Carboxylic acid

A carboxylic acid is a member of a homologous series whose molecules contain the carboxyl group, –COOH, and which behaves as a weak acid. Its general formula is CₙH₂ₙ₊₁COOH, and ethanoic acid, CH₃COOH, is the example you meet most often. Malay: asid karboksilik · Chinese: 羧酸.

The –COOH group is what gives the series its acidic character. In water a carboxylic acid ionises only partly, releasing a small proportion of hydrogen ions, H⁺, which is exactly why it is classed as a weak acid rather than a strong one. Even so, it still shows all the typical reactions of an acid: it reacts with a reactive metal to give a salt and hydrogen gas, with a carbonate to give a salt, water and carbon dioxide, and with a base to give a salt and water in neutralisation.

Example. Ethanoic acid is the acid in vinegar. Added to magnesium it fizzes gently as hydrogen is released, and added to sodium carbonate it produces bubbles of carbon dioxide that turn limewater milky. Because it only partly ionises, it fizzes less vigorously than a strong acid of the same concentration would.

Confusion to avoid. Do not mix up a carboxylic acid with an alcohol. Both contain oxygen and both can come from ethanol, but the alcohol has the neutral –OH group while the acid has the acidic –COOH group; only the acid turns blue litmus red. Keep “weak acid” separate from “dilute acid” too, weak refers to partial ionisation, not to concentration.

Carboxylic acids sit at the oxidised end of the chapter’s chemistry: oxidising ethanol produces ethanoic acid, and reacting that acid with an alcohol produces an ester, so this term links the alcohols, the oxidation reaction and esterification together across Carbon Compounds in SPM Chemistry.