Structural and energetic aspects of the protonation of phenol, catechol, resorcinol, and hydroquinone.

Abstract : The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol.
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Guy Bouchoux, Dirk Defaye, T. Mcmahon, A. Likholyot, O. Mó, et al.. Structural and energetic aspects of the protonation of phenol, catechol, resorcinol, and hydroquinone.. Chemistry - A European Journal, Wiley-VCH Verlag, 2002, 8 (13), pp.2900-2009. ⟨10.1002/1521-3765(20020703)8:13<2900::AID-CHEM2900>3.0.CO;2-T⟩. ⟨hal-00918102⟩

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