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

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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.

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