Esters
Edexcel International GCSE ChemistryΒ· 4.38Cβ4.43CΒ· 18 min read
1. Ester Structure and Functional Groupβ β βββHL onlyβ± 3 min
Ester Linkage
The covalent functional group defining all esters, formed when a carboxylic acid reacts with an alcohol
Example:
In ethyl ethanoate, the ester linkage connects the ethyl group from ethanol to the ethanoate group from ethanoic acid
All esters follow the general formula RCOOR', where R is an alkyl group (or hydrogen) from the parent carboxylic acid, and R' is an alkyl group from the parent alcohol. Displayed formulae must explicitly show all bonds, including the C=O double bond and C-O single bond in the linkage.
Draw the displayed formula of the ester functional group, showing open bonding sites for attached alkyl groups.
- 1
Draw a central carbon atom with a double bond to one oxygen atom
- 2
Add a single bond from the same carbon atom to a second oxygen atom
- 3
Mark the two open bonding sites: one on the central carbon (attaches to the acid-derived alkyl group) and one on the single-bonded oxygen (attaches to the alcohol-derived alkyl group)
Which functional group is present in all esters?
A -OH
B -COOH
C -COO-
D -C=C-
Reveal answer
C βA is the alcohol functional group, B is the carboxylic acid functional group, D is the alkene functional group.
2. Ester Naming and Formula Writingβ β β ββHL onlyβ± 5 min
Ester naming follows a fixed two-part rule, regardless of the size of the parent reactants. You will be expected to derive ester names from given reactants, and identify parent reactants from given ester names.
Name the ester formed when methanol reacts with propanoic acid, and write its structural formula.
- 1
Extract the alkyl group from the parent alcohol: methanol β methyl
- 2
Replace the -oic acid suffix of the parent acid with -oate: propanoic acid β propanoate
- 3
Combine the two parts to get the ester name: methyl propanoate
- 4
Write the structural formula: CHβCHβCOOCHβ
Identify the parent alcohol and carboxylic acid used to form butyl ethanoate.
- 1
The first part of the ester name (butyl) comes from the alcohol: butanol
- 2
The second part of the ester name (ethanoate) comes from the carboxylic acid: ethanoic acid
Exam tip:
Ester names reverse the order of the reactants: the alcohol-derived group comes first, followed by the acid-derived -oate suffix.
3. Formation of Ethyl Ethanoateβ β β ββHL onlyβ± 4 min
Ethyl ethanoate is the most commonly tested ester, formed via a reversible condensation reaction between ethanol and ethanoic acid, catalysed by concentrated sulfuric acid. One molecule of water is produced as a by-product.
Write the full balanced symbol equation for the formation of ethyl ethanoate, including the catalyst.
- 1
Write the formulae of reactants: ethanoic acid (CHβCOOH) and ethanol (CβHβ OH)
- 2
Write the formulae of products: ethyl ethanoate (CHβCOOCβHβ ) and water (HβO)
- 3
Verify atom balance: 4 C, 10 H, and 3 O atoms on both sides of the equation
- 4
Add concentrated HβSOβ above the reaction arrow to indicate the catalyst
4. Properties and Uses of Estersβ β βββHL onlyβ± 2 min
Esters have two key physical properties that drive their commercial uses: they are highly volatile (evaporate easily at room temperature) and have distinctive sweet, fruity smells.
Food flavourings: Used to replicate the taste and smell of fruits like strawberry, banana, and pineapple in processed foods, confectionery and drinks
Perfumes: Their volatility means fragrant ester molecules travel easily through air to reach the nose, making them ideal for scented products
Explain why esters are used in the production of perfumes.
- 1
Esters have distinct, pleasant fruity smells that are desired in perfumes
- 2
Esters are volatile, so the scented molecules evaporate easily and can be detected by the wearer and people nearby
5. Practical Preparation of Ethyl Ethanoateβ β β β βHL onlyβ± 4 min
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You are expected to recall the simple school lab method for preparing a small sample of ethyl ethanoate, including key safety precautions.
Add equal volumes of ethanol and ethanoic acid to a boiling tube
Add 3-4 drops of concentrated sulfuric acid catalyst to the mixture
Warm the tube gently in a 60Β°C hot water bath for 5 minutes (avoid open flames, as ethanol is flammable)
Pour the resulting mixture into a beaker of cold sodium carbonate solution to neutralise unreacted acid and remove impurities
An insoluble, sweet-smelling oily layer of ethyl ethanoate will form on the surface of the solution
Give one safety precaution for this practical, and explain why it is required.
- 1
Use a hot water bath instead of direct Bunsen burner heating
- 2
Ethanol is highly flammable, so open flames risk ignition of the reactant mixture
6. Common Pitfalls
Wrong move:
Naming esters in the same order as reactants (e.g. ethanol + ethanoic acid β ethanoic ethyl ester)
Why:
Ester names reverse the order of parent reactants, with the alcohol-derived group first.
Correct move:
Name the alcohol alkyl group first, followed by the acid-derived -oate suffix.
Wrong move:
Drawing the ester linkage with incorrect bonding, e.g. -CO-OH or -O-CO- with missing double bond
Why:
The ester functional group is explicitly -COO- with a C=O double bond adjacent to a C-O single bond.
Correct move:
Show all bonds in displayed formulae, including the C=O double bond in the ester linkage.
Wrong move:
Omitting water as a product in esterification balanced equations
Why:
Esterification is a condensation reaction that eliminates one molecule of water per ester molecule formed.
Correct move:
Always include HβO as a product when writing esterification equations.
Wrong move:
Stating dilute sulfuric acid is used as the esterification catalyst
Why:
Only concentrated sulfuric acid acts as an effective catalyst, and also absorbs water to shift equilibrium towards ester products.
Correct move:
Explicitly state 'concentrated sulfuric acid' when naming the catalyst.
Wrong move:
Confusing ester uses with carboxylic acid uses (e.g. stating esters are used as cleaning agents)
Why:
Esters have sweet smells and volatility, while carboxylic acids have acidic cleaning properties.
Correct move:
Only recall the two specified uses: food flavourings and perfumes.
7. Quick Reference Cheatsheet
Concept | Key Exam Fact |
|---|---|
Ester functional group | -COO- (ester linkage) |
Ester naming rule | Alkyl (alcohol) + -oate (carboxylic acid) |
Ethyl ethanoate reactants | Ethanol + ethanoic acid |
Esterification catalyst | Concentrated sulfuric acid |
Key ester properties | Volatile, sweet/fruity smell |
Ester uses | Food flavourings, perfumes |
Practical test for ester | Sweet oily layer on sodium carbonate solution |
8. Frequently Asked
Do I need to state the specific catalyst for esterification?
Yes, you must explicitly name concentrated sulfuric acid for full marks, not just a generic 'acid catalyst'.
Is ester hydrolysis included in the 4CH1 syllabus?
No, only ester formation, properties, uses and naming are required for this sub-topic.
What order do I name the parts of an ester?
The first part of the name comes from the parent alcohol (alkyl group), the second part comes from the parent carboxylic acid with the -oate suffix.
Going deeper
What's Next
Now you have mastered ester content for Edexcel IGCSE 4CH1 Paper 2C, you are ready to progress to related organic chemistry sub-topics. Esters are a core building block for understanding condensation polymerisation, where ester linkages connect monomer units to form polyesters, a common synthetic material. You should also consolidate your knowledge of alcohols and carboxylic acids, as exam questions frequently combine content from these three sub-topics in longer structured responses. Be sure to practice writing displayed formulae for esters and balanced esterification equations, as these are common high-mark questions on Paper 2C.