Substituent effects. Part 11. Anomalous dissociation constants of benzoic acids in water–organic solvent mixtures. An extended Hammett equation comprising the hydrophobic constant as an additional parameter

Abstract

Thermodynamic dissociation constants of benzoic acids show deviations from the Hammett equation in ethanol–water and, more strongly, in t-butyl alcohol–water, with maxima at about 40% ethanol–water (16 mol%) and 32% t-butyl alcohol–water (8 mol%). The extended equation Δ=ρσ+hπ, where π is Hansch's hydrophobic constant, covers the experimental data very well; h varies from 0 to – 0.16. Similar behaviour is exemplified for other systems. In the solvents 80% methyl Cellosolve and 65% dimethyl sulphoxide the Hammett equation is followed closely. Some practical implications and an attempt at rationalization are given. Related data from the literature, including recent work of Fan and Jiang, are discussed briefly.