Esters are organic compounds with a distinctive sweet or fruity smell, formed when a carboxylic acid reacts with an alcohol in the presence of an acid catalyst, releasing water. The reaction is reversible and the ester is the desired product. Ethyl ethanoate, made from ethanol and ethanoic acid, is one of the commonest GCSE examples.
What is an ester?
An ester is an organic compound containing the functional group –COO– (the ester linkage), formed by a condensation reaction between a carboxylic acid and an alcohol. The –OH of the acid and the H of the alcohol's –OH combine to form water, which is eliminated as the ester bond forms.
Esters are widely found in nature: fats and oils are esters of glycerol and long-chain fatty acids; many fruit flavours and flower scents arise from naturally occurring esters; and esters are manufactured industrially as solvents, flavourings, plasticisers, and biodiesel.
How is an ester formed?
The reaction is called esterification:
Alcohol + Carboxylic acid ⇌ Ester + Water
Conditions:
- An acid catalyst — concentrated sulfuric acid (H₂SO₄) is most commonly used
- Warming — the reaction is slow at room temperature
- The reaction is reversible (shown by ⇌) — an equilibrium mixture is established
The reverse reaction — breaking an ester back into an alcohol and carboxylic acid using water — is called hydrolysis.
How do you name an ester?
Ester names have two parts:
- The first word comes from the alcohol, with the ending changed from -ol to -yl (e.g. ethanol → ethyl)
- The second word comes from the carboxylic acid, with the ending changed from -oic acid to -oate (e.g. ethanoic acid → ethanoate)
General rule: [alkyl from alcohol] [anoate from acid]
Naming examples:
| Alcohol | Carboxylic acid | Ester name | Smell/use |
|---|---|---|---|
| Methanol | Ethanoic acid | Methyl ethanoate | Solvent, glue |
| Ethanol | Ethanoic acid | Ethyl ethanoate | Solvent, nail polish remover |
| Ethanol | Propanoic acid | Ethyl propanoate | — |
| Propanol | Ethanoic acid | Propyl ethanoate | Pear drops flavouring |
| Methanol | Butanoic acid | Methyl butanoate | Apple flavouring |
How do you write esterification equations?
Worked example — making ethyl ethanoate:
Ethanol + Ethanoic acid → Ethyl ethanoate + Water
CH₃CH₂OH + CH₃COOH ⇌ CH₃COOCH₂CH₃ + H₂O
Or using shorter notation:
C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O
Worked example — making methyl propanoate:
Methanol + Propanoic acid ⇌ Methyl propanoate + Water
CH₃OH + C₂H₅COOH ⇌ C₂H₅COOCH₃ + H₂O
At GCSE, you may be given the names of the alcohol and acid and asked to name the ester, or given the structural formulae and asked to identify which bond formed between the two reactants.
What are the uses of esters?
| Use | Example ester(s) | Why esters are suitable |
|---|---|---|
| Perfumes and cosmetics | Many natural esters, e.g. linalyl acetate | Sweet/fruity aromas; volatile (evaporate readily) |
| Food flavourings | Methyl butanoate (apple), pentyl ethanoate (banana) | Non-toxic (at food concentrations), distinctive fruit smells |
| Solvents | Ethyl ethanoate | Dissolves many organic substances; used in nail polish remover, adhesives, inks |
| Plasticisers | Phthalate esters | Make plastics more flexible without weakening them |
| Biodiesel | Fatty acid methyl esters (FAME) | Made by reacting vegetable oils with methanol; renewable fuel |
How does hydrolysis reverse esterification?
Hydrolysis breaks an ester apart using water (in the presence of acid or alkali catalyst):
Ester + Water → Alcohol + Carboxylic acid (acid hydrolysis)
Ester + Alkali → Alcohol + Carboxylate salt (alkaline hydrolysis, also called saponification)
Saponification is the basis of soap-making: animal fats (esters of glycerol and fatty acids) are treated with hot sodium hydroxide solution, producing glycerol and the sodium salts of fatty acids (soaps).
At GCSE, you need to know that hydrolysis is the reverse of esterification and produces an alcohol and an acid (or its salt with alkali).
How do you predict the products of esterification?
A reliable method:
- Identify the alcohol — note the alkyl group (the part without the –OH).
- Identify the carboxylic acid — note the acyl group (the part without the –OH).
- The ester is formed by joining these two groups through the –COO– linkage.
- Water is always the other product.
The bond that forms is between the carbonyl carbon of the acid and the oxygen of the alcohol. The bond that breaks is the O–H of the acid and the O–H of the alcohol — the two oxygens end up on opposite sides of the ester linkage.
Frequently asked questions
Why does the esterification reaction not go to completion?
Esterification is a reversible reaction. As the ester and water form, they can react with each other in the reverse direction (hydrolysis) to regenerate the alcohol and carboxylic acid. The system reaches a dynamic equilibrium at which the forward and reverse rates are equal. To obtain more ester, you can remove it as it forms (e.g. by distillation), add excess of one of the reactants, or remove water (e.g. by adding a drying agent).
How do I tell an ester name from an acid or alcohol name in GCSE exams?
Esters always have a two-word name where the first word ends in -yl (e.g. methyl, ethyl, propyl) and the second word ends in -oate (e.g. methanoate, ethanoate, propanoate). Alcohols end in -ol (ethanol, propanol) and carboxylic acids end in -oic acid (ethanoic acid, propanoic acid). If you see a compound with "-yl" and "-oate" in its name, it is an ester.
Why are esters used as solvents in the chemical industry?
Esters such as ethyl ethanoate are effective solvents because they can dissolve a wide range of organic substances, including resins, varnishes, fats, and many polymers. They evaporate readily at room temperature (they are volatile), leaving behind the dissolved material as it dries. They are also less toxic than many traditional solvents and are therefore preferred in paints, varnishes, nail polish removers, and adhesives where the solvent needs to evaporate in an enclosed or domestic setting.
What is the structural difference between an ester and a carboxylic acid?
Both contain a –COO– group but arranged differently. A carboxylic acid has –COOH (a carbonyl C=O and a hydroxyl –OH on the same carbon). An ester has –COO– (the carbonyl carbon is bonded to an oxygen that is itself bonded to a carbon of the alkyl group from the alcohol). The key distinction is that in an ester there is no free –OH on the carbonyl carbon. This means esters cannot hydrogen-bond in the same way as acids, making them more volatile and giving them lower boiling points than the corresponding carboxylic acid.
For Socratic GCSE chemistry with Professor Curie — tracing ester formation from functional groups to industrial applications at the particle level — visit aitutors.me.