Esters
Summary: Esters (RCOOR’) are formed by reacting carboxylic acids with alcohols in an esterification reaction (condensation, H2SO4 catalyst, warm). They have characteristic fruity smells and are used as flavourings, fragrances, and solvents. Tags: igcse chemistry organic-chem Created: 2026-07-14 Last Updated: 2026-07-16
Learning Objectives
By the end of this topic, you should be able to:
- Describe the esterification reaction: carboxylic acid + alcohol ⇌ ester + water
- State the conditions: concentrated H2SO4 catalyst, warm
- Name esters from the parent alcohol and carboxylic acid
- Draw structural formulae of esters from their names and vice versa
- Describe the physical properties of esters (volatile, fruity smells)
- State the uses of esters (perfumes, flavourings, solvents)
Content
1. The Ester Functional Group
Esters have the functional group -COO- (a carbonyl group bonded to an oxygen bonded to another carbon):
O
||
R - C - O - R'
Where:
- R comes from the carboxylic acid (the acyl group)
- R’ comes from the alcohol (the alkyl group)
2. Esterification — The Reaction
General equation:
Carboxylic acid + Alcohol ⇌ Ester + Water
The reaction is a condensation reaction (two molecules join with the elimination of a small molecule — water).
Conditions:
- Concentrated sulfuric acid (H2SO4) as a catalyst
- Warm the mixture (not boil — esters are volatile and would escape)
- The reaction is reversible (equilibrium), so an excess of one reactant (usually the alcohol, which is cheaper) is used to drive the equilibrium towards the ester
Why concentrated H2SO4?
- Acts as a catalyst (speeds up the reaction)
- Also acts as a dehydrating agent — removes water produced, shifting equilibrium to the right, increasing ester yield
3. Naming Esters
An ester’s name has two parts:
First part (from the alcohol) → alkyl group name → -yl ending
Second part (from the carboxylic acid) → carboxylate name → -oate ending
| Alcohol | Name Part 1 | Carboxylic Acid | Name Part 2 | Ester Name |
|---|---|---|---|---|
| Methanol (CH3OH) | methyl | Ethanoic acid (CH3COOH) | ethanoate | Methyl ethanoate |
| Ethanol (C2H5OH) | ethyl | Ethanoic acid (CH3COOH) | ethanoate | Ethyl ethanoate |
| Ethanol (C2H5OH) | ethyl | Methanoic acid (HCOOH) | methanoate | Ethyl methanoate |
| Propanol (C3H7OH) | propyl | Ethanoic acid (CH3COOH) | ethanoate | Propyl ethanoate |
| Butanol (C4H9OH) | butyl | Ethanoic acid (CH3COOH) | ethanoate | Butyl ethanoate |
| Methanol (CH3OH) | methyl | Propanoic acid (C2H5COOH) | propanoate | Methyl propanoate |
| Ethanol (C2H5OH) | ethyl | Butanoic acid (C3H7COOH) | butanoate | Ethyl butanoate |
| Pentanol (C5H11OH) | pentyl | Ethanoic acid (CH3COOH) | ethanoate | Pentyl ethanoate |
Naming rule: The C=O bond in the ester comes from the acid. The O-R’ bond comes from the alcohol. The alcohol part is named FIRST in the ester name.
4. Drawing Esters
Method: From Name to Structural Formula
Example: Draw ethyl ethanoate.
- Identify the parts: “ethyl” = from ethanol (C2H5), “ethanoate” = from ethanoic acid (CH3COOH)
- Draw the acid part with C=O: CH3-C(=O)-
- Attach the alcohol part through the O: CH3-C(=O)-O-C2H5
- Full structural formula: CH3COOC2H5
Example: Draw methyl propanoate.
- “methyl” from methanol (CH3); “propanoate” from propanoic acid (C2H5COOH)
- Structural formula: C2H5COOCH3
Method: From Structural Formula to Name
Example: Name CH3CH2COOCH3.
- Find the C=O group — the chain on the C=O side is from the acid: CH3CH2-C(=O)- → propanoate
- The group on the -O- side is from the alcohol: -O-CH3 → methyl
- Name: methyl propanoate
5. Specific Examples with Equations
Ethyl Ethanoate (the most commonly examined ester)
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Ethanoic acid + Ethanol ⇌ Ethyl ethanoate + Water
- Conditions: Concentrated H2SO4, warm
- Smell: Pear drops / nail varnish remover / fruity
Methyl Ethanoate
CH3COOH + CH3OH ⇌ CH3COOCH3 + H2O
Ethyl Methanoate
HCOOH + C2H5OH ⇌ HCOOC2H5 + H2O
6. Properties of Esters
| Property | Detail |
|---|---|
| Smell | Characteristic fruity / sweet smells (each ester has a distinct smell) |
| Volatility | Volatile (evaporate easily at room temperature) — this is why you can smell them |
| Boiling point | Lower than parent carboxylic acid (no hydrogen bonding between ester molecules, while carboxylic acids form dimers) |
| Solubility | Short-chain esters are slightly soluble in water; longer-chain esters are insoluble |
| Flammability | Flammable (like most organic compounds) |
Reason for lower boiling points than carboxylic acids: Esters cannot form hydrogen bonds between their own molecules (they have no -OH group for H-bond donation). Carboxylic acids form strong hydrogen bonds (dimers), giving them much higher boiling points.
7. Uses of Esters
| Use | Explanation | Examples |
|---|---|---|
| Perfumes / Fragrances | Volatile — evaporate easily so the smell reaches your nose. Fruity smells are pleasant | Ethyl ethanoate (pear), pentyl ethanoate (banana), ethyl butanoate (pineapple) |
| Food Flavourings | Small quantities added to foods to give specific fruity flavours | Methyl butanoate (apple), octyl ethanoate (orange) |
| Solvents | Good solvents for organic compounds; used in glues, paints, printing inks, nail varnish remover | Ethyl ethanoate (common lab/industrial solvent) |
| Plasticisers | Added to polymers to make them more flexible | Phthalate esters in PVC |
Worked Examples (Exam-Style)
Example 1: Naming an Ester
Question: Name the ester formed from ethanol and propanoic acid. [1 mark]
Solution:
- Alcohol part: ethanol → ethyl
- Acid part: propanoic acid → propanoate
- Ester name: ethyl propanoate ✓
Example 2: Drawing from Name
Question: Draw the structural formula of methyl ethanoate. Show all bonds. [2 marks]
Solution:
- Methyl (from CH3OH) + ethanoate (from CH3COOH)
- Structural formula: CH3COOCH3
- Or displayed:
H O
| ||
H - C - C - O - C - H
| |
H H
Example 3: Writing the Equation
Question: Write the balanced equation for the reaction between methanol and ethanoic acid. [2 marks]
Solution: CH3OH + CH3COOH ⇌ CH3COOCH3 + H2O ✓
(Methanol + Ethanoic acid ⇌ Methyl ethanoate + Water)
Example 4: Property Explanation
Question: Explain why esters are used in perfumes. [2 marks]
Solution:
- Esters are volatile (evaporate easily) ✓ — so the smell travels through the air to the nose ✓
- Esters have pleasant, fruity smells ✓ (any two of these three marking points)
Practice Questions
-
Write a balanced equation for the reaction between ethanoic acid and ethanol. Include the catalyst above the arrow. [2 marks]
-
Name the ester with the formula HCOOC2H5. [1 mark]
-
Give the structural formula of propyl ethanoate. [1 mark]
-
State two properties of esters that make them useful as flavourings. [2 marks]
-
Ethyl butanoate smells of pineapple. Draw its structural formula, given that it is formed from ethanol and butanoic acid. [2 marks]
-
Explain why concentrated sulfuric acid is used in the preparation of esters. [2 marks]
Key Facts to Memorise
- General formula: RCOOR’ (R from acid, R’ from alcohol)
- Esterification: Carboxylic acid + Alcohol ⇌ Ester + Water (condensation reaction)
- Conditions: Concentrated H2SO4 catalyst, warm
- H2SO4 acts as a catalyst AND a dehydrating agent (removes water, increases yield)
- Naming: First part from alcohol (-yl), second part from acid (-oate)
- Properties: Volatile, fruity/sweet smells, lower bp than carboxylic acids, flammable
- Uses: Perfumes, food flavourings, solvents
- Most commonly tested ester: ethyl ethanoate (from ethanol + ethanoic acid)
Common Misconceptions
| Students often think… | But the correct understanding is… |
|---|---|
| ”The alcohol gives the -oate part of the name” | The ALCOHOL gives the -yl (first) part. The ACID gives the -oate (second) part. e.g., ethyl ethanoate = ethanol + ethanoic acid |
| ”Esterification is the same as neutralisation because it makes a ‘salt-like’ compound” | Esterification is a condensation reaction making an ester + water. Neutralisation is acid + base making salt + water. They are different types of reactions |
| ”Esters have high boiling points because they contain a C=O group” | Esters have LOWER boiling points than the parent carboxylic acids because esters cannot form hydrogen bonds between their own molecules |
| ”The H2SO4 is only a catalyst” | H2SO4 is a catalyst AND a dehydrating agent — it removes water, shifting equilibrium to increase ester yield |
| ”The ester name is ‘ethyl ethanoic acid‘“ | The name ends in -oate (e.g., ethyl ethanoate), NOT -oic acid. The -oic acid name is for the parent carboxylic acid |
Key Concepts from Past Papers
Definitions You MUST Know (Exact Mark Scheme Wording)
- Esterification: the reaction between a carboxylic acid and an alcohol to form an ester and water
- Condensation reaction: a reaction in which two molecules join together with the elimination of a small molecule (usually water)
- Volatile: evaporates easily at room temperature
Recurring Mark Scheme Answers
- “Concentrated sulfuric acid acts as a catalyst AND as a dehydrating agent”
- “Ethyl ethanoate = ethanol + ethanoic acid”
- “Esters are used in perfumes because they are volatile and have pleasant/fruity smells”
- “The reaction is reversible; an excess of alcohol is used to increase the yield of ester”
- (property on which fractional distillation) depends is the boiling point AND 1 mark each for any two of:
- idea of (fractional distillation) column
- petroleum heated / petroleum vaporised
- petroleum vapour enters at bottom of fractionating column
- temperatures high at bottom of column and low at the top
- different fractions / compounds move different distances
- vapour turns to liquid in condenser / vapour turns to liquid when it reaches a certain height
- with methane (aqueous) bromine remains orange with ethene (aqueous) bromine decolourised
Common Question Types
- Short recall (state/give/name): “Name the ester formed from methanol and ethanoic acid”
- Extended writing (describe/explain/suggest): “Describe how ethyl ethanoate can be prepared in the laboratory”
- Drawing/structural formula: “Draw the displayed formula of methyl propanoate”
- Equation writing: “Write the equation for the formation of ethyl ethanoate”
- Property explanation: “Explain why esters are used in perfumes”
Exam Tips
- This topic appears in 27 papers in the database
- Total Q+A entries: 65
- Average marks per question: 1.0
- Always show the C=O double bond clearly when drawing esters — a common mark-losing error is drawing C-O instead of C=O
- For the naming question, remember: alcohol first (-yl), acid second (-oate)
- The equation MUST include the reversible arrow (⇌) — the reaction is an equilibrium, not a one-way reaction
- When asked about uses of esters, “volatile” and “fruity smell” are the two key marking points for perfumes/flavourings
Keywords from Past Papers
methane, ethene, point, bonds, carbon, fractions, energy, hydrocarbons, boiling, idea, structure, water, alcohol, addition, poly
Related Notes
- Organic Chemistry Fundamentals — Functional groups, homologous series, naming
- Alkanes — Saturated hydrocarbons
- Alkenes — Unsaturated hydrocarbons (C=C)
- Alcohols — -OH functional group, ethanol production
- Carboxylic Acids — -COOH functional group (needed for esterification)
- Polymers — Condensation polymers (polyesters like terylene)
- IGCSE-Chem-Index — Full IGCSE Chemistry index
Sources
- OpenStax Chemistry 2e — [Chapter 20: Organic Chemistry (Esters)], Rice University (free, CC BY 4.0)
- BBC Bitesize GCSE Chemistry — [Esters], BBC (free educational resource)
- Cambridge IGCSE Chemistry 0620 — Syllabus Section 11: Organic Chemistry (Esters), Cambridge Assessment International Education
- CK-12 Chemistry for High School — [Chapter 25: Esters], CK-12 Foundation (free, CC BY-NC 3.0)
Past Paper Sources
- 0620/32 Feb/March 2017: Q33(b)(iv) (1m), Q33(c)(iii) (1m)
- 0620/32 Feb/March 2018: Q66(a)(i) (1m)
- 0620/32 Feb/March 2019: Q55(d)(iii) (1m)
- 0620/32 Feb/March 2020: Q11(a)(ii) (1m), Q11(a)(v) (1m)
- 0620/32 Feb/March 2021: Q66(c)(i) (1m)
- 0620/32 Feb/March 2022: Q22(a)(ii) (2m), Q77(b)(i) (1m)
- 0620/32 Feb/March 2023: Q66(a)(iv) (2m)
- 0620/32 May/June 2018: Q11(a)(v) (1m), Q33(a)(iv) (1m), Q33(c)(ii) (0m)
- 0620/33 May/June 2016: Q44(c)(iv) (1m), Q66(e)(i) (1m)
- 0620/33 May/June 2017: Q44(a)(ii) (1m)
- 0620/33 May/June 2019: Q11(a)(i) (2m), Q11(a)(iv) (1m), Q11(a)(vi) (1m) (+3 more)
- 0620/33 May/June 2021: Q44(a)(iii) (1m), Q44(d)(i) (1m), Q44(d)(ii) (1m) (+1 more)