Is esterification a reversible reaction?

Yes, esterification is a reversible reaction. It reaches a state of dynamic equilibrium. To maximise the yield of the ester, we use a dehydrating agent (like concentrated sulfuric acid) to remove water, which shifts the equilibrium to the product side according to Le Chatelier's principle.

Why is the reaction mixture heated under reflux?

Heating under reflux allows the reaction to be carried out at the boiling point of the solvent for an extended period without losing volatile reactants or products. The vapours are condensed and returned to the reaction flask, ensuring the reaction can proceed at a high temperature to increase the rate, without boiling dry.

What is the difference in products between acid and alkaline hydrolysis?

In acid hydrolysis, the products are the original carboxylic acid and alcohol. In alkaline hydrolysis, the products are the salt of the carboxylic acid (a carboxylate) and the alcohol. This is because the carboxylic acid formed reacts with the alkali present.

Can tertiary alcohols be used to make esters?

While it is possible, the esterification of tertiary alcohols with carboxylic acids is very difficult and slow due to steric hindrance around the hydroxyl group. They are more likely to undergo an elimination reaction (dehydration) in the presence of hot, concentrated acid.

Biodiesel is a fuel produced from renewable sources such as vegetable oils or animal fats. These fats and oils are large esters (triglycerides). They are reacted with a simple alcohol (usually methanol) in a process called transesterification to produce fatty acid methyl esters (FAMEs), which is the primary component of biodiesel.

Where can I learn Esters for Cambridge 9701?

This free lesson on MarkScheme covers syllabus point 18.2 (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/9701/18-2-esters.

Esters 9701 — Free A-Level Chemistry

Esters

Esters are a fascinating family of organic compounds responsible for many of the natural and artificial smells and flavours we encounter daily, from the scent of pineapples to bananas. In this lesson, we'll delve into the chemistry of these aromatic molecules, learning how they are synthesised, named, and how they react.

BY THE END, YOU CAN…

1Recall the formation of an ester from a carboxylic acid and an alcohol using a concentrated acid catalyst.
2Name esters using the IUPAC 'alkyl alkanoate' convention.
3Describe and write equations for the hydrolysis of esters in both acidic (reversible) and alkaline (irreversible) conditions.
4State the uses of esters, including in perfumes, food flavourings, and as solvents.

In simple terms

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

Esters: Building with Alcohols and Acids

Esters are fragrant organic compounds formed when carboxylic acids and alcohols react in a process called esterification. This reaction is reversible and can be broken down by hydrolysis, with different outcomes in acid or alkali.

THINK OF IT LIKE…

Imagine you have two different types of building blocks, one representing an alcohol (let's say it has a 'peg') and one a carboxylic acid (with a 'hole'). To join them, you need a special connector tool (a catalyst) and some energy (heat). When they click together to form the ester, a small water block is released. This new structure can be carefully taken apart to get the original blocks back, or broken permanently with a different tool (alkali) to yield a modified acid block and the original alcohol block.

1
Ester: R–COO–R′ from acid + alcohol (conc. H₂SO₄, heat).
2
Naming: alkyl alkanoate (e.g. methyl methanoate).
3
Hydrolysis: acid → reverse esterification; alkali → carboxylate + alcohol.
4
Uses: fragrances, solvents, biodiesel (methyl esters).

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.

$\rightleftharpoons$

Tap a symbol — great for exam definitions

Full topic notes

Formal explanation with the rigour you need for the exam.

Formation of Esters: Esterification

Esters are formed in a condensation reaction between a carboxylic acid and an alcohol. This specific type of condensation reaction is called esterification. The reaction is typically slow at room temperature, so it is carried out by heating the mixture under reflux. A strong acid catalyst is also required to increase the rate of reaction.

Carboxylic Acid + Alcohol $\rightleftharpoons$ Ester + Water \ RCOOH + R'OH $\rightleftharpoons$ RCOOR' + H₂O

Reactants: Carboxylic acid and an alcohol.
Conditions: Heat under reflux with a concentrated sulfuric acid ( $H_2SO_4$ ) or concentrated hydrochloric acid ( $HCl$ ) catalyst.
Reaction Type: Condensation and it is reversible.
Role of Catalyst: Concentrated $H_2SO_4$ acts as both a catalyst and a dehydrating agent, removing water to shift the equilibrium position to the right and increase the yield of the ester.

Naming Esters

The name of an ester is derived from the parent alcohol and carboxylic acid. The naming system follows the pattern 'alkyl alkanoate'. The first part of the name, the 'alkyl' group, comes from the alcohol. The second part, the 'alkanoate', comes from the carboxylic acid. For example, the ester formed from methanol and ethanoic acid is called methyl ethanoate.

Hydrolysis of Esters

Hydrolysis is the chemical breakdown of a substance by water. Esters can be hydrolysed back into a carboxylic acid and an alcohol. This process can be carried out under acidic or alkaline conditions, which lead to different outcomes.

Acid Hydrolysis: This is the reverse of esterification. The ester is heated under reflux with a dilute acid (e.g., dilute $H_2SO_4$ ). The reaction is reversible and produces the carboxylic acid and the alcohol. An equilibrium mixture is formed. \n $RCOOR' + H_2O \rightleftharpoons RCOOH + R'OH$
Alkaline Hydrolysis (Saponification): The ester is heated under reflux with a strong alkali (e.g., sodium hydroxide solution). This reaction is irreversible and goes to completion. The products are the salt of the carboxylic acid (a carboxylate) and the alcohol. \n $RCOOR' + OH^- \rightarrow RCOO^- + R'OH$

EXAM TIP

A common exam question asks why alkaline hydrolysis is irreversible. The carboxylic acid formed initially is immediately neutralised by the excess alkali to form a carboxylate salt. This salt is stable and does not react with the alcohol to reform the ester, thus driving the reaction to completion.

Uses of Esters

The properties of esters make them useful for a variety of commercial applications. Short-chain esters are volatile and have pleasant, fruity smells, making them ideal for use in perfumes, food flavourings (e.g., pear drops, pineapple), and cosmetics. Their ability to dissolve many non-polar organic compounds also makes them excellent industrial solvents, used in products like paints, varnishes, and adhesives (e.g., ethyl ethanoate). Furthermore, esters of long-chain fatty acids with methanol (biodiesel) are becoming an important renewable fuel source.

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 propanoic acid. \n(a) Draw the displayed formula of the ester formed. \n(b) Name the ester.

1
(a) First, identify the structures of the reactants. Butan-1-ol is $CH_3CH_2CH_2CH_2OH$ and propanoic acid is $CH_3CH_2COOH$ . The reaction removes the –OH from the carboxylic acid and the –H from the alcohol's hydroxyl group to form water. The remaining parts join together. \n The ester link is –COO–. The part from the acid is $CH_3CH_2CO-$ and the part from the alcohol is $-OCH_2CH_2CH_2CH_3$ . \n Displayed formula: \n H H O H H H H \n | | || | | | | \n H-C-C-C-O-C-C-C-C-H \n | | | | | | \n H H H H H H \n (b) To name the ester, we use the 'alkyl alkanoate' format. \n The alkyl part comes from the alcohol, butan-1-ol, so it is 'butyl'. \n The alkanoate part comes from the carboxylic acid, propanoic acid, so it is 'propanoate'. \n Therefore, the name of the ester is butyl propanoate.

✓ FULLY WORKED

EXAMPLE 2

An ester has the molecular formula $C_4H_8O_2$ . It is formed from methanol. \n(a) Identify the carboxylic acid used to form this ester. \n(b) Write an equation for the alkaline hydrolysis of this ester using sodium hydroxide.

1
(a) The general formula of the ester is RCOOR'. We know the alcohol is methanol ( $CH_3OH$ ), so the R' group is methyl ( $CH_3$ ). \n The formula is $RCOOCH_3$ . The total formula is $C_4H_8O_2$ . \n Let's subtract the atoms from the $-COOCH_3$ part: $C_2H_3O_2$ . \n $C_4H_8O_2 - C_2H_3O_2 = C_2H_5$ . \n So, the R group is an ethyl group, $C_2H_5$ . \n The carboxylic acid is therefore $C_2H_5COOH$ , which is propanoic acid. \n The ester is methyl propanoate. \n (b) Alkaline hydrolysis of methyl propanoate ( $CH_3CH_2COOCH_3$ ) with sodium hydroxide ( $NaOH$ ) will produce the sodium salt of the carboxylic acid and the alcohol. \n Products: Sodium propanoate and methanol. \n Equation: \n $CH_3CH_2COOCH_3(aq) + NaOH(aq) \rightarrow CH_3CH_2COONa(aq) + CH_3OH(aq)$

✓ FULLY WORKED

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 general structure of an ester functional group?

TAP OR SPACE TO FLIP

ANSWER

The ester functional group is –COO–. The general formula is RCOOR', where R and R' are alkyl or aryl groups. The R group comes from the carboxylic acid and the R' group comes from the alcohol.

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 sulfuric acid ( $H_2SO_4$ ) or concentrated hydrochloric acid ( $HCl$ ) catalyst.
✓
Reaction Type: Condensation and it is reversible.
✓
Role of Catalyst: Concentrated $H_2SO_4$ acts as both a catalyst and a dehydrating agent, removing water to shift the equilibrium position to the right and increase the yield of the ester.

Practice — then mark it

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

Real past paper[0]

Practice Questions: 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 Practice Questions: 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

← PREVIOUS18.1 Carboxylic acids NEXT →19.1 Primary amines

Esters

BY THE END, YOU CAN…

1Recall the formation of an ester from a carboxylic acid and an alcohol using a concentrated acid catalyst.
2Name esters using the IUPAC 'alkyl alkanoate' convention.
3Describe and write equations for the hydrolysis of esters in both acidic (reversible) and alkaline (irreversible) conditions.
4State the uses of esters, including in perfumes, food flavourings, and as solvents.

In simple terms

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

Esters: Building with Alcohols and Acids

THINK OF IT LIKE…

1
Ester: R–COO–R′ from acid + alcohol (conc. H₂SO₄, heat).
2
Naming: alkyl alkanoate (e.g. methyl methanoate).
3
Hydrolysis: acid → reverse esterification; alkali → carboxylate + alcohol.
4
Uses: fragrances, solvents, biodiesel (methyl esters).

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.

$\rightleftharpoons$

Tap a symbol — great for exam definitions

Full topic notes

Formal explanation with the rigour you need for the exam.

Formation of Esters: Esterification

Carboxylic Acid + Alcohol $\rightleftharpoons$ Ester + Water \ RCOOH + R'OH $\rightleftharpoons$ RCOOR' + H₂O

Reactants: Carboxylic acid and an alcohol.
Conditions: Heat under reflux with a concentrated sulfuric acid ( $H_2SO_4$ ) or concentrated hydrochloric acid ( $HCl$ ) catalyst.
Reaction Type: Condensation and it is reversible.
Role of Catalyst: Concentrated $H_2SO_4$ acts as both a catalyst and a dehydrating agent, removing water to shift the equilibrium position to the right and increase the yield of the ester.

Naming Esters

Hydrolysis of Esters

Acid Hydrolysis: This is the reverse of esterification. The ester is heated under reflux with a dilute acid (e.g., dilute $H_2SO_4$ ). The reaction is reversible and produces the carboxylic acid and the alcohol. An equilibrium mixture is formed. \n $RCOOR' + H_2O \rightleftharpoons RCOOH + R'OH$
Alkaline Hydrolysis (Saponification): The ester is heated under reflux with a strong alkali (e.g., sodium hydroxide solution). This reaction is irreversible and goes to completion. The products are the salt of the carboxylic acid (a carboxylate) and the alcohol. \n $RCOOR' + OH^- \rightarrow RCOO^- + R'OH$

EXAM TIP

Uses of Esters

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 propanoic acid. \n(a) Draw the displayed formula of the ester formed. \n(b) Name the ester.

1
(a) First, identify the structures of the reactants. Butan-1-ol is $CH_3CH_2CH_2CH_2OH$ and propanoic acid is $CH_3CH_2COOH$ . The reaction removes the –OH from the carboxylic acid and the –H from the alcohol's hydroxyl group to form water. The remaining parts join together. \n The ester link is –COO–. The part from the acid is $CH_3CH_2CO-$ and the part from the alcohol is $-OCH_2CH_2CH_2CH_3$ . \n Displayed formula: \n H H O H H H H \n | | || | | | | \n H-C-C-C-O-C-C-C-C-H \n | | | | | | \n H H H H H H \n (b) To name the ester, we use the 'alkyl alkanoate' format. \n The alkyl part comes from the alcohol, butan-1-ol, so it is 'butyl'. \n The alkanoate part comes from the carboxylic acid, propanoic acid, so it is 'propanoate'. \n Therefore, the name of the ester is butyl propanoate.

✓ FULLY WORKED

EXAMPLE 2

1
(a) The general formula of the ester is RCOOR'. We know the alcohol is methanol ( $CH_3OH$ ), so the R' group is methyl ( $CH_3$ ). \n The formula is $RCOOCH_3$ . The total formula is $C_4H_8O_2$ . \n Let's subtract the atoms from the $-COOCH_3$ part: $C_2H_3O_2$ . \n $C_4H_8O_2 - C_2H_3O_2 = C_2H_5$ . \n So, the R group is an ethyl group, $C_2H_5$ . \n The carboxylic acid is therefore $C_2H_5COOH$ , which is propanoic acid. \n The ester is methyl propanoate. \n (b) Alkaline hydrolysis of methyl propanoate ( $CH_3CH_2COOCH_3$ ) with sodium hydroxide ( $NaOH$ ) will produce the sodium salt of the carboxylic acid and the alcohol. \n Products: Sodium propanoate and methanol. \n Equation: \n $CH_3CH_2COOCH_3(aq) + NaOH(aq) \rightarrow CH_3CH_2COONa(aq) + CH_3OH(aq)$

✓ FULLY WORKED

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 general structure of an ester functional group?

TAP OR SPACE TO FLIP

ANSWER

The ester functional group is –COO–. The general formula is RCOOR', where R and R' are alkyl or aryl groups. The R group comes from the carboxylic acid and the R' group comes from the alcohol.

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 sulfuric acid ( $H_2SO_4$ ) or concentrated hydrochloric acid ( $HCl$ ) catalyst.
✓
Reaction Type: Condensation and it is reversible.
✓
Role of Catalyst: Concentrated $H_2SO_4$ acts as both a catalyst and a dehydrating agent, removing water to shift the equilibrium position to the right and increase the yield of the ester.

Practice — then mark it

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

Real past paper[0]

Practice Questions: 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 Practice Questions: 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

← PREVIOUS18.1 Carboxylic acids NEXT →19.1 Primary amines

Revise Esters for Cambridge 9701 (18.2)

In simple terms

Explore the concept

Key formulas

Full topic notes

Formation of Esters: Esterification

Naming Esters

Hydrolysis of Esters

Uses of Esters

Worked examples

How it all connects

Glossary

Quick check

Revision flashcards

Key takeaways

Practice — then mark it

Practice Questions: 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

Uses of Esters

Worked examples

How it all connects

Glossary

Quick check

Revision flashcards

Key takeaways

Practice — then mark it

Practice Questions: Esters

Extra simulations & links

Frequently asked

Checkpoint

Reading it isn’t knowing it — prove it.

Discuss Esters