Hydrogen Bonding

The presence of a hydrogen bond donor near a carboxyl group may act to enhance its acidity, as demonstrated by the three isomeric hydroxybenzoic acids and three isomeric benzenedicarboxylic acids (phthalic acids) shown in the following table. Compared with benzoic acid, the meta and para-isomers display expected changes in acidity, due to combined inductive and resonance effects. However, the ortho isomers are both roughly 15 times more acidic, even though the hydroxyl and carboxyl substituents have opposite influences in the para-location.

Intramolecular hydrogen bonding of an ortho OH donor to the carbonyl oxygen of the carboxyl group, acting as an acceptor, increases the positive charge on the carbonyl carbon and consequently the acidity of the carboxyl OH. This is illustrated for salicylic acid in the following diagram. Phthalic acid engages in a similar seven-membered cyclic hydrogen bond with a similar outcome. This intramolecular hydrogen bonding also explains the decreased acidity of the remaining acidic function - that is the phenolic OH in salicylic acid and the second carboxyl group in phthalic acid.

The stereoisomeric 2-butenedioic acids, maleic and fumaric acid display a similar behavior. The cis isomer, maleic acid, has pK_a¹ = 2.00 and pK_a² = 6.50. This contrasts with the values for the trans-isomer, fumaric acid, pK_a¹ = 3.00 and pK_a² = 4.50.