Acidity of carboxylic acids: resonance delocalization or induction?

Abstract

On the basis of calculated acidities of the vinylogues of formic acid and vinyl alcohol, Dewar and Krull have concluded that the higher acidity of carboxylic acids relative to alcohols is due to stabilization of the conjugate anion by resonance delocalization. More detailed calculations on some of the compounds that they have considered shows indeed that delocalization of charge in the anion plays a role in the acidity of buta-1,3-dien-1-ol 1 relative to vinyl alcohol and of 3-hydroxyprop-2-enal 2 relative to formic acid. However, these calculations show that this conclusion is not applicable to the relative acidities of ethanol, vinyl alcohol and formic acid or of 2 relative to 1. These are due to the charge distribution in the neutral molecule, in keeping with the ideas proposed by Siggel et al.