Why are carboxylic acids stronger acids than alcohols? The electrostatic theory of Siggel-Thomas revisited

The electrostatic explanation of Siggel and Thomas for the acidity differences between series of acids was investigated critically. Acidities and electrostatic potentials at the protons of XOH, XNH2, and XCH3 derivatives (X = H, CH3, HCO, NO2, and Fl were calculated at the Becke3LYP/6-311 +G** level. Final state (anion) relaxation energies were obtained as proposed by Siggel and Thomas. Examination of initial and final state contributions revealed deficiencies in the Siggel-Thomas method as the final-state (relaxation) energies still retain contributions from the electronic structure of the initial state. Hence, this relaxation energy is not reliable as a measure of resonance stabilisation in anion. The application of Siggel-Thomas approach in two opposite directions-dissociation of neutral acid and protonation of an anion-leads to contradictory conclusions about whether the acidity difference between methanol and formic acid is determined by the neutral acid or anion. Hence, this scheme cannot be used to determine the initial state (neutral acid) and final state (anion) contributions to acidity as proposed by Siggel and Thomas. (C) 2000 Elsevier Science B.V. All rights reserved.