4.3Unit 2 · Organic Chemistry
Esters — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
5 min read 4 parts Lesson 3 of 4 1 quiz
Lesson summary
Recognise esters from their names, formulae and functional group.Ester functional group–COO–.Ethanol + ethanoic acid ⇌ ethyl ethanoate + water.CH3COOH + C2H5OH → CH3COOC2H5 + H2O.Catalysta few drops of concentrated sulfuric acid.Esters are volatile with sweet, fruity smells.Usesf…
- What are esters?
- Making esters
- Where the atoms go
- Naming esters
- Practical: preparation of ethyl ethanoate
- Properties and uses
Section 1 of 4Esters
What are esters?
Esters are a homologous series containing the –COO– functional group. They are formed when an alcohol reacts with a carboxylic acid. The simplest example you need is ethyl ethanoate, CH3COOC2H5.Making esters:
alcohol + carboxylic acid → ester + water.- A few drops of concentrated sulfuric acid act as a catalyst.
- The reaction is reversible, so the yield is not 100%.
- Water is lost as the molecules join, so this is a condensation reaction.
Where the atoms go:
the –OH from the acid and the H from the alcohol's –OH combine to form water; the remaining parts join through the new C–O bond.Naming esters:
the first word comes from the alcohol (ethanol → ethyl), the second from the acid (ethanoic acid → ethanoate). In the formula the acid part is written first: CH3COO– then –C2H5.Practical: preparation of ethyl ethanoate:
warm ethanol and ethanoic acid with a few drops of concentrated sulfuric acid in a water bath.- Pour into sodium carbonate solution to neutralise acids, then waft to detect the fruity smell.
- Distillation can separate the ester.
Safety:
concentrated sulfuric acid is corrosive; ethanol and esters are flammable, so a water bath or electric heater is used instead of a Bunsen flame. Wear eye protection.Properties and uses:
esters are volatile liquids with pleasant, sweet smells. They are used as food flavourings, in perfumes and cosmetics, and as solvents (e.g. in nail-varnish remover and glues).Working backwards:
to find the reactants for an ester, the -yl word gives the alcohol and the -oate word gives the acid. Methyl propanoate, CH3CH2COOCH3, comes from methanol and propanoic acid.Distilling as it forms:
esters have low boiling points, so they evaporate first. Distilling the ester off as it forms removes it from the mixture, which stops the reverse reaction and improves the yield.Purifying the distillate:
the distillate may also contain some ethanoic acid, sulfuric acid and ethanol. Add sodium carbonate solution until fizzing stops to remove the acids; add calcium chloride solution to remove ethanol.General structure:
an ester can be written R–COO–R′, where R comes from the acid and R′ from the alcohol. Esters are oily liquids that do not mix well with water.
Section 2 of 4Process & sequence
- Mix ethanol and ethanoic acid in a boiling tube or flask.
- Add a few drops of concentrated sulfuric acid as a catalyst.
- Warm gently in a water bath (no flames: ethanol is flammable) for a few minutes.
- Pour the mixture into a beaker of sodium carbonate solution to neutralise leftover acid.
- Carefully smell by wafting: a sweet, fruity odour shows an ester.
- For a purer sample, the ester can be distilled off and collected.
Preparing ethyl ethanoate — write these steps in this order; each step is a marking point.
- Mix ethanol and ethanoic acid
- Add conc. sulfuric acid catalyst
- Warm in water bath
- Pour into sodium carbonate
- Waft to smell fruity ester
Section 3 of 4Compare
| Alcohol | Carboxylic acid | Ester | Formula |
|---|---|---|---|
| Methanol | Ethanoic acid | Methyl ethanoate | CH3COOCH3 |
| Ethanol | Ethanoic acid | Ethyl ethanoate | CH3COOC2H5 |
| Ethanol | Methanoic acid | Ethyl methanoate | HCOOC2H5 |
| Propanol | Ethanoic acid | Propyl ethanoate | CH3COOC3H7 |
- Name = alcohol part (-yl) first, then acid part (-oate).
Section 4 of 4Worked examples
Name the ester formed from propanol and methanoic acid and write its formula.
- Propanol gives 'propyl'; methanoic acid gives 'methanoate'. Ester: propyl methanoate, HCOOC3H7.
Explain why the reaction mixture is poured into sodium carbonate solution.
- Sodium carbonate neutralises unreacted ethanoic acid and the sulfuric acid catalyst, so their sharp smell does not hide the fruity ester smell.
Marked practiceQuizzes for this lesson
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