Carboxylic acids are characterized by the carboxyl group (-COOH), which combines a carbonyl and a hydroxyl moiety. The acidity of carboxylic acids (pKa ~ 4–5) arises from resonance stabilization of the conjugate base (carboxylate ion), where the negative charge is delocalized over both oxygen atoms. Deprotonation by a base such as NaOH or NaHCO₃ generates the carboxylate salt, which is water-soluble and serves as a handle for purification.
Structure and Acidity Trends
The stability of the carboxylate anion makes carboxylic acids substantially more acidic than alcohols (pKa ~ 16) or phenols (pKa ~ 10). Electron-withdrawing groups (EWGs) such as -NO₂, -Cl, or -CF₃ at the α-position increase acidity by stabilizing the conjugate base through inductive effects. Conversely, electron-donating groups reduce acidity. The inductive effect diminishes with distance: a chloro substituent at the α-position raises acidity more than at the β- or γ-position. Aromatic carboxylic acids (e.g., benzoic acid, pKa ~ 4.2) follow similar substituent effects governed by Hammett σ constants.
Synthesis of Carboxylic Acids
Oxidation of primary alcohols or aldehydes with strong oxidants (KMnO₄, K₂Cr₂O₇, CrO₃/H₂SO₄) yields carboxylic acids. Hydrolysis of nitriles (RCN) under acidic or basic conditions provides another route, proceeding through the amide intermediate. Grignard reagents react with CO₂ (dry ice) to form carboxylic acids after acidic workup — a one-carbon homologation. Alkylbenzenes with a benzylic hydrogen undergo oxidation with KMnO₄ or Na₂Cr₂O₇ to benzoic acid derivatives.
Physical Properties
Carboxylic acids form strong intermolecular hydrogen bonds, existing as cyclic dimers in the gas phase and in nonpolar solvents. This dimerization elevates their boiling points relative to alcohols and aldehydes of comparable molecular weight. Low-molecular-weight carboxylic acids (formic, acetic, propionic) are miscible with water due to hydrogen bonding with water, while longer alkyl chains decrease aqueous solubility.
Reactions of Carboxylic Acids
The hydroxyl group of the carboxyl can be replaced by nucleophiles through acyl substitution. Thionyl chloride (SOCl₂) or PCl₅ converts carboxylic acids to acyl chlorides, which are much more reactive toward nucleophiles. Fischer esterification (RCOOH + R’OH, H₂SO₄ catalyst) produces esters in an equilibrium process driven by removal of water. Direct amidation typically requires activation of the carboxyl group (via acyl chloride or coupling reagent). Anhydrides are formed by dehydration of two carboxylic acid molecules, either thermally or via acyl chloride-carboxylate coupling.
Decarboxylation and Reduction
β-Keto acids and malonic acid derivatives undergo thermal decarboxylation via a cyclic six-membered transition state, releasing CO₂. This reaction is exploited in the malonic ester synthesis for alkylation and subsequent decarboxylation to substituted acetic acids. Reduction of carboxylic acids to primary alcohols requires strong reducing agents such as LiAlH₄ or borane (BH₃·THF), as NaBH₄ is insufficient. Borane offers the advantage of selectively reducing the carboxyl group in the presence of other reducible functional groups.
