Pergamon 0040-4020(95)00143-3 Tetrahedron Vol. 51, No. 14, pp. 4021.-4032, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0040-4020/95 $9.50+0.00 Acidity of Alkyl Substituted Alcohols : Are Alkyl Groups Electron-donating or Electron-withdrawing ? F. De Proft, W. Langenaeker and P. Geerlings* Eenheid Algemene Chemie (ALGC),FaculteitWetenschappen, Vrije Universiteit Brussel, Pleinlaan2, 1050 Brussels,Belgium. Abstract : Two important functional group properties, the group electronegativity and hardness, are calculated for a number of alkyl groups. The results indicate that alkyl groups become less electronegativeand hard with increasinggroup size. The calculatedproperties are used in a study of the inversion of the alkyl alcohol acidity scale when going from aqueous solution to the gas phase. Finally, the Sanderson electronegativityequalization principle using functional group properties is shown to be a valuabletool in describing the charge distribution of these molecularsystems. INTRODUCTION In describing the effects of structure on chemical reactivity, organic chemists like to use functional group properties (substituent constants) to predict charge transfer between different regions of a molecule. Many of the derived substituent constants result from acid-base equilibria in solution, thus being a measure of the intrinsic properties of the functional group modulated by the solvent. When the acid-base equilibrium is studied in the gas-phase however, the observed acidity and basicity sequences can be traced back to the intrinsic properties of the molecule. In the gas-phase, a number of well known and "traditional" solution acidity or basicity sequences are inverted 1-5. Most of these involve alkyl substituted molecules ; well known examples are the acidities of alkyl substituted alcohols or carboxylic acids and the basicities of alkyl amines. In aqueous solution, the acidity or basicity of these systems decreases with increasing alkyl group size : this finding is in accordance with the traditional view of alkyl groups as being electron-releasing. In the gas-phase, the opposite trend occurs, thus giving the impression that alkyl groups act as electron-withdrawing species. In the past, a number of studies (theoretical and experimental) were dedicated to this apparant anomaly between the behaviour of alkyl groups in solution and in the gas-phase 6-20. The attention has been focussed mainly on the basicity of the alkyl amines and considerably less studies were devoted to the case of acidity, e.g. the carboxylic acid or alcohol acidity scales. Some differences can however be expected to exist between 4021