Esters: core idea

· Esters are organic compounds containing the ester functional group, written generally as RCOOR′.
· Esters are closely linked to carboxylic acids and alcohols because they can be made from them and hydrolysed back into related products.
· Key CIE reaction types: condensation, esterification, and hydrolysis.
· General ester formula: RCOOR′, where R and R′ are alkyl groups, e.g. CH₃COOCH₂CH₃.
· Exam focus: know the reagents, conditions, products, and how to write balanced organic equations.

This diagram shows the overall esterification reaction. A carboxylic acid reacts with an alcohol to form an ester and water. This is the key formation reaction required for CIE 18.2. Source

Making esters: condensation / esterification

· Esters are produced by a condensation reaction between a carboxylic acid and an alcohol.
· Required reagent/condition: concentrated H₂SO₄ catalyst.
· General equation: RCOOH + R′OH ⇌ RCOOR′ + H₂O.
· This is called esterification because an ester is formed.
· It is a condensation reaction because water is eliminated.
· Example: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O.
· Product example: ethanoic acid + ethanol → ethyl ethanoate + water.
· Exam tip: the –OH from the carboxylic acid and H from the alcohol combine to form H₂O.

This diagram shows that ester formation and acid hydrolysis are reversible processes. In esterification, a carboxylic acid + alcohol form an ester + water. In hydrolysis, the ester is broken back down. Source

Hydrolysis of esters: dilute acid

· Hydrolysis means breaking a compound using water.
· Esters undergo acid hydrolysis using dilute acid and heat.
· General equation: RCOOR′ + H₂O ⇌ RCOOH + R′OH.
· Products: carboxylic acid and alcohol.
· Conditions: dilute acid, usually dilute H₂SO₄ or dilute HCl, and heat.
· Acid hydrolysis is usually shown as reversible.
· Example: CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH.
· Product example: ethyl ethanoate → ethanoic acid + ethanol.

Hydrolysis of esters: dilute alkali

· Esters undergo alkaline hydrolysis using dilute alkali and heat.
· Common alkali: NaOH(aq).
· General equation: RCOOR′ + NaOH → RCOO⁻Na⁺ + R′OH.
· Products: carboxylate salt and alcohol.
· Example: CH₃COOCH₂CH₃ + NaOH → CH₃COO⁻Na⁺ + CH₃CH₂OH.
· Product example: ethyl ethanoate + sodium hydroxide → sodium ethanoate + ethanol.
· Exam tip: with alkali, do not write the carboxylic acid as the direct product; write the carboxylate salt.
· If acid is later added, the carboxylate ion can be converted into the carboxylic acid.

This diagram shows ester hydrolysis under basic conditions. The ester bond is broken to form an alcohol and a carboxylate salt. This matches the CIE requirement to describe ester hydrolysis by dilute alkali and heat. Source

Acid vs alkali hydrolysis

· Dilute acid + heat: ester → carboxylic acid + alcohol.
· Dilute alkali + heat: ester → carboxylate salt + alcohol.
· Acid hydrolysis equation uses H₂O as a reactant.
· Alkaline hydrolysis equation uses OH⁻ or NaOH as a reactant.
· Key exam distinction: acid hydrolysis gives RCOOH, but alkaline hydrolysis gives RCOO⁻Na⁺.

Common exam equation patterns

· Formation: carboxylic acid + alcohol ⇌ ester + water.
· Formation with structures: RCOOH + R′OH ⇌ RCOOR′ + H₂O.
· Acid hydrolysis: RCOOR′ + H₂O ⇌ RCOOH + R′OH.
· Alkaline hydrolysis: RCOOR′ + NaOH → RCOO⁻Na⁺ + R′OH.
· Always check whether the question says dilute acid or dilute alkali before writing products.

Practical and exam wording

· Concentrated H₂SO₄ is used as a catalyst in ester formation.
· Heat is required for ester hydrolysis.
· Condensation means a small molecule, here water, is removed.
· Hydrolysis means a bond is broken using water or aqueous conditions.
· Esters are often tested through reaction routes, especially linking alcohols, carboxylic acids, and esters.