Additive substituent effects on the ionization of meta-substituted phenols

Abstract

Thermodynamic ionization constants of 3-ethoxyphenol; 3,5-diethoxyphenol, 3,5-dichlorophenol, 3,5-dibromophenol, 3,5-diiodophenol, and 3,5-dinitrophenol have been measured spectrophotometrically within the temperature range 5-60°, and the thermodynamic functions of ionization ΔG₂₅, ΔH₂₅, ΔS₂₅, and ΔC_p.25 calculated. These results, in conjunction with others measured previously, indicate that, for this reaction series, substituent effects on the free energies of ionization are precisely additive, and on the entropies of ionization closely additive. An assessment of the results in terms of Hepler's internal/external enthalpy theory is made.