The influence of the solvent on organic reactivity. Part I. Kinetics and mechanism of the reactions of carboxylic acids with diazodiphenylmethane in donor aprotic solvents

Abstract

Apparent second-order rate coefficients have been determined for the reactions at 37 °C of diazodiphenylmethane with benzoic acid in 19 donor aprotic solvents. Values determined for various initial concentrations of acid, c, are analysed in terms of the equation k(obs)=k₀+k′c. Reactions of acetic acid and of deuteriated benzoic and acetic acid (PhCO₂D and MeCO₂D) have also been studied. We conclude that k₀ corresponds to the reversible formation of a complex between monomeric acid and diazodiphenylmethane and its subsequent breakdown to products, while k′ corresponds to the reaction of a small amount of highly reactive acid in the form of an open-chain dimer. The effect of a wide range of aprotic solvents on k₀ and k′ is interpreted in terms of the following solvent parameters: a function of dielectric constant and one of refractive index, and a measure of solvent nucleophilicity and one of electrophilicity. On including results from an earlier study, multiple linear regression of log k₀ on the four parameters gives an excellent correlation for 24 solvents (R= 0·975, s= 0·209). A similar analysis for the log k′ values for ten donor aprotic solvents is less satisfactory. Correlation analysis for log k₀ and log k′ values of the reaction in 11 ketones is very unsatisfactory: the hoped-for advantages of considering a homogeneous set of aprotic solvents were thus not realised. Apparent second-order rate coefficients have also been determined for the reactions at 37 °C of diazodiphenylmethane with substituted benzoic acids in seven donor aprotic solvents. Values of k₀ and k′, and of the Hammett reaction constant for the two types of rate coefficient are discussed.