Why is concentrated sulfuric acid used in esterification instead of dilute acid?

Concentrated sulfuric acid serves two purposes. Firstly, it is a strong acid catalyst that increases the rate of reaction. Secondly, it is a powerful dehydrating agent. By removing the water produced during the reaction, it shifts the position of the equilibrium to the right (Le Chatelier's principle), increasing the yield of the ester.

Are fats and oils esters?

Yes, fats and oils are natural esters. They are triesters formed from one molecule of glycerol (propane-1,2,3-triol) and three molecules of long-chain carboxylic acids, known as fatty acids. The hydrolysis of these esters with sodium hydroxide is the traditional method for making soap.

Can an ester be formed from a phenol instead of an alcohol?

Yes, but the reaction is different. Phenols are less reactive than alcohols. They do not readily undergo esterification with carboxylic acids. Instead, they react with more reactive acid derivatives, such as acid anhydrides or acyl chlorides, to form phenyl esters.

Why are esters used as solvents?

Many simple esters are volatile liquids that can dissolve a wide range of organic compounds. They are polar due to the C=O and C-O bonds but lack the hydrogen bonding of alcohols or carboxylic acids, making them good solvents for polar and non-polar substances alike. Ethyl ethanoate, for example, is a common solvent in nail polish remover and for decaffeinating coffee.

Where can I learn Esters for Cambridge 9701?

This free lesson on MarkScheme covers syllabus point 33.2 (Esters) for Cambridge A-Level Chemistry. It includes visual revision, key points, and links to real past-paper questions with mark-scheme marking at markscheme.app/courses/9701/33-2-esters.

Is Esters (33.2) on 9701 free to study?

Yes. The Chemistry 9701 course on MarkScheme is 100% free — including premium visual lessons, exam tips, and past-paper marking. No paywall for Esters.

Esters 9701 — Free A-Level Chemistry

Esters

Esters are a fascinating and fragrant family of organic compounds. They are responsible for many of the natural and artificial smells and flavours we encounter daily, from the sweet scent of pineapples to the aroma of perfumes. Beyond their sensory properties, esters are vital as industrial solvents and as the building blocks for important polymers like polyesters.

BY THE END, YOU CAN…

1Describe the formation of esters from carboxylic acids and alcohols using an acid catalyst.
2Explain the hydrolysis of esters under both acidic (reversible) and alkaline (irreversible) conditions.
3State the uses of esters, including as food flavourings, perfumes, and solvents.
4Describe the formation of polyesters through condensation polymerisation of a dicarboxylic acid and a diol.

In simple terms

A friendly intro before the formal notes — no formulas yet.

Making and Breaking Esters

Esters are formed when a carboxylic acid and an alcohol react, losing a water molecule. This reaction can be reversed (hydrolysis), either partially with acid or completely with alkali to form a salt.

THINK OF IT LIKE…

Imagine two different Lego bricks, one representing an alcohol and one a carboxylic acid. To join them, you snap off a small piece from each (forming a water molecule) and click the main bricks together to make an ester. You can break them apart again using water (hydrolysis). If you use an alkaline solution, it's like using a special tool that breaks them apart permanently and changes one of the bricks into a salt, so it can't easily rejoin.

1
The ester functional group, –COO–, is formed by reacting a carboxylic acid (–COOH) with an alcohol (–OH) under reflux with a concentrated H₂SO₄ catalyst.
2
Hydrolysis is the reverse reaction. Acid hydrolysis is a reversible equilibrium, whereas alkaline hydrolysis (saponification) is irreversible, forming a carboxylate salt and an alcohol.
3
Esters are named with the alkyl group from the alcohol first, followed by the carboxylate part from the acid ending in '-oate'. For example, ethanol + ethanoic acid → ethyl ethanoate, which smells like nail polish remover.
4
Polyesters, like Terylene, are condensation polymers formed from monomers with two functional groups: a dicarboxylic acid and a diol. This links to polymerisation chemistry.

Explore the concept

Use the live diagram and synced steps — play it or tap a step card to walk through.

Key formulas

Tap any symbol to reveal exactly what it means and its units.

$\text{R-COOH} + \text{R'-OH} \rightleftharpoons \text{R-COOR'} + \text{H}_2\text{O}$

$\text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} \stackrel{\text{H}^+}{\rightleftharpoons} \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH}$

$n \text{ HOOC-C}_6\text{H}_4\text{-COOH} + n \text{ HO-CH}_2\text{CH}_2\text{-OH} \rightarrow [- \text{OC-C}_6\text{H}_4\text{-CO-OCH}_2\text{CH}_2\text{O} -]_n + 2n \text{ H}_2\text{O}$

Full topic notes

Formal explanation with the rigour you need for the exam.

Formation of Esters: Esterification

Esterification is a condensation reaction between a carboxylic acid and an alcohol. A molecule of water is eliminated as the ester is formed. The reaction is typically slow at room temperature, so it is carried out by heating the mixture under reflux. A strong acid catalyst, usually concentrated sulfuric acid, is required to achieve a reasonable rate of reaction and yield.

General Equation: $\text{R-COOH} + \text{R'-OH} \rightleftharpoons \text{R-COOR'} + \text{H}_2\text{O}$ (Carboxylic Acid + Alcohol $\rightleftharpoons$ Ester + Water)

Reactants: Carboxylic acid and an alcohol.
Conditions: Heat under reflux with a concentrated H₂SO₄ catalyst.
Reaction Type: Reversible condensation reaction.
Catalyst Role: H₂SO₄ acts as a catalyst and a dehydrating agent, shifting the equilibrium to favour the products.

Naming Esters

The systematic name of an ester is derived from its parent alcohol and carboxylic acid. The first part of the name is the alkyl group from the alcohol. The second part is the name of the parent carboxylic acid, but the ending '-oic acid' is replaced with '-oate'. For example, the ester formed from methanol and propanoic acid is called methyl propanoate.

Hydrolysis of Esters

Hydrolysis is the chemical breakdown of a compound due to reaction with water. For esters, this means splitting the ester linkage to re-form the parent carboxylic acid and alcohol. This process can be catalysed by either acid or alkali, with significantly different outcomes.

Acid Hydrolysis: $\text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} \stackrel{\text{H}^+}{\rightleftharpoons} \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH}$

Alkaline Hydrolysis: $\text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{OH}^- \rightarrow \text{CH}_3\text{COO}^- + \text{CH}_3\text{CH}_2\text{OH}$

Acid Hydrolysis: This is the reverse of esterification. The ester is heated under reflux with a dilute aqueous acid (e.g., dilute H₂SO₄ or HCl). The reaction is reversible and establishes an equilibrium. Products: Carboxylic acid + Alcohol.
Alkaline Hydrolysis (Saponification): The ester is heated under reflux with an aqueous alkali (e.g., NaOH). The reaction is irreversible and goes to completion. Products: Carboxylate salt + Alcohol. The term 'saponification' means 'soap-making', as this reaction is used to make soap from fats and oils (which are natural esters).

EXAM TIP

A very common mistake is confusing the products of acid and alkaline hydrolysis. Remember: Alkaline hydrolysis produces a salt. The reaction is irreversible because the carboxylate anion formed is stable and does not react with the alcohol. In acid hydrolysis, you get the carboxylic acid back, and the reaction is reversible.

Polyesters

Polyesters are polymers formed through condensation polymerisation. This involves reacting monomers that each have two functional groups. For a polyester, the monomers are typically a dicarboxylic acid (a molecule with two –COOH groups) and a diol (a molecule with two –OH groups). Each time an ester link is formed, a small molecule, usually water, is eliminated. This process repeats to form a long polymer chain.

Formation of Terylene (showing repeating unit): $n \text{ HOOC-C}_6\text{H}_4\text{-COOH} + n \text{ HO-CH}_2\text{CH}_2\text{-OH} \rightarrow [- \text{OC-C}_6\text{H}_4\text{-CO-OCH}_2\text{CH}_2\text{O} -]_n + 2n \text{ H}_2\text{O}$

Monomers: A dicarboxylic acid and a diol.
Example: Terylene (also known as PET) is formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
Linkage: The monomers are joined by ester linkages (–COO–).
By-product: Water is eliminated during the polymerisation.

Worked examples

See the formulas applied — reveal one step at a time, like the exam.

EXAMPLE 1

An ester is formed from butan-1-ol and ethanoic acid. (a) Draw the displayed formula of this ester. (b) Give its systematic name.

1
(a) First, identify the structures of the reactants. Butan-1-ol is $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$ and ethanoic acid is $\text{CH}_3\text{COOH}$ . The reaction removes the –OH from the carboxylic acid and the –H from the alcohol's hydroxyl group.

EXAMPLE 2

A 1.76 g sample of an ester, methyl propanoate ( $\text{CH}_3\text{CH}_2\text{COOCH}_3$ ), is hydrolysed by heating with excess aqueous sodium hydroxide. Calculate the maximum mass of sodium propanoate that can be formed. ( $A_r$ values: C=12.0, H=1.0, O=16.0, Na=23.0)

1
Step 1: Write the balanced equation. $\text{CH}_3\text{CH}_2\text{COOCH}_3 + \text{NaOH} \rightarrow \text{CH}_3\text{CH}_2\text{COONa} + \text{CH}_3\text{OH}$

How it all connects

The big idea sits in the middle — tap a linked idea to explore the link.

MAIN IDEAEsters

Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.

Glossary

Try to recall each definition before you reveal it.

Quick check

Answer in your head first — then tap to check. No pressure.

Revision flashcards

Flip the card. Test yourself before the exam.

QUESTION · 1 / 12

What is the functional group of an ester?

TAP OR SPACE TO FLIP

ANSWER

The ester functional group is –COO–, sometimes written as –COOR where R is an alkyl group.

TAP OR SPACE TO FLIP BACK

1 / 12 · ← → space

Key takeaways

Review these before you close the topic — retrieval beats re-reading.

✓
Reactants: Carboxylic acid and an alcohol.
✓
Conditions: Heat under reflux with a concentrated H₂SO₄ catalyst.
✓
Reaction Type: Reversible condensation reaction.
✓
Catalyst Role: H₂SO₄ acts as a catalyst and a dehydrating agent, shifting the equilibrium to favour the products.

Practice — then mark it

The whole point: a real Cambridge question, marked mark-by-mark.

Real past paper[0]

Test Your Knowledge on Esters

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

Extra simulations & links

PhET, GeoGebra and other curated tools — open in a new tab.

Frequently asked

✓

Checkpoint

One marked question is worth ten re-reads — close the loop before you move on.

Checkpoint

Reading it isn’t knowing it — prove it.

Before you move on: do Test Your Knowledge on Esters on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.

Attempt & get marked →Takes about a minute · you’ll land back on this lesson

Esters

BY THE END, YOU CAN…

1Describe the formation of esters from carboxylic acids and alcohols using an acid catalyst.
2Explain the hydrolysis of esters under both acidic (reversible) and alkaline (irreversible) conditions.
3State the uses of esters, including as food flavourings, perfumes, and solvents.
4Describe the formation of polyesters through condensation polymerisation of a dicarboxylic acid and a diol.

In simple terms

A friendly intro before the formal notes — no formulas yet.

Making and Breaking Esters

THINK OF IT LIKE…

1
The ester functional group, –COO–, is formed by reacting a carboxylic acid (–COOH) with an alcohol (–OH) under reflux with a concentrated H₂SO₄ catalyst.
2
Hydrolysis is the reverse reaction. Acid hydrolysis is a reversible equilibrium, whereas alkaline hydrolysis (saponification) is irreversible, forming a carboxylate salt and an alcohol.
3
Esters are named with the alkyl group from the alcohol first, followed by the carboxylate part from the acid ending in '-oate'. For example, ethanol + ethanoic acid → ethyl ethanoate, which smells like nail polish remover.
4
Polyesters, like Terylene, are condensation polymers formed from monomers with two functional groups: a dicarboxylic acid and a diol. This links to polymerisation chemistry.

Explore the concept

Use the live diagram and synced steps — play it or tap a step card to walk through.

Key formulas

Tap any symbol to reveal exactly what it means and its units.

$\text{R-COOH} + \text{R'-OH} \rightleftharpoons \text{R-COOR'} + \text{H}_2\text{O}$

$\text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} \stackrel{\text{H}^+}{\rightleftharpoons} \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH}$

$n \text{ HOOC-C}_6\text{H}_4\text{-COOH} + n \text{ HO-CH}_2\text{CH}_2\text{-OH} \rightarrow [- \text{OC-C}_6\text{H}_4\text{-CO-OCH}_2\text{CH}_2\text{O} -]_n + 2n \text{ H}_2\text{O}$

Full topic notes

Formal explanation with the rigour you need for the exam.

Formation of Esters: Esterification

General Equation: $\text{R-COOH} + \text{R'-OH} \rightleftharpoons \text{R-COOR'} + \text{H}_2\text{O}$ (Carboxylic Acid + Alcohol $\rightleftharpoons$ Ester + Water)

Reactants: Carboxylic acid and an alcohol.
Conditions: Heat under reflux with a concentrated H₂SO₄ catalyst.
Reaction Type: Reversible condensation reaction.
Catalyst Role: H₂SO₄ acts as a catalyst and a dehydrating agent, shifting the equilibrium to favour the products.

Naming Esters

Hydrolysis of Esters

Acid Hydrolysis: $\text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O} \stackrel{\text{H}^+}{\rightleftharpoons} \text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH}$

Alkaline Hydrolysis: $\text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{OH}^- \rightarrow \text{CH}_3\text{COO}^- + \text{CH}_3\text{CH}_2\text{OH}$

Acid Hydrolysis: This is the reverse of esterification. The ester is heated under reflux with a dilute aqueous acid (e.g., dilute H₂SO₄ or HCl). The reaction is reversible and establishes an equilibrium. Products: Carboxylic acid + Alcohol.
Alkaline Hydrolysis (Saponification): The ester is heated under reflux with an aqueous alkali (e.g., NaOH). The reaction is irreversible and goes to completion. Products: Carboxylate salt + Alcohol. The term 'saponification' means 'soap-making', as this reaction is used to make soap from fats and oils (which are natural esters).

EXAM TIP

Polyesters

Monomers: A dicarboxylic acid and a diol.
Example: Terylene (also known as PET) is formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
Linkage: The monomers are joined by ester linkages (–COO–).
By-product: Water is eliminated during the polymerisation.

Worked examples

See the formulas applied — reveal one step at a time, like the exam.

EXAMPLE 1

An ester is formed from butan-1-ol and ethanoic acid. (a) Draw the displayed formula of this ester. (b) Give its systematic name.

1
(a) First, identify the structures of the reactants. Butan-1-ol is $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$ and ethanoic acid is $\text{CH}_3\text{COOH}$ . The reaction removes the –OH from the carboxylic acid and the –H from the alcohol's hydroxyl group.

EXAMPLE 2

1
Step 1: Write the balanced equation. $\text{CH}_3\text{CH}_2\text{COOCH}_3 + \text{NaOH} \rightarrow \text{CH}_3\text{CH}_2\text{COONa} + \text{CH}_3\text{OH}$

How it all connects

The big idea sits in the middle — tap a linked idea to explore the link.

MAIN IDEAEsters

Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.

Glossary

Try to recall each definition before you reveal it.

Quick check

Answer in your head first — then tap to check. No pressure.

Revision flashcards

Flip the card. Test yourself before the exam.

QUESTION · 1 / 12

What is the functional group of an ester?

TAP OR SPACE TO FLIP

ANSWER

The ester functional group is –COO–, sometimes written as –COOR where R is an alkyl group.

TAP OR SPACE TO FLIP BACK

1 / 12 · ← → space

Key takeaways

Review these before you close the topic — retrieval beats re-reading.

✓
Reactants: Carboxylic acid and an alcohol.
✓
Conditions: Heat under reflux with a concentrated H₂SO₄ catalyst.
✓
Reaction Type: Reversible condensation reaction.
✓
Catalyst Role: H₂SO₄ acts as a catalyst and a dehydrating agent, shifting the equilibrium to favour the products.

Practice — then mark it

The whole point: a real Cambridge question, marked mark-by-mark.

Real past paper[0]

Test Your Knowledge on Esters

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

Extra simulations & links

PhET, GeoGebra and other curated tools — open in a new tab.

Frequently asked

✓

Checkpoint

One marked question is worth ten re-reads — close the loop before you move on.

Checkpoint

Reading it isn’t knowing it — prove it.

Before you move on: do Test Your Knowledge on Esters on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.

Attempt & get marked →Takes about a minute · you’ll land back on this lesson

Revise Esters for Cambridge 9701 (33.2)

In simple terms

Explore the concept

Key formulas

Full topic notes

Formation of Esters: Esterification

Naming Esters

Hydrolysis of Esters

Polyesters

Worked examples

How it all connects

Glossary

Quick check

Revision flashcards

Key takeaways

Practice — then mark it

Test Your Knowledge on Esters

Extra simulations & links

Frequently asked

Checkpoint

Reading it isn’t knowing it — prove it.

In simple terms

Explore the concept

Key formulas

Full topic notes

Formation of Esters: Esterification

Naming Esters

Hydrolysis of Esters

Polyesters

Worked examples

How it all connects

Glossary

Quick check

Revision flashcards

Key takeaways

Practice — then mark it

Test Your Knowledge on Esters

Extra simulations & links

Frequently asked

Checkpoint

Reading it isn’t knowing it — prove it.

Discuss Esters