THEORETICAL-STUDY OF THE INVERSION OF THE ALCOHOL ACIDITY SCALE IN AQUEOUS-SOLUTION - TOWARD AN INTERPRETATION OF THE ACID-BASE BEHAVIOR OF ORGANIC-COMPOUNDS IN SOLUTION

Some homologous series of organic compounds present an acid-base behavior in solution very different from that in the gas phase. The series formed by MeOH to t-BuOH is a representative example of these series with a gas-phase acidity order (t-BuOH > i-PrOH > EtOH > MeOH) following the greater ability of the methyl group relative to hydrogen to stabilize the charged centers while in solution the final ordering is just the opposite. In this paper, the change in the alcohol's acidity scale when passing from the gas phase to solution is studied. Gas-phase energies are calculated at the MP4 level with the 6-31G* basis set and an sp diffuse function added on the oxygen atom. Calculated deprotonation free energies are in very good agreement with the experimental values. Solvation energies are obtained in the framework of the continuum model using a quantum description of the solute charge distribution. The main contribution to the change in the acidity scale in solution is the electrostatic component of the solvation energy of the basic forms, and the charge delocalization produced by the progressive substitution of hydrogen atoms by methyl groups can be used to understand the order in solution. Our results, which reproduce quite well experimental ordering and magnitude, seem to indicate that a simple electrostatic argument could explain the origin of this inversion and can be employed to rationalize the acid-base behavior of some homologous series.