Abstract
Rate coefficients have been determined for the reactions at 30 °C of diazodiphenylmethane with benzoic acid in 10 alcohols so that, with earlier work, results are now available for this reaction in 20 alcohols of very varied structures. The kinetic solvent effect can be interpreted in terms of the dielectric constant of the solvent and the influence of hydrogen bonding (solvent basicity or acidity). Multiple linear correlation of log k with the Kirkwood function of dielectric constant and the Taft σ* value for the alkyl group of the alcohol is highly successful (r= 0·983), but an additional parameter is necessary to express the rate enhancing effect of branching at the β-carbon atom of the alcohol. The most useful third parameter is nγH, the number of γ-hydrogen atoms in the alcohol molecule, giving r= 0·991.Apparent second-order rate coefficients have also been determined for the reactions at 37 °C of diazodiphenylmethane with benzoic acid in 22 aprotic solvents. Values determined for a range of initial concentrations of acid have been extrapolated to obtain values of k0, the rate coefficient at [acid]= 0. The effect of solvent on k0 can be interpreted in terms of the effect of the dielectric constant and the basicity of the solvent, but multiple linear correlation of log k0 with a suitable function of dielectric constant and a measure of solvent basicity (two are available) is only moderately successful (r=ca. 0·91). Attempts to improve the correlations by including the refractive index as a measure of ‘dynamic dielectric constant’ have met with little success. Detailed examination of the pattern of results suggests the relevance of other factors but at present these cannot be incorporated as additional parameters in the correlations.In both protic and aprotic solvents the specific role of the solvent is more important than its general electrostatic effect.
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