Study of the Hammett equation for the ionisation of substituted benzoic acids and phenols in dioxan–water mixtures

Abstract

The thermodynamic pK values of 17 substituted benzoic acids and 21 substituted phenols were determined in 20, 40, 60, and 80% v/v dioxan–water at 30 ± 0.1 °C and µ0.1M(NaClO4) by the Calvin–Bjerrum potentiometric titration technique. The ‘practical’ pK values of substituted benzoic acids were also determined in 85 and 90% v/v dioxan–water mixtures. The Hammett substituent constants were calculated and the data for the meta- and para-derivatives were utilized to give Hammett reaction constants. It was observed that dρ/dD(D= dielectric constant) was not regular. In the case of the benzoic acids, the ρ values decrease above 0.44 mole fraction dioxan. The ρ values are, however, insensitive to the medium for ortho-substituted compounds. Attempts have been made to explain these observations by considering the structure of the dioxan–water solvent.