The separation of polar and steric effects. Part 14. Kinetics of the reactions of benzoic acid and of ortho-substituted benzoic acids with diazodiphenylmethane in various alcohols

Abstract

The main results now discussed are rate coefficients for the reactions of benzoic acid and 32 ortho-substituted benzoic acids with diazodiphenylmethane (DDM) at 30.0 °C in 11 alcohols including 2-methoxyethanol. The reaction involves a rate-determining proton transfer. The results have been subjected to correlation analysis in two ways. (a) For the reactions of a given acid in the various alcohols, the log k values are correlated through multiple regression on appropriate solvent parameters. (b) For reactions in a given alcohol, the log k values for the various acids are correlated by using the appropriate form of the extended Hammett equation involving inductive, resonance, and steric parameters. Analyses of type (a), as in earlier work, involve the σ* value of the group R in the alcohol ROH, the Kirkwood function of dielectric constant, f(ε)=(ε– 1)/(2ε+ 1) and n_γH, the number of hydrogen atoms in the γ-position in the alcohol. These correlations yield information on the influence of ortho-substituents on the relative importance of specific and non-specific solvent effects. In analyses of type (b) the correlation equations are established with a limited set of substituents (up to 18), whose characteristic parameters σ_I, σ_R, and v(determined by Charton) are well established, and are unlikely to show effects due to hydrogen-bonding or substituent conformation. The results for other substituents are then interpreted by comparing log k(calc). from a correlation expression with log k(obs.).