13.3 : 羧酸的酸度

Carboxylic acids exhibit significant acidity in the presence of bases, forming carboxylate anions.

The pK_a values of most carboxylic acids—ranging from 4-5—are much lower compared to other compounds such as alcohols, that have pK_a values around 16-18.

The lower pK_a or higher acidity of carboxylic acids results from the higher stability of its corresponding carboxylate anion.

The electron-withdrawing –C=O group exerts an inductive effect through the C–O σ bond, stabilizing the anion.

Furthermore, the negative charge on the carboxylate oxygen is delocalized over the –C=O oxygen, forming equivalent resonance structures with the resonance hybrid having a total negative charge dispersed over two oxygen atoms.

The two equivalent C–O bonds of the carboxylate ion have one-and-a-half bond order, indicating delocalization of charge over both oxygens.

In contrast, the conjugate-base of alcohols—the alkoxide ion—has no resonance structure, which leads to decreased stability, in turn making carboxylic acids more acidic than alcohols.