Last updated: 24 August 2026
Esters are often introduced as products of a carboxylic acid reacting with an alcohol, but that simple description is only the beginning. The more useful A-Level idea is to see an ester as an acyl group joined through oxygen to another carbon group. Once you recognise that pattern, both ester formation and ester hydrolysis become much easier to predict.
The two syllabus examples, ethyl ethanoate and phenyl benzoate, are chosen for a reason. Ethyl ethanoate shows the familiar reversible esterification of a carboxylic acid with an alcohol, while phenyl benzoate shows why a more reactive acyl chloride is useful when the oxygen nucleophile is phenol. Hydrolysis then runs the chemistry in the opposite direction, with acidic and alkaline conditions giving importantly different products.
Try these five questions before reading the chapter. Focus on the –COO– functional group, how esters are formed and how acidic and alkaline hydrolysis differ.
- 5 diagnostic questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
What Is an Ester?
An ester is a carboxylic acid derivative containing the functional group –COO–. A useful general representation is R–C(=O)–O–R′, where the group attached directly to the carbonyl carbon comes from the carboxylic acid side and the group attached through oxygen comes from the alcohol or phenol side.
In the A-Level syllabus, the two key examples are ethyl ethanoate and phenyl benzoate. They contain the same ester linkage but are commonly prepared by different routes, which makes them useful for understanding how the reactivity of the starting compounds controls the synthetic method.
Key principle: an ester contains the linkage R–C(=O)–O–R′. When you analyse an ester, first locate the C=O bond and then identify the group attached through the single-bonded oxygen.
Recognising the two sides of an ester
The carbonyl-containing side corresponds to the carboxylic acid from which the ester can be considered to be derived. The group attached to the single-bonded oxygen corresponds to the alcohol or phenol component. Seeing these two fragments makes both naming and hydrolysis much easier.
Ethyl ethanoate, CH₃COOCH₂CH₃: the CH₃COO– part corresponds to ethanoic acid, while the –CH₂CH₃ group attached through oxygen corresponds to ethanol.
Phenyl benzoate, C₆H₅COOC₆H₅: the C₆H₅COO– part corresponds to benzoic acid, while the phenyl group attached through oxygen corresponds to phenol.
Naming simple esters
Ester names are written in two parts. The first part names the alkyl or aryl group attached to the single-bonded oxygen. The second part is the carboxylate name derived from the acid.
For ethyl ethanoate, “ethyl” comes from ethanol and “ethanoate” comes from ethanoic acid. For phenyl benzoate, “phenyl” comes from phenol and “benzoate” comes from benzoic acid.
Forming Esters from Carboxylic Acids and Alcohols
A carboxylic acid reacts with an alcohol to form an ester and water. This is a condensation reaction because two molecules join and a small molecule, water, is eliminated. It is also called esterification.
For the syllabus example ethyl ethanoate, ethanoic acid reacts with ethanol:
Reagents and conditions
The standard conditions are concentrated H₂SO₄ as an acid catalyst and heat, commonly under reflux. Reflux allows the mixture to be heated for an extended period without continuously losing volatile reactants or products.
| Feature | What to state | Why it matters |
|---|---|---|
| Reagent | Carboxylic acid + alcohol | These provide the two fragments of the ester. |
| Catalyst | Concentrated H₂SO₄ | Provides acidic catalysis for esterification. |
| Condition | Heat, usually under reflux | Raises the reaction rate while retaining volatile substances. |
Why the equilibrium arrow matters
Esterification is reversible. The ester and water formed can react in the reverse direction to regenerate the carboxylic acid and alcohol. This means heating makes equilibrium establish faster, but it does not by itself force complete conversion to ester.
Exam point: do not replace the equilibrium arrow with a one-way arrow for carboxylic acid–alcohol esterification. The reversibility is central to understanding both ester formation and acid-catalysed hydrolysis.
Forming Esters from Acyl Chlorides
Acyl chlorides are much more reactive than carboxylic acids. They react readily with alcohols and phenols to form esters, usually at room temperature. In the simplest equation, the other product is HCl.
The syllabus example is the preparation of phenyl benzoate from benzoyl chloride and phenol:
Why phenyl benzoate is prepared from an acyl chloride
Phenol is a weaker nucleophile than a typical aliphatic alcohol because one of the oxygen lone pairs can be delocalised into the benzene ring. As a result, phenol does not readily form an ester by direct reaction with a carboxylic acid. The much more reactive benzoyl chloride provides a suitable acylating agent.
Ethyl ethanoate: ethanoic acid + ethanol, concentrated H₂SO₄, heat. This route is reversible and produces water.
Phenyl benzoate: benzoyl chloride + phenol, typically at room temperature. The direct acyl chloride reaction is rapid and produces HCl.
What if NaOH is present during benzoylation?
In some laboratory preparations, phenol is first treated with NaOH to form phenoxide, which is a stronger nucleophile. The base also neutralises the HCl produced. In that case, an overall equation can be written as:
For syllabus questions, follow the reagents actually given. The essential chemistry is still acylation of phenol by benzoyl chloride to form phenyl benzoate.
Acid Hydrolysis of Esters
Hydrolysis breaks the ester linkage using water. Under acidic conditions, the reaction is essentially the reverse of acid-catalysed esterification. Use aqueous acid, such as dilute H₂SO₄ or dilute HCl, together with heat, commonly under reflux.
For ethyl ethanoate:
For phenyl benzoate:
Key principle: acid hydrolysis is reversible. The products are the corresponding carboxylic acid and alcohol or phenol, and the system can establish an equilibrium.
Base Hydrolysis of Esters
Esters can also be hydrolysed by heating with aqueous NaOH or KOH. This reaction is often called alkaline hydrolysis or saponification. The crucial difference from acid hydrolysis is that the acid-derived product is present as a carboxylate salt.
For ethyl ethanoate:
Why alkaline hydrolysis is effectively irreversible
The carboxylic acid that would be produced by hydrolysis is immediately deprotonated in strongly alkaline solution to form a carboxylate ion. That carboxylate ion does not readily react with the alcohol to regenerate the ester under these conditions. This removes the acid from the esterification equilibrium and drives the hydrolysis forward.
Do not write the free carboxylic acid as the final acid-derived product of alkaline hydrolysis. With NaOH, write the sodium carboxylate; with KOH, write the potassium carboxylate.
Obtaining the carboxylic acid after base hydrolysis
If the question requires the free carboxylic acid, first carry out the alkaline hydrolysis and then acidify the carboxylate salt using a strong aqueous acid such as dilute HCl.
Ethyl ethanoate + NaOH gives sodium ethanoate and ethanol. If ethanoic acid is required, acidify the sodium ethanoate solution after hydrolysis so that CH₃COO⁻ is protonated to CH₃COOH.
A special point for phenyl benzoate
Phenyl benzoate releases a phenolic group on hydrolysis. Phenol is itself acidic enough to react with strong alkali. Therefore, when excess aqueous NaOH is used, the strongly alkaline mixture contains both sodium benzoate and sodium phenoxide.
This is an important refinement of the generic rule “ester + NaOH gives carboxylate + alcohol”. If the hydroxy product is a phenol rather than an ordinary alcohol, excess NaOH can deprotonate it as well.
Acid Hydrolysis and Base Hydrolysis Compared
The easiest way to keep the two hydrolysis routes straight is to focus on the protonation state of the acid-derived product. Acidic conditions give the carboxylic acid, whereas alkaline conditions give its carboxylate salt.
| Feature | Acid hydrolysis | Base hydrolysis |
|---|---|---|
| Reagent | Aqueous acid, e.g. dilute H₂SO₄ or HCl | Aqueous NaOH or KOH |
| Condition | Heat, commonly under reflux | Heat, commonly under reflux |
| Acid-derived product | Carboxylic acid | Carboxylate salt |
| Reversibility | Reversible | Effectively irreversible |
Ethyl Ethanoate and Phenyl Benzoate Compared
The two named syllabus examples are worth comparing because together they show the main formation and hydrolysis chemistry of esters.
| Feature | Ethyl ethanoate | Phenyl benzoate |
|---|---|---|
| Formula | CH₃COOCH₂CH₃ | C₆H₅COOC₆H₅ |
| Typical formation route | Ethanoic acid + ethanol, conc. H₂SO₄, heat | Benzoyl chloride + phenol |
| Acid hydrolysis | Ethanoic acid + ethanol | Benzoic acid + phenol |
| Base hydrolysis | Ethanoate salt + ethanol | Benzoate salt; with excess strong alkali, phenoxide is also formed |
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Writing a one-way arrow for carboxylic acid–alcohol esterification | This esterification is reversible and establishes an equilibrium. | Use ⇌ for carboxylic acid + alcohol ⇌ ester + water. |
| Writing a free carboxylic acid as the final product of hydrolysis with NaOH | The carboxylic acid is deprotonated in alkaline solution. | Write the carboxylate salt first; acidify afterwards only if the free acid is required. |
| Trying to prepare phenyl benzoate directly from benzoic acid and phenol | Phenol is a weaker nucleophile and does not readily undergo direct esterification with a carboxylic acid. | Use benzoyl chloride as the acylating agent for phenol. |
| Forgetting that excess NaOH can also deprotonate phenol formed from a phenyl ester | Phenol is appreciably acidic and reacts with strong alkali. | For phenyl benzoate with excess NaOH, recognise sodium benzoate and sodium phenoxide as the strongly alkaline products. |
What you should be able to do
Recognise an ester
Identify the –COO– linkage and separate the acid-derived and alcohol- or phenol-derived parts of the molecule.
Name simple esters
Relate the first part of the ester name to the group attached through oxygen and the second part to the carboxylic acid.
Form ethyl ethanoate
State ethanoic acid, ethanol, concentrated H₂SO₄ and heat, and show the reaction as reversible.
Form phenyl benzoate
Use benzoyl chloride and phenol, and recognise acyl chloride ester formation as a rapid acylation reaction.
Distinguish the two hydrolysis routes
Predict carboxylic acid + alcohol or phenol under acidic conditions, and carboxylate salts under alkaline conditions.
Handle acidification correctly
Convert a carboxylate salt into the free carboxylic acid by adding aqueous acid after alkaline hydrolysis when required.
These ten questions test whether you can connect ester structure with naming, preparation, hydrolysis and the different behaviour of ethyl ethanoate and phenyl benzoate.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Questions Students Often Ask About Esters
Most ester questions become much easier once you keep track of which fragment came from the acid side, which came from the alcohol or phenol side, and whether the reaction mixture is acidic or alkaline.
Why can ethyl ethanoate be made from ethanoic acid and ethanol, but phenyl benzoate is usually made from benzoyl chloride and phenol?
Ethanol is sufficiently nucleophilic to undergo acid-catalysed esterification with ethanoic acid. Phenol is a weaker nucleophile because an oxygen lone pair can be delocalised into the benzene ring, so direct esterification with a carboxylic acid is much less favourable. Benzoyl chloride is far more reactive and therefore readily acylates phenol.
Is concentrated H₂SO₄ used up when ethyl ethanoate is prepared?
No. Its key syllabus role is as an acid catalyst, so it participates in the reaction pathway but is regenerated. The esterification itself is reversible, which is why the equation should still be written with an equilibrium arrow.
Why does acid hydrolysis give a carboxylic acid but base hydrolysis gives a carboxylate salt?
In acidic solution, the acid-derived product remains protonated as RCOOH. In alkaline solution, any RCOOH formed is deprotonated by OH⁻ to give RCOO⁻, so the final acid-derived product is a sodium or potassium carboxylate salt.
Why is alkaline hydrolysis treated as irreversible?
The carboxylic acid product is converted into its carboxylate ion in alkaline solution. The carboxylate ion does not readily react with the alcohol to reform the ester under those conditions, so the reverse esterification is effectively prevented.
What extra point should I remember when phenyl benzoate is hydrolysed with excess NaOH?
Hydrolysis first produces the benzoate part and a phenolic part. Because phenol is acidic enough to react with strong alkali, excess NaOH also converts phenol into phenoxide. The overall strongly alkaline products can therefore be sodium benzoate and sodium phenoxide, with two moles of NaOH consumed per mole of phenyl benzoate.
Ester chemistry is mainly a story of making and breaking the –COO– linkage. Once you can identify the two sides of an ester, the reagents, products and equilibrium behaviour become much easier to predict: carboxylic acid + alcohol gives a reversible esterification, acyl chlorides provide a more reactive route, and hydrolysis depends on whether the medium is acidic or alkaline.