Acidic Nature of Carboxylic Acids

A carboxylic acid, $\text{R-COOH}$, combines a carbonyl group and a hydroxyl group on the same carbon -- and it is precisely this combination, not either group in isolation, that gives carboxylic acids their characteristic and comparatively strong acidity. A carboxylic acid ionises in water,

and how far this equilibrium lies to the right -- how strong an acid the compound is -- depends almost entirely on how well the resulting carboxylate anion, $\text{RCOO}^-$, is able to stabilise the negative charge it is left holding.

Why the carboxylate anion is so well stabilised. The negative charge on a carboxylate ion is not confined to a single oxygen; it is delocalised equally over both oxygens by resonance, since the two carbon-oxygen bonds in the carboxylate become exactly equivalent (each with a bond order of $1.5$, intermediate between a single and a double bond) once the ion forms -- experimentally, the two $\text{C-O}$ bond lengths in a carboxylate are found to be identical, direct evidence for this equal delocalisation. This is a more effective, lower-energy way of spreading out negative charge than the delocalisation available to a phenoxide ion (where, as covered in the Alcohols/Phenols/Ethers chapter, resonance can only push charge onto specific ring carbons — a less electronegative, less charge-accommodating atom than oxygen — and only at the ortho and para positions, not uniformly). An alkoxide ion (the conjugate base of a simple alcohol) has no comparable resonance delocalisation at all -- the negative charge sits entirely on one oxygen with no assistance. …