Carboxylic acid: compound containing the carboxyl group -COOH, general formula RCOOH. Weaker acid than mineral acids but still gives H⁺ in water. Carbonyl compounds contain C=O.
Ester: sweet-smelling compound from an acid and an alcohol, general formula RCOOR′, containing the -COO- group. Esterification: carboxylic acid + alcohol ⇌ ester + water with conc. H₂SO₄ as catalyst (a condensation reaction, since a small molecule, water, is lost).
Formation of ethanoic acid
Chemical oxidation: ethanol is oxidised by acidified aqueous potassium manganate(VII) (or potassium dichromate). CH₃CH₂OH + [O] → CH₃CHO (ethanal) + H₂O, then CH₃CHO + [O] → CH₃COOH. Overall CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. Fast, but needs a strong oxidising agent. The purple permanganate is decolourised. KMnO₄ is the oxidising agent.
Bacterial oxidation: acetobacter bacteria (enzymes) oxidise ethanol in air (aerobic) to ethanoic acid, giving vinegar (4 to 8% acetic acid, about 5% commonly). Slow, but uses renewable resources and is environmentally friendly.
Reactions of ethanoic acid
With a base: CH₃COOH + NaOH → CH₃COONa + H₂O (sodium ethanoate, salt and water).
With a carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ (effervescence). With a bicarbonate: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. CO₂ turns limewater milky.
With an active metal: 2CH₃COOH + 2Na → 2CH₃COONa + H₂ (also Mg, Ca, K). Propanoic acid with sodium gives sodium propanoate and hydrogen. Methanoic acid with NaHCO₃ gives sodium methanoate, water and CO₂.
Acids: methanoic acid in textile industry; ethanoic acid for plastics, esters and aspirin; vinegar for pickles and salad; sodium salts of fatty acids for soap and detergents; citric acid and benzoic acid (sodium benzoate) as food preservatives; tartaric acid in baking powder; ascorbic acid is vitamin C; solvents; perfumes and food essences.
Esters: flavouring and fragrance (isopentyl acetate banana, octyl acetate orange, ethyl butyrate pineapple, methyl butyrate apple, isobutyl formate raspberry), solvents for dyes, glues, inks and nail polish remover, car paints and varnishes, polyesters for bottles and clothing fibres, cosmetics (soften and smooth skin), aspirin (an ester) as fever reducer and painkiller.
Past-paper questions with model answers
What is esterification? Give an example having a balanced equation (B, 1x3 and 2026 M, 1+2): reaction of a carboxylic acid with an alcohol in the presence of conc. H₂SO₄ giving an ester and water. Example CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O.
Balanced chemical reaction between ethanoic acid and methanol, name the products (D, 1+2): same as above; methyl ethanoate and water.
General word equation for esterification with an example (M): carboxylic acid + alcohol → ester + water.
2026 Section A MCQ: C₂H₅OH + 2[O] with KMnO₄/H₂SO₄ gives CH₃COOH. 2025: functional group in carboxylic acids is -COOH; vinegar turns blue litmus red.