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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.
Chapman et al. (1974) studied this question.