The hydroxyl hydrogen of a carboxylic acid is quite acidic. This results in a negative charge that
remains after the hydrogen is removed and resonance stabilization occurs between both of the
electronegative oxygen atoms. Delocalization of the negative charge results in a very stable
carboxylate anion, which is demonstrated in Figure 8.5.
Figure 8.5. Carboxylate Anion Stability
The negative charge from deprotonation is stabilized through resonance.The more stable the conjugate base is, the easier it is for the proton to leave, and thus, the stronger
the acid. Carboxylic acids are relatively acidic, with pKa values on the order of 4.8 for ethanoic acid
and 4.9 for propanoic acid. However, keep in mind that although these are quite acidic for organic
compounds, they do not compare to strong acids like HCl (pKa = –8.0) or even (pKa = 1.99).
Remember, lower pKa values indicate stronger acids.
Substituents on carbon atoms near a carboxyl group influence anion stability and therefore affect
acidity. Groups like –NO 2 or halides are electron-withdrawing and increase acidity. In contrast, –NH 2
or –OCH 3 are electron-donating groups that destabilize the negative charge, decreasing the acidity
of the compound. The closer the substituent groups are to the carboxyl group, the greater the effect
will be.
In dicarboxylic acids, each –COOH group influences the other –COOH group. Carboxylic acids are
electron-withdrawing due to the electronegative oxygen atoms they contain. The net result is that
dicarboxylic acids are more acidic than the analogous monocarboxylic acids. However, when one
proton is removed from the molecule, the carboxylate anion is formed, resulting in an immediate
decrease in the acidity of the remaining carboxylic acid. This makes sense because if the second
group were deprotonated, it would create a doubly charged species with two negative charges
repelling each other. Due to this instability, the second proton is actually less acidic (harder to
remove) than the analogous proton of a monocarboxylic acid.
β-dicarboxylic acids are dicarboxylic acids in which each carboxylic acid is positioned on the β-
carbon of the other; in other words, there are two carboxylic acids separated by a single carbon.
These compounds are notable for the high acidity of the α-hydrogens located on the carbon