ROLE OF PI-ELECTRON DELOCALIZATION IN THE ENHANCED ACIDITY OF CARBOXYLIC-ACIDS AND ENOLS RELATIVE TO ALCOHOLS

The relative importance of pi-electron delocalization and induction to the enhanced acidity of carboxylic acids and enols compared to alcohols is examined. These generic classes of molecules are represented by formic acid, vinyl alcohol, and ethanol, respectively. An ab initio valence bond method is applied to calculate the energetic effect of pi -delocalization in a direct way. The calculations are performed at three different levels of theory. The results are found to be insensitive to the degree of electron correlation. It is concluded that the major geometry changes that the carboxylic acid and enol undergo by loosing their acidic proton are partly-not entirely-explained by pi-delocalization. Both inductive effects and pi-delocalization are found to contribute, with equal importance, to the acidity of the carboxylic acid. The effect of pi-delocalization is predominant in the enol. The recent controversy between the traditional view on the enhanced acidity, entirely based on resonance theory, and a new challenging view, which primarily invokes inductive effects, is discussed.