4.1Unit 2 · Organic Chemistry
Alcohols — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 1 of 4 1 quiz
Lesson summary
Recognise alcohols from their names, formulae and the –OH group.General formula of alcoholsCnH2n+1OH; functional group –OH.FermentationC6H12O6 → 2C2H5OH + 2CO2.HydrationC2H4 + H2O → C2H5OH (300 °C, 60–70 atm, H3PO4).Complete combustionC2H5OH + 3O2 → 2CO2 + 3H2O.Acidified potassiu…
- What are alcohols?
- Properties of alcohols
- Oxidation by combustion
- Oxidation by microbes
- Oxidation by potassium dichromate(VI)
- Manufacture by fermentation
Section 1 of 4Alcohols
What are alcohols?
Alcohols are a homologous series with the hydroxyl functional group, –OH, and general formula CnH2n+1OH.- Their names end in –ol: methanol, ethanol, propanol, butanol.
- Propanol and butanol can have the –OH on different carbons (isomers).
Properties of alcohols:
short-chain alcohols are colourless liquids that mix with water. They are used as fuels, solvents (e.g. in perfumes) and ethanol is found in alcoholic drinks.Oxidation by combustion:
ethanol burns in plenty of oxygen with a clean blue flame to give carbon dioxide and water. Here all of the ethanol is oxidised.Oxidation by microbes:
when an alcoholic drink is left open to air, bacteria use oxygen from the air to oxidise ethanol to ethanoic acid. This is why wine turns into vinegar.Oxidation by potassium dichromate(VI):
heating ethanol with acidified potassium dichromate(VI) oxidises it to ethanoic acid. The dichromate is reduced and changes colour from orange to green, which can be used to show an alcohol is present.Manufacture by fermentation:
yeast converts glucose into ethanol and carbon dioxide at 30–40 °C in anaerobic conditions.- Too cold is slow; too hot denatures the enzymes.
- Oxygen must be excluded or ethanol would be oxidised to ethanoic acid.
- The dilute ethanol is concentrated by fractional distillation.
Manufacture by hydration of ethene:
ethene (from cracking crude oil fractions) reacts with steam at about 300 °C and 60–70 atm over a phosphoric acid catalyst. It is a continuous process giving pure ethanol and has a 100% atom economy.Comparing methods:
fermentation uses renewable resources and low temperatures but is slow, a batch process, and needs distillation.- Hydration is fast and continuous and gives pure product but uses a finite resource and needs high energy input.
Solutions of alcohols:
short-chain alcohols dissolve in water to give neutral solutions (pH 7), unlike carboxylic acids, which give acidic solutions.Naming position:
the full names of the third and fourth alcohols are propan-1-ol and butan-1-ol; the 1 shows the –OH is on the end carbon. Propanol and butanol are accepted in the exam.Spirit burners:
ethanol burns with an almost invisible blue flame and little smell, so it is used as the fuel in laboratory spirit burners.Microbial oxidation equation:
bacteria in air carry out aerobic oxidation: ethanol + oxygen → ethanoic acid + water, C2H5OH + O2 → CH3COOH + H2O.Dichromate oxidation equation:
the oxidising agent is written as [O]: CH3CH2OH + 2[O] → CH3COOH + H2O. The reaction is slow, so the mixture is heated for some time with a condenser above the flask so volatile liquids do not escape (reflux).Separating products of hydration:
the mixture leaving the reactor contains ethanol, steam and unreacted ethene.- Cooling condenses ethanol and water, while ethene (b.p. about −100 °C) stays a gas and is recycled.
- Ethanol (b.p.
- 78 °C) is then separated from water by fractional distillation.
Section 2 of 4Process & sequence
- Dissolve glucose (sugar) in warm water in a flask.
- Add yeast, which contains enzymes that catalyse fermentation.
- Fit an airlock so carbon dioxide escapes but air (oxygen) cannot enter.
- Keep the mixture at about 30–40 °C for several days.
- Fermentation stops when ethanol concentration reaches about 15%, poisoning the yeast.
- Filter off the yeast, then fractionally distil to concentrate the ethanol.
Making ethanol by fermentation — write these steps in this order; each step is a marking point.
- Glucose solution plus yeast
- Keep at 30-40 C
- Exclude oxygen with airlock
- Ethanol and CO2 form
- Filter off yeast
- Fractionally distil ethanol
Section 3 of 4Compare
| Feature | Fermentation | Hydration of ethene |
|---|---|---|
| Raw material | Sugars (glucose) from crops – renewable | Ethene from crude oil – non-renewable |
| Conditions | 30–40 °C, yeast, no oxygen | 300 °C, 60–70 atm, phosphoric acid catalyst |
| Rate / process | Slow, batch process | Fast, continuous process |
| Purity of product | Impure (about 15%), needs distillation | Pure ethanol |
| Other product | Carbon dioxide | None (addition reaction) |
| Name | Formula | Structure (condensed) |
|---|---|---|
| Methanol | CH3OH | CH3OH |
| Ethanol | C2H5OH | CH3CH2OH |
| Propanol | C3H7OH | CH3CH2CH2OH |
| Butanol | C4H9OH | CH3CH2CH2CH2OH |
- General formula CnH2n+1OH.
Section 4 of 4Worked examples
Calculate the maximum mass of ethanol from 90 g of glucose by fermentation. (Mr: C6H12O6 = 180, C2H5OH = 46)
- Moles glucose = 90 ÷ 180 = 0.50 mol. From C6H12O6 → 2C2H5OH + 2CO2, moles ethanol = 2 × 0.50 = 1.0 mol. Mass = 1.0 × 46 = 46 g.
Suggest why a country with lots of farmland but no oil would choose fermentation.
- Sugar crops can be grown locally and are renewable, so no crude oil needs to be imported.
- The low temperature also needs less energy, even though the process is slower and needs distillation.
- Sugar crops can be grown locally and are renewable, so no crude oil needs to be imported.
Marked practiceQuizzes for this lesson
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