Carboxylic acids and estersAQA A-Level Chemistry: Revision notes

A carboxylic acid contains the carboxyl group, –COOH (a C=O and an OH on the same carbon), for example ethanoic acid, CH₃COOH. An ester contains –COO– with a carbon group on each side, RCOOR′, for example ethyl ethanoate, CH₃COOC₂H₅.

Name an ester from the alcohol first, then the acid: the alkyl group from the alcohol and the –oate from the acid. Methyl butanoate, CH₃CH₂CH₂COOCH₃, is made from methanol and butanoic acid.

Carboxylic acids are weak acids: they only partly dissociate in water, RCOOH ⇌ RCOO⁻ + H⁺. They are still strong enough to react with carbonates and liberate carbon dioxide, which is the standard test:

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Observation: effervescence (bubbles of gas that turn limewater milky). Alcohols do not do this, so the test distinguishes carboxylic acids from alcohols.

A carboxylic acid and an alcohol react, in the presence of an acid catalyst (concentrated sulfuric acid), to form an ester and water. The reaction is reversible:

RCOOH + R′OH ⇌ RCOOR′ + H₂O

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl ethanoate)

Esters are used as solvents, plasticisers, perfumes and food flavourings, because many are volatile with pleasant smells.

Esters are hydrolysed (split by water) by heating with acid or with alkali.

Acidic hydrolysis (heat with dilute sulfuric or hydrochloric acid): a reversible reaction giving a carboxylic acid and an alcohol. CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH

Alkaline hydrolysis (heat with aqueous sodium hydroxide): goes to completion, giving the salt of the carboxylic acid and the alcohol. CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

Adding dilute acid to the salt then gives the free carboxylic acid.

Vegetable oils and animal fats are esters of propane-1,2,3-triol (glycerol) with three long-chain carboxylic acids. Heating them with aqueous sodium hydroxide hydrolyses the ester links, giving glycerol and the sodium salts of the long-chain carboxylic acids, which are soap.

(C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃

Worked example: 8.90 g of glyceryl tristearate (Mr = 890) is 0.0100 mol. The ratio is 1 : 3, so 0.0300 mol sodium stearate (Mr = 306) forms, a mass of 0.0300 × 306 = 9.18 g.

Biodiesel is a mixture of methyl esters of long-chain carboxylic acids. It is produced by reacting vegetable oils with methanol in the presence of a catalyst. The other product is propane-1,2,3-triol (glycerol).

The reaction replaces the glycerol part of each ester with methyl groups, giving smaller ester molecules that can be used as a fuel. This differs from hydrolysis with sodium hydroxide, which gives soap.