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Carboxylic acids are a broad class of organic compounds characterized by the carboxyl group (-COOH) attached to a carbon framework (R-COOH). They are weak acids; in water they donate a proton from the carboxyl group, forming carboxylate anions. Acidity and reactivity vary with the substituent on the R group, but the acidic proton is a defining feature.

The carboxyl group exhibits resonance between two canonical forms, which stabilizes the carboxylate anion. This resonance contributes to relatively high boiling points and predictable reactivity. Short-chain carboxylic acids are typically miscible with water, while longer chains reduce water solubility and increase hydrophobic character.

Common examples include formic acid (HCOOH), acetic acid (CH3COOH), propionic acid, and benzoic acid. They occur naturally in foods and metabolism; fatty acids are components of lipids, while acetic and related acids have industrial and fermentation origins.

Preparation and reactions: industrially, carboxylic acids are produced by oxidation of primary alcohols or aldehydes, or by hydrolysis of nitriles and esters. Reactions include acid-base to form salts, esterification with alcohols to give esters (Fischer esterification), and reduction to primary alcohols. They can be converted to acyl chlorides using reagents such as thionyl chloride, enabling further transformations. Decarboxylation and decarboxylative coupling are also common under specific conditions.

Applications include solvents, flavorings and preservatives, and building blocks for polymers such as polyesters. Safety concerns center on corrosivity and irritation; some carboxylic acids are toxic at high concentrations. Proper handling, ventilation, and protective equipment are advised in industrial and laboratory contexts.