The influence of the solvent on organic reactivity. Part II. Kinetics and mechanism of the reactions of carboxylic acids with diazodiphenylmethane in aprotic solvents in which the acids may be dimeric

Apparent second-order rate coefficients have been determined for the reactions at 37 °C of diazodiphenylmethane with benzoic acid in 11 aprotic solvents in which the dimeric form of the acid predominates. Depending on the solvent chosen, k(obs) shows one of several forms of dependence upon the initial concentration of acid, c. The mechanism previously developed for the reaction in donor aprotic solvents is applied and adapted to the situation in which the acid is largely dimeric. Besides processes involving reactions of monomer and dimer, the kinetics suggest the participation of a reactive multimer, probably a trimer and possibly a tetramer. Established or inferred approximate values for K21, the dissociation constant of dimer in the various solvents, are used to analyse the kinetics in terms of the equation k(obs)=α[HA]/c+β[HA]2/c+γ[HA]3/c+δ[HA]4/c, where the coefficient of [HA]n characterises the reaction of (HA)n(the cubic and/or quartic terms are not always needed). The variety of ways in which k(obs) may vary with concentration arises from the different solvent, structural, and isotopic effects on the coefficients.