions of diazodiphenylmethane with benzoic acid or 2,4-dinitrophenol in aprotic solvents

Abstract

The kinetics of the reactions between diazodiphenylmethane and benzoic acid at 37 °C have been studied in a further 13 aprotic solvents. Values of k₀, the limiting second-order rate coefficient at [acid]= 0 are tabulated. log k₀ values were subjected to multiple correlation with solvent parameters, results from this and earlier work being combined for 43 solvents in all. In addition to updating the analysis of the type advocated by Koppel and Palm, we also examined possible improvements in this and the application of alternative solvent parameters. We conclude that the B parameter is still the most useful available measure of solvent basicity in the correlation analysis of the present reactions, but there is a case for using the E_T parameter rather than separate solvent electrophilicity (E) and dielectric [(ε– 1)/(2ε+ 1)] parameters. The kinetics of the reactions between diazodiphenylmethane and 2,4-dinitrophenol in 38 aprotic solvents at 30 °C have also been studied. In contrast to the reaction with benzoic acid, observed second-order rate coefficients always showed little dependence on concentration. The spread of values of rate coefficients over the range of solvents studied was only 180-fold, compared with 3 200-fold for benzoic acid. An analysis of the Koppel–Palm type is presented for the reaction in 31 solvents in which 2,4-dinitrophenol is believed to be present in the chelated form, but this analysis does not apply to the highly basic solvents in which the chelate ring is broken. In contrast to the reaction with benzoic acid, the dielectric properties of the reaction medium are more important than solvent basicity in governing log k. However, simple regression of log k on E_T is remarkably successful.