carboxylryhmää

The carboxyl group is a common functional group in organic chemistry, written as -COOH. It consists of a carbonyl group (C=O) bonded to a hydroxyl group (–OH) on the same carbon. The carbon is sp2-hybridized and the group is typically planar. Resonance between two contributing structures gives the carboxyl group partial double-bond character and a tendency to exist as either the neutral carboxylic acid or as the deprotonated carboxylate (-COO−) anion.

In aqueous solutions, simple carboxylic acids are weak acids with a typical pKa around 4–5, meaning they donate a proton to water to form carboxylate and hydronium. Electron-withdrawing substituents increase acidity, while electron-donating groups decrease it. In biological contexts, carboxyl groups are often deprotonated at physiological pH, forming negatively charged carboxylate groups that participate in salt bridges and metal binding.

Occurrence and significance: carboxyl groups occur widely in chemistry and biology, for example in fatty acids, nucleotides, and the amino and terminal groups of amino acids. They are essential in metabolism and in biochemical pathways. Carboxyl groups define many derivatives, including esters, anhydrides, and amides, by reaction with alcohols, carboxylic acid derivatives, or amines.

Common reactions: esterification converts -COOH to esters using alcohols (usually with acid catalysis). Amide formation occurs by coupling with amines, often via activated derivatives. Decarboxylation is the loss of CO2 and occurs in certain β-keto acids and related systems. Reduction can convert carboxyl groups to primary alcohols, using strong reducing agents.