Compilation and critical evaluation of structure-reactivity parameters and equations:: Part 2.: Extension of the Hammett σ scale through data for the ionization of substituted benzoic acids in aqueous organic solvents at 25°C

Data have been compiled for the pK values in certain aqueous organic solvents at 25 degrees C of about 70 meta- or para-substituted benzoic acids, with a view to proposing values of Hammett constants, sigma, in the case of substituents for which values cannot be based on the ionization of substituted benzoic acids in water (Part 1). The emphasis of the work is on the use of data for the apparent pK values of the acids in 1:1-EtOH-H2O, i.e. a solvent made up of equal volumes of ethanol and water, A calibration equation is derived relating substituent effects measured in this solvent to those measured in water. On the basis of this equation sigma values are proposed for 25 substituents, although some of the values are subject to caveats, and several further substituents are also discussed.