Overview
Carboxylic acids are the strongest organic acids. However, their acidic strength is much less than mineral acids like HCl. Carboxylic acids ionize in water and readily lose the hydroxyl proton to form a resonance-stabilized carboxylate ion.
The acid dissociation constant (Ka) or pKa value indicates the extent of ionization, reflecting the moderate acidic strength of carboxylic acids. For simple carboxylic acids, the Ka values are around 10−5, and the pKa values are in the range of 4–5. In comparison, alcohols and phenol are less acidic than carboxylic acids.
The higher acidity of carboxylic acids is due to the higher stability of its conjugate base—the carboxylate anion. In the carboxylate anion, the negative charge on the carboxylate oxygen is reduced by the electron-withdrawing effect of the carbonyl group, thereby stabilizing the anion. However, no such inductive effect is observed for alkoxides, resulting in their lower stability and acidity.
As illustrated below, in the resonance stabilization of acetate ions, the negative charge is delocalized on both of the oxygen atoms instead of residing on one, as in the case of ethoxide ions. In the case of phenoxide, the charge delocalization occurs on one electronegative oxygen atom and three carbon atoms instead of two electronegative oxygen atoms. Thus, phenols are less acidic than carboxylic acids but more acidic than alcohols.
Procedure
Carboxylic acids exhibit significant acidity in the presence of bases, forming carboxylate anions.
The pKa values of most carboxylic acids—ranging from 4-5—are much lower compared to other compounds such as alcohols, that have pKa values around 16-18.
The lower pKa 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.