Carboxilcsoport

Carboxilcsoport, or carboxyl group, is a functional group in organic chemistry with the formula −COOH. It consists of a carbonyl carbon (C=O) bonded to a hydroxyl group (−OH) and attached to a carbon skeleton. The group is acidic: in carboxylic acids, the hydroxyl proton can be donated, producing a carboxylate anion (−COO−). The negative charge in the anion is delocalized over the two oxygens by resonance, which stabilizes the conjugate base.

In most molecules the carboxyl group is written as R−COOH, where R is any carbon-containing substituent. The carbonyl carbon is sp2-hybridized and participates in a range of reactions. The acidity and reactivity can be modulated by substituents on R; electron-withdrawing groups increase acidity. The group is central to many organic transformations and to the properties of the resulting compounds.

Occurrence and importance: carboxyl groups are found in carboxylic acids, amino acids (which carry both a carboxyl and an amino group), fatty acids, and numerous metabolites and drugs. They influence solubility, polarity, and reactivity, enabling key processes such as esterification and amidation.

Reactions and derivatives: carboxylic acids react with alcohols to form esters (esterification), with amines to form amides, and with reducing agents to yield primary alcohols. They can be converted to acyl chlorides with reagents like thionyl chloride, and they can undergo decarboxylation under heat or catalytic conditions.

Physical properties: the group imparts polarity and hydrogen-bonding capacity, contributing to relatively high boiling points for small acids and to water solubility that decreases as the carbon chain length increases. The pKa of typical aliphatic carboxylic acids is around 4–5.