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Apparent second-order rate coefficients have been determined for the reactions a t 37 "C of diazodiphenylmethane with benzoic acid in 1 9 donor aprotic solvents. Values determined for various initial concentrations of acid, c, are analysed in terms of the equation k(obs) = ko + k'c. Reactions of acetic acid and of deuteriated benzoic and acetic acid (PhC02D and MeC0,D) have also been studied. We conclude that k, 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 ko 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 ko 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 k, 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 ko and k', and of the Hammett reaction constant for the two types of rate coefficient are discussed.

Dr. K. Bowden (personal communication) and his co-workers have also observed a similar effect for phenylacetic acid reacting with diazodiphenylmethane in ethyl acetate, but not in dimethyl sulphoxide.