Alcohols — A-Level Chemistry Revision

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What is Alcohols?

Alcohols are organic compounds containing the hydroxyl (-OH) functional group. They can be classified as primary, secondary, or tertiary depending on the number of carbon atoms attached to the carbon bearing the -OH group. Their properties are dominated by the polar O-H bond, which allows for hydrogen bonding, leading to relatively high boiling points and solubility in water. Key reactions include combustion, oxidation (with different products for primary, secondary, and tertiary alcohols), and elimination (dehydration) to form alkenes.

Board notes: All boards cover the classification, naming, and characteristic reactions of alcohols. AQA often includes questions on the production of ethanol by fermentation and its use as a biofuel. Edexcel may focus on the different methods for oxidising primary alcohols to obtain either an aldehyde or a carboxylic acid. OCR frequently tests the mechanism of elimination (dehydration) reactions.

Worked example

Common mistakes

• 1Confusing the oxidation products of different types of alcohols. Primary alcohols can be oxidised to aldehydes (with distillation) or further to carboxylic acids (with reflux). Secondary alcohols are oxidised to ketones. Tertiary alcohols are resistant to oxidation.
• 2Forgetting the specific conditions required for different reactions. For example, oxidation requires an oxidising agent like acidified potassium dichromate(VI), while elimination requires concentrated sulfuric or phosphoric acid and heat.
• 3Incorrectly naming alcohols, especially more complex branched structures. The principal chain must be the longest one that contains the -OH group, and the carbon atom attached to the -OH group should be given the lowest possible number.

Alcohols exam questions

Exam-style questions for Alcohols with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• Why are alcohols soluble in water?

  Short-chain alcohols are soluble in water because the hydroxyl (-OH) group can form hydrogen bonds with water molecules. However, as the length of the non-polar hydrocarbon chain increases, the solubility decreases.

• How can you distinguish between a primary, secondary, and tertiary alcohol in the lab?

  You can use a mild oxidising agent like acidified potassium dichromate(VI). When warmed, primary and secondary alcohols will be oxidised, and the solution will turn from orange to green. Tertiary alcohols will not react, and the solution will remain orange.