Organic chemistry: alcohols — structure, properties and uses

What you'll learn

This revision guide covers everything you need to know about alcohols for AQA GCSE Chemistry. You'll learn how to identify the alcohol functional group, name and draw the first three alcohols, understand their physical and chemical properties, and explain their important uses in industry and everyday life. This topic connects to organic chemistry fundamentals and prepares you for questions worth 4-6 marks in your exams.

Key terms and definitions

Alcohol — an organic compound containing the hydroxyl functional group (-OH) attached to a carbon atom

Functional group — an atom or group of atoms responsible for the characteristic chemical reactions of a compound

Homologous series — a family of organic compounds with the same general formula and similar chemical properties, differing by CH₂

Hydroxyl group — the -OH functional group that defines alcohols

Aqueous solution — a solution where water is the solvent, often used when dissolving alcohols

Combustion — a chemical reaction where a substance reacts with oxygen, releasing energy as heat and light

Oxidation — a chemical reaction where a substance gains oxygen or loses hydrogen; alcohols can be oxidised to carboxylic acids

Fermentation — the anaerobic breakdown of glucose by yeast to produce ethanol and carbon dioxide

Core concepts

Structure and naming of alcohols

Alcohols belong to a homologous series with the general formula CₙH₂ₙ₊₁OH. The defining feature is the hydroxyl group (-OH) bonded to a carbon atom in the chain.

The first three alcohols you must know are:

Methanol (CH₃OH)

• One carbon atom
• Structural formula: CH₃OH
• Displayed formula shows C bonded to three H atoms and one OH group

Ethanol (C₂H₅OH)

• Two carbon atoms
• Structural formula: CH₃CH₂OH or C₂H₅OH
• Most commonly encountered alcohol in GCSE exams

Propanol (C₃H₇OH)

• Three carbon atoms
• Structural formula: CH₃CH₂CH₂OH or C₃H₇OH
• Follows the same pattern as methanol and ethanol

The naming system follows a pattern: the stem (meth-, eth-, prop-) indicates the number of carbon atoms, whilst the suffix -anol identifies the compound as an alcohol.

When drawing displayed formulae, ensure you show:

• All carbon-carbon bonds
• All carbon-hydrogen bonds
• The carbon-oxygen bond in the hydroxyl group
• The oxygen-hydrogen bond in the hydroxyl group

Physical properties of alcohols

Alcohols have distinctive physical properties that differ from alkanes with similar numbers of carbon atoms.

Solubility in water

The first three alcohols (methanol, ethanol, propanol) dissolve completely in water to form neutral aqueous solutions. This high solubility occurs because:

• The hydroxyl group can form hydrogen bonds with water molecules
• The -OH group is polar, making alcohols polar molecules
• "Like dissolves like" — polar alcohols dissolve in polar water

As the carbon chain lengthens beyond propanol, solubility decreases because the non-polar hydrocarbon portion becomes more significant.

Boiling points

Alcohols have relatively high boiling points compared to alkanes with similar molecular mass:

• Methanol: 65°C
• Ethanol: 78°C
• Propanol: 97°C

These elevated boiling points result from hydrogen bonding between alcohol molecules. The -OH groups allow hydrogen bonds to form, requiring more energy to separate molecules during boiling.

Volatility

Methanol and ethanol are volatile liquids at room temperature, meaning they evaporate easily. This property makes them useful as solvents and fuels.

Chemical reactions of alcohols

Alcohols undergo several important reactions at GCSE level.

Combustion reactions

Alcohols burn in oxygen to produce carbon dioxide and water, releasing energy. These are combustion reactions.

Complete combustion of ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Complete combustion of methanol: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

Key points about alcohol combustion:

• Produces large amounts of energy (exothermic reactions)
• Alcohols burn with a clean, pale blue flame
• Products are always carbon dioxide and water (in complete combustion)
• Used as a basis for alcohol fuels

Oxidation reactions

Alcohols can be oxidised to form carboxylic acids using oxidising agents such as acidified potassium dichromate or acidified potassium manganate(VII).

Oxidation of ethanol produces ethanoic acid: C₂H₅OH + 2[O] → CH₃COOH + H₂O

or written with full formulae: ethanol + oxygen → ethanoic acid + water

This oxidation reaction occurs:

• When alcoholic drinks are left open to air (microbes catalyse the reaction)
• When acidified potassium dichromate is added to an alcohol and warmed
• During the industrial production of vinegar (ethanoic acid solution)

Observable changes during oxidation with acidified potassium dichromate:

• Colour changes from orange to green
• The alcohol is oxidised
• The dichromate ion is reduced

Production of ethanol

Ethanol can be produced by two main methods, each with advantages and disadvantages.

Fermentation

Fermentation is the breakdown of glucose by yeast in the absence of oxygen (anaerobic respiration).

Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ glucose → ethanol + carbon dioxide

Conditions required:

• Temperature: 25-35°C (optimum around 30°C)
• Absence of oxygen (anaerobic conditions)
• Yeast present as a catalyst
• Aqueous solution of glucose

Key features of fermentation:

• Stops when ethanol concentration reaches approximately 15% (alcohol kills yeast)
• Renewable process using plant-based glucose from sugar cane or grains
• Produces ethanol for alcoholic drinks
• Relatively slow process
• Requires fractional distillation to concentrate the ethanol

Hydration of ethene

Ethanol can be produced industrially by reacting ethene with steam in the presence of a catalyst.

Equation: C₂H₄ + H₂O → C₂H₅OH ethene + steam → ethanol

Conditions required:

• Temperature: 300°C
• Pressure: 60-70 atmospheres
• Phosphoric acid catalyst

Key features of hydration:

• Fast, continuous process
• Produces pure ethanol directly
• Uses ethene from crude oil (non-renewable)
• Requires high temperatures and pressures (energy-intensive)
• 100% atom economy in theory

Comparing the two methods

Factor	Fermentation	Hydration of ethene
Raw material	Glucose (renewable)	Ethene (non-renewable)
Rate	Slow (batch process)	Fast (continuous)
Purity	Impure (~15%) requires distillation	Pure product
Conditions	Low temperature, atmospheric pressure	High temperature and pressure
Sustainability	Carbon neutral if using plant sources	Depends on finite crude oil

Uses of alcohols

Ethanol applications

Ethanol is the most commercially important alcohol with diverse uses:

• Alcoholic drinks: beer, wine, spirits contain ethanol produced by fermentation
• Solvent: dissolves substances that don't dissolve in water (e.g., perfumes, aftershaves, ink)
• Fuel: bioethanol added to petrol reduces dependence on fossil fuels; used pure in some countries
• Chemical feedstock: used to manufacture other organic compounds

Methanol applications

Methanol has important industrial uses but is highly toxic:

• Fuel: can be used in racing cars and as a petrol additive
• Chemical feedstock: used to produce other chemicals including plastics
• Solvent: in industry for paints and resins

Health and safety considerations

• Ethanol in drinks is toxic in large quantities; causes liver damage with prolonged use
• Methanol is extremely toxic; even small amounts can cause blindness or death
• Both alcohols are flammable and should be kept away from naked flames
• All alcohols should be handled in well-ventilated areas due to volatile nature

Alcohols and the environment

Biofuels

Ethanol produced from fermentation of plant material is considered a biofuel:

Advantages:

• Carbon neutral cycle: CO₂ released during combustion was absorbed during plant growth
• Renewable resource (can grow more crops)
• Reduces dependence on crude oil
• Can be mixed with petrol for use in existing engines

Disadvantages:

• Land used for fuel crops could grow food crops
• Energy needed to harvest and process crops
• Fertilisers for crops may have environmental impacts
• Not truly carbon neutral when considering agricultural processes

Worked examples

Example 1: Drawing and naming alcohols (2 marks)

Question: Draw the displayed formula of propanol and state its molecular formula.

Answer:

    H   H   H
    |   |   |
H — C — C — C — O — H
    |   |   |
    H   H   H

Molecular formula: C₃H₇OH or C₃H₈O

Mark scheme:

• 1 mark for correct displayed formula showing all atoms and bonds
• 1 mark for correct molecular formula

Example 2: Comparing production methods (4 marks)

Question: Ethanol can be produced by fermentation of glucose or by hydration of ethene. Compare these two methods in terms of:

• Rate of production
• Sustainability of raw materials

Answer:

Rate of production:

• Hydration of ethene is a fast, continuous process (✓)
• Fermentation is a slow, batch process (✓)

Sustainability:

• Fermentation uses glucose from plants which is a renewable resource (✓)
• Hydration uses ethene from crude oil which is non-renewable/finite (✓)

Mark scheme: 1 mark for each valid comparison point (maximum 4)

Example 3: Oxidation reaction (3 marks)

Question: When ethanol is oxidised using acidified potassium dichromate solution, ethanoic acid is formed.

a) Write a balanced equation for this reaction using [O] to represent the oxidising agent. (2 marks)

b) State the colour change observed during this reaction. (1 mark)

Answer:

a) C₂H₅OH + 2[O] → CH₃COOH + H₂O (✓✓)

Alternative acceptable: ethanol + 2[O] → ethanoic acid + water

b) Orange to green (✓)

Mark scheme:

• 2 marks for correct balanced equation (1 mark if unbalanced but correct substances)
• 1 mark for correct colour change from orange to green

Common mistakes and how to avoid them

• Confusing alcohols with alkanes: Remember alcohols contain the -OH functional group; alkanes contain only C-H and C-C bonds. Always check for the hydroxyl group.

• Incorrect naming: The suffix is -anol, not -ane. Methanol, ethanol, propanol — not methane-ol. The -anol ending indicates the alcohol functional group.

• Missing atoms in displayed formulae: When drawing alcohols, students often forget to show all hydrogen atoms or draw the OH group incorrectly. Count carefully: propanol has 8 hydrogen atoms total (7 on carbons, 1 in OH group).

• Muddling fermentation conditions: Fermentation requires the absence of oxygen, not the presence of it. Yeast performs anaerobic respiration. If oxygen is present, yeast will respire aerobically instead, producing CO₂ and water rather than ethanol.

• Incomplete combustion equations: In complete combustion, alcohols always produce CO₂ and H₂O. Balance equations carefully, ensuring oxygen atoms on both sides match.

• Forgetting the catalyst in industrial processes: The hydration of ethene requires a phosphoric acid catalyst at high temperature and pressure. Don't confuse these conditions with fermentation conditions.

Exam technique for "Organic chemistry: alcohols — structure, properties and uses"

• "Compare" questions require you to make direct comparisons between two things. Use comparative language like "whereas," "however," or "in contrast." For comparing fermentation and hydration, discuss both methods for each point. A comparison of rates earns no marks if you only discuss one method.

• Drawing displayed formulae questions award marks for correct bonds and atoms. Show every bond as a line, include all atoms (especially hydrogen), and ensure the -OH group is clearly attached to a carbon atom. Check your diagram shows the correct number of each atom type.

• "Suggest why" or "Explain" command words require reasons or mechanisms. For example, explaining why alcohols dissolve in water needs mention of the polar hydroxyl group and hydrogen bonding — stating "they just do" earns zero marks.

• Equation questions may accept word equations or symbol equations at GCSE, but balanced symbol equations often earn more marks. Always check your balancing by counting atoms on each side. State symbols are sometimes required — read the question carefully.

Quick revision summary

Alcohols are organic compounds containing the -OH hydroxyl functional group, forming a homologous series with general formula CₙH₂ₙ₊₁OH. The first three members are methanol, ethanol and propanol. They dissolve in water, have relatively high boiling points due to hydrogen bonding, and undergo combustion and oxidation reactions. Ethanol is produced industrially by hydration of ethene or by fermentation of glucose using yeast. Both methods have advantages: fermentation uses renewable resources whilst hydration is faster. Alcohols are used as fuels, solvents and in alcoholic drinks.