AQA A Level Chemistry 3.3.5 Alcohols Exam Questions

About AQA A Level Chemistry 3.3.5 Alcohols

Alcohols link hydrocarbons to carbonyl compounds and acids in the AQA A Level organic course. You need to compare fermentation and the hydration of ethene for producing ethanol, including the debate about whether bioethanol is carbon neutral. You classify alcohols as primary, secondary or tertiary, and use acidified potassium dichromate(VI) to oxidise them to aldehydes, carboxylic acids or ketones. You also dehydrate alcohols to alkenes. Required practical 5 (distillation of a product) and Required practical 6 (tests for alcohols, aldehydes, alkenes and carboxylic acids) apply here. Examiners test oxidation equations using [O], apparatus choices and explanations of observations.

What you need to know

• Production of ethanol Fermentation: C6H12O6 → 2C2H5OH + 2CO2, using yeast at about 35 °C without oxygen. Hydration of ethene uses steam and a phosphoric acid catalyst at high temperature and pressure; the mechanism is electrophilic addition.
• Carbon neutrality Bioethanol is sometimes called carbon neutral because the CO2 released on burning was absorbed by the plants during photosynthesis. In practice, farming, transport and processing also release CO2.
• Classification Primary alcohols have the OH carbon bonded to one other carbon, secondary to two and tertiary to three.
• Oxidation Acidified K2Cr2O7 turns from orange to green. Primary alcohols give aldehydes (distil off as formed) and then carboxylic acids (heat under reflux with excess oxidant). Secondary alcohols give ketones. Tertiary alcohols are not oxidised.
• Tests for aldehydes Tollens' reagent gives a silver mirror and Fehling's solution changes from blue to a brick-red precipitate with aldehydes but not with ketones.
• Dehydration Heating with concentrated phosphoric or sulfuric acid eliminates water to form an alkene. This offers a route to polymers that does not rely on crude oil.

Common mistakes in 3.3.5 Alcohols

• Heating under reflux when the aldehyde is wanted. Use distillation to remove the aldehyde as it forms, before it is oxidised further.
• Describing the dichromate colour change the wrong way round. It goes from orange to green.
• Forgetting to balance oxidation equations with [O] and water, e.g. CH3CH2OH + 2[O] → CH3COOH + H2O.
• Stating that tertiary alcohols can be oxidised to ketones. Tertiary alcohols do not react with acidified dichromate.
• Claiming bioethanol is completely carbon neutral without mentioning CO2 from farming, fertilisers, transport and distillation.

Try three 3.3.5 Alcohols exam-style questions