What does carboxyl mean?
Carboxyl is the name given to the univalent chemical group –COOH, made up of a carbonyl (C=O) and a hydroxyl (–OH) sharing the same carbon atom. It is the defining feature of carboxylic acids such as acetic acid (the acid in vinegar) and citric acid, and it also appears in every amino acid, where it reacts with an amino group to form the peptide bonds that build proteins. Because the group readily releases its hydrogen ion, it gives these molecules their acidic character. The word belongs firmly to the vocabulary of chemistry, appearing most often in scientific texts as 'the carboxyl group' rather than alone. Understanding it unlocks a large family of everyday substances, from fats and soaps to fruit acids and pharmaceuticals, all unified by this small but chemically powerful structure.
nounA univalent chemical group, –COOH, consisting of a carbonyl group attached to a hydroxyl group; it characterises organic acids known as carboxylic acids.
- The univalent functional group –COOH that defines carboxylic acids.
"The amino acid glycine contains both an amino group and a carboxyl group."
"Acetic acid owes its acidity to the single carboxyl group attached to its methyl group."
"In peptide synthesis, chemists couple the carboxyl of one amino acid to the amino group of another to form a peptide bond."
Rarely pluralised in practice; chemists typically refer to individual carboxyl groups within a molecule rather than counting them as separate nouns.
"Citric acid's structure contains three carboxyl groups."
Every vinegar molecule, every amino acid in your body — and every fatty acid in butter — carries the same tiny signature: one carboxyl group.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of carboxyl
Carboxyl was coined in nineteenth-century organic chemistry from 'carbon' combined with 'oxyl', a shortened form of 'oxygen', reflecting the group's two constituent elements. It was formed on the model of related terms like 'hydroxyl' and 'carbonyl'. Its derivatives include the adjective 'carboxylic' and the noun 'carboxylate', denoting salts and esters of carboxylic acids.
Related word forms
How carboxyl is actually used
Strictly a technical term of chemistry, used almost exclusively in scientific writing and education. It usually appears with 'group' when naming the moiety explicitly.
Easily confused with carboxyl
A carbonyl group is only C=O, whereas a carboxyl group is a carbonyl bonded to a hydroxyl (–COOH).
A hydroxyl group is just –OH attached to a carbon or other atom, while a carboxyl group additionally includes the carbonyl carbon.