Effects of substituents on the thermodynamic functions of ionisation of meta-substituted phenols

Abstract

Thermodynamic ionisation constants of m-methoxyphenol, m-nitrophenol, and m-hydroxybenzaldehyde have been determined by a spectrophotometric technique within the range 5–60°. Values for the standard free energy, enthalpy, and entropy changes have been calculated for the ionisation of each phenol. For a series of eight meta-substituted phenols the Hammett equation is closely obeyed over the range 10–55°, and the reaction parameter ρ is independent of temperature. The results are assessed in terms of a solvation-dependent mechanism.