Karboksyyl

Karboksyyl, commonly referred to in English as the 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) on the same carbon. This arrangement gives the group distinctive acidity and reactivity, and it is the defining feature of carboxylic acids.

Structure and properties: The carboxyl carbon is double-bonded to oxygen and single-bonded to hydroxyl; resonance between forms distributes negative charge in the carboxylate anion when the group is deprotonated. In water, many carboxylic acids are weak acids, with pKa values typically around 4–5 for simple examples. The group is highly polar and capable of hydrogen bonding, which enhances solubility for small carboxylic acids, while larger hydrophobic substituents reduce overall solubility.

Reactions: The carboxyl group participates in esterification with alcohols to form esters and water, and in amidation with amines to form amides. It can be converted to acid chlorides or anhydrides using appropriate reagents. Deprotonation yields carboxylate salts (RCOO−). Decarboxylation, the loss of CO2, occurs under certain heating conditions or in specific substrate contexts (e.g., β-keto acids).

Nomenclature and examples: When the carboxyl group is the highest-priority functional group, the parent compound is named as a carboxylic acid (suffix -carboxylic acid). Examples include acetic acid (ethanoic acid), benzoic acid, and formic acid.

Biological and industrial relevance: Carboxyl groups are ubiquitous in biomolecules such as amino acids and fatty acids, and they play a central role in polymer chemistry, catalysis, and organic synthesis.