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Alkenes, Alcohols and Carboxylic Acids » Alcohols

What you'll learn this session

Study time: 30 minutes

AQA spec: 4.7.2.3

The -OH group and the first four alcohols
How alcohols react with sodium, water, oxygen and oxidising agents
The main uses of alcohols
How ethanol is made by fermentation using yeast

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What is an alcohol?

Alcohols are a family of compounds that all contain the functional group -OH. This one group gives every alcohol its typical reactions. Alcohols form a homologous series, so each one is one CH₂ bigger than the one before.

The first four members are methanol, ethanol, propanol and butanol. Notice that every name ends in -ol. That ending is how you recognise an alcohol from its name.

Alcohol	Carbon atoms	Formula
Methanol	1	CH₃OH
Ethanol	2	CH₃CH₂OH
Propanol	3	CH₃CH₂CH₂OH
Butanol	4	CH₃CH₂CH₂CH₂OH

You can also write ethanol as C₂H₅OH. In a displayed formula, the -OH group is drawn as an oxygen atom joined to a hydrogen atom, and the oxygen is joined to a carbon atom.

Key terms:

Alcohol: a compound that contains the -OH functional group.

Spotting an alcohol

To recognise an alcohol, look for the -OH group on a carbon chain. Look for a name that ends in -ol, too.

CH₃CH₂OH is an alcohol (ethanol).
C₃H₇OH is an alcohol (propanol).
CH₃CH₃ is not an alcohol. It has no -OH group.

You only need to know the names of the first four alcohols.

Reactions of alcohols

Fizzing sodium in ethanol gives off hydrogen - gentler than sodium in water, but the same idea

⚛ With sodium

Sodium fizzes in an alcohol and hydrogen gas is given off. The reaction is gentler than sodium with water.

💧 Added to water

Methanol, ethanol and propanol mix completely with water to give a clear solution. Butanol is less soluble because its carbon chain is longer.

🔥 Burning in air

Alcohols burn well. Complete combustion gives carbon dioxide and water.

⚖ With an oxidising agent

Alcohols are oxidised to carboxylic acids. For example, ethanol becomes ethanoic acid.

Burning alcohols

When an alcohol burns in plenty of air, it makes carbon dioxide and water. The reaction gives out heat.

Ethanol + oxygen → carbon dioxide + water

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

These are the combustion equations for the other three:

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O
2C₃H₇OH + 9O₂ → 6CO₂ + 8H₂O
C₄H₉OH + 6O₂ → 4CO₂ + 5H₂O

Good to know

You only need to write balanced equations for the combustion of alcohols. For the other reactions, a description is enough.

Worked example

Balance the equation for burning propanol.
Start with C₃H₇OH + O₂ → CO₂ + H₂O.
1. Carbon: 3 carbon atoms, so 3CO₂.
2. Hydrogen: C₃H₇OH has 8 hydrogen atoms, so 4H₂O.
3. Oxygen on the right: 6 + 4 = 10. The alcohol already has 1, so oxygen gas must supply 9. That is 4.5O₂.
4. Double everything to remove the half: 2C₃H₇OH + 9O₂ → 6CO₂ + 8H₂O.

Oxidising alcohols

An oxidising agent adds oxygen to a molecule. When an alcohol reacts with an oxidising agent, it is oxidised and a carboxylic acid is made. This is why wine left open to the air can turn sour: the ethanol is slowly oxidised to ethanoic acid, the acid in vinegar.

Ethanol → ethanoic acid

You will meet carboxylic acids in the next lesson.

Uses of alcohols

Fuels: alcohols burn well and give out heat. Ethanol is used as a fuel, for example mixed with petrol.
Solvents: alcohols dissolve many substances that water cannot. Ethanol is used as a solvent in perfumes and aftershaves.
Drinks: ethanol is the alcohol in alcoholic drinks such as beer and wine.

Making ethanol by fermentation

Those bubbles are yeast hard at work: glucose → ethanol + carbon dioxide, in warm conditions with no air

Fermentation makes an aqueous solution of ethanol from a sugar solution, using yeast. Yeast is a living organism. It contains enzymes that turn sugar into ethanol and carbon dioxide.

Glucose → ethanol + carbon dioxide

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

You need to know the conditions for fermentation:

Yeast is added to the sugar solution.
A warm temperature, about 30 °C to 40 °C. This suits the yeast enzymes.
No oxygen (anaerobic). The mixture is kept in a closed container with a gap for carbon dioxide to escape.

If it is too cold, the yeast works too slowly. If it is too hot, the enzymes are damaged and fermentation stops.

The result is a weak solution of ethanol in water. Ethanol can be separated from water by fractional distillation.

Key terms:

Fermentation: using yeast to turn sugar into ethanol and carbon dioxide without oxygen.

Common mistakes

1. Forgetting the oxygen already in the alcohol when balancing combustion equations.
2. Saying fermentation needs oxygen. It must be kept air-free.
3. Giving a temperature that is too high. Yeast works best at about 30 °C to 40 °C.
4. Calling any compound with a long name an alcohol. Look for -OH and an -ol ending.

Exam-style question

Ethanol can be made by fermentation. (a) Name the substance added to the sugar solution. (b) Give two conditions needed. (c) Describe what you would see when sodium is added to ethanol. (d) Write the balanced equation for the complete combustion of ethanol.

Model answer

(a) Yeast.
(b) A temperature of about 30 °C to 40 °C; no oxygen (anaerobic).
(c) The sodium fizzes (bubbles of hydrogen gas are given off).
(d) C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Exam tip

If a question asks you to describe a reaction of an alcohol, say what you would see: fizzing for sodium and a clear solution for the smaller alcohols in water. For an oxidising agent, say the alcohol is oxidised to a carboxylic acid.

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