7.7 Alcohols

The -OH functional group, the names and formulae of methanol to butanol, the properties, reactions and uses of alcohols, and making ethanol by fermentation and concentrating it by fractional distillation.

Separate only AQA Edexcel OCR 1 min read

The alcohols

Key terms

Alcohol: an organic compound containing the functional group –OH (hydroxyl).

Alcohols form a homologous series. Their names end in -ol.

Displayed formulae of methanol, ethanol, propanol and butanol, each with an -OH group, and fermentation apparatus: sugar solution with yeast in a warm flask, with the carbon dioxide turning limewater milky

The first four alcohols
Name	Formula
methanol	CH₃OH
ethanol	C₂H₅OH
propanol	C₃H₇OH
butanol	C₄H₉OH

Properties and reactions

Dissolve in water to form neutral solutions (the smaller alcohols mix completely).
Burn in air: ethanol + oxygen → carbon dioxide + water. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
React with sodium: fizzing, giving hydrogen (more slowly than water does).
Are oxidised by oxidising agents (such as acidified potassium dichromate) or by microbes in the air to form carboxylic acids: ethanol → ethanoic acid. This is why wine left open turns to vinegar.

Uses

Fuels, such as ethanol mixed with petrol, and methanol
Solvents, for example in perfumes and hand gels
Ethanol is the alcohol in alcoholic drinks

Making ethanol by fermentation

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

Yeast provides the enzymes.
Temperature about 25–35 °C: too cold and it is slow; too hot and the enzymes are denatured.
No air (anaerobic), otherwise the ethanol is oxidised to ethanoic acid. An air lock lets carbon dioxide out but keeps air out.

Fermentation stops at about 15% ethanol, because the ethanol kills the yeast. A concentrated solution of ethanol is obtained by fractional distillation: ethanol (boiling point 78 °C) evaporates before water (100 °C).

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