Carboxylic acids: core reactions

· Carboxylic acids contain the –COOH / carboxyl group and behave as weak acids in organic reactions.
· General formula: RCOOH.
· In equations, carboxylic acids often form carboxylate salts, written as RCOO⁻ M⁺.
· Key exam focus: know how carboxylic acids are made, their acid reactions, esterification, and reduction.

The diagram shows the carboxyl group, –COOH, which defines carboxylic acids. It contains both a carbonyl C=O and a hydroxyl O–H group attached to the same carbon atom. Source

Making carboxylic acids

· Primary alcohol → carboxylic acid by oxidation.
· Reagents: acidified K₂Cr₂O₇ or acidified KMnO₄.
· Conditions: refluxing to ensure full oxidation.
· Example: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.
· Aldehyde → carboxylic acid by oxidation using acidified K₂Cr₂O₇ / KMnO₄ and refluxing.
· Example: CH₃CHO + [O] → CH₃COOH.
· Nitrile → carboxylic acid by hydrolysis with dilute acid or dilute alkali, then acidification.
· Example: CH₃CN → CH₃COOH after hydrolysis and acidification.
· Ester → carboxylic acid by hydrolysis with dilute acid or dilute alkali and heat, followed by acidification if alkali is used.
· Exam trap: alkaline hydrolysis gives a carboxylate salt first, so acidification is needed to obtain the carboxylic acid.

Acid reactions of carboxylic acids

· With reactive metals: carboxylic acid → salt + hydrogen gas.
· General equation: 2RCOOH + 2Na → 2RCOONa + H₂.
· Observation: effervescence due to H₂(g).
· With alkalis: carboxylic acid + alkali → salt + water.
· General equation: RCOOH + NaOH → RCOONa + H₂O.
· With carbonates: carboxylic acid + carbonate → salt + water + carbon dioxide.
· General equation: 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂.
· Observation: effervescence due to CO₂(g); CO₂ turns limewater milky.

Esterification

· Carboxylic acid + alcohol ⇌ ester + water.
· Conditions: concentrated H₂SO₄ catalyst and heat / reflux.
· General equation: RCOOH + R′OH ⇌ RCOOR′ + H₂O.
· Example: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O.
· Product is an ester, often with a sweet / fruity smell.
· Exam wording: esterification is a condensation reaction because water is eliminated.
· The reaction is reversible, so exam questions may also link this to ester hydrolysis.

This mechanism diagram shows how a carboxylic acid reacts with an alcohol under acidic conditions to form an ester. For CIE 18.1, focus on the overall reaction, reagents, and conditions rather than memorising the full mechanism unless required by your teacher. Source

Reduction of carboxylic acids

· Carboxylic acids can be reduced to primary alcohols.
· Reagent: LiAlH₄.
· General equation: RCOOH + 4[H] → RCH₂OH + H₂O.
· Example: CH₃COOH → CH₃CH₂OH.
· Exam trap: NaBH₄ does not reduce carboxylic acids under normal syllabus conditions; use LiAlH₄.

Reaction summary: learn this route map

· Primary alcohol → aldehyde → carboxylic acid using acidified K₂Cr₂O₇ / KMnO₄.
· Primary alcohol → carboxylic acid directly if heated under reflux with excess oxidising agent.
· Nitrile → carboxylic acid by hydrolysis, then acidification.
· Ester → carboxylic acid by hydrolysis, then acidification if alkaline conditions are used.
· Carboxylic acid → ester with alcohol + concentrated H₂SO₄.
· Carboxylic acid → primary alcohol with LiAlH₄.
· Carboxylic acid → salt with metal / alkali / carbonate.

Exam equation patterns

· Metal reaction: 2CH₃COOH + 2Na → 2CH₃COONa + H₂.
· Neutralisation: CH₃COOH + NaOH → CH₃COONa + H₂O.
· Carbonate reaction: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.
· Esterification: CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O.
· Reduction: CH₃COOH + 4[H] → CH₃CH₂OH + H₂O.

The diagram shows the partial dissociation of a carboxylic acid to form a carboxylate ion. This explains why carboxylic acids react with metals, alkalis, and carbonates to form salts. Source