OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.5 Alcohols Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.5 Alcohols: what you need to know, common mistakes and exam-style questions with answers. Covers classifying alcohols, oxidation with acidified dichromate, distillation and reflux, aldehyde tests and elimination to alkenes.
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About OxfordAQA International A Level Chemistry 3.3.5 Alcohols
Alcohols is a Unit 2 organic topic with a strong practical focus. You need to classify alcohols as primary, secondary or tertiary, and describe their oxidation using acidified potassium dichromate(VI): primary alcohols to aldehydes or carboxylic acids depending on the method, secondary alcohols to ketones, and tertiary alcohols not oxidised. You need Fehling's and Tollens' tests to tell aldehydes from ketones, and the acid-catalysed elimination of alcohols to form alkenes. Required practicals 4 (distilling a product from a reaction) and 5 (tests for alcohols, aldehydes, alkenes and carboxylic acids) belong here. It links to halogenoalkanes, alkenes and carbonyl chemistry at A2.
What you need to know
- Classification In a primary alcohol the carbon bonded to OH is attached to one other carbon; secondary to two; tertiary to three.
- Oxidation of primary alcohols With acidified potassium dichromate(VI), primary alcohols form aldehydes if the aldehyde is distilled off as it forms. Heating under reflux with excess oxidising agent gives carboxylic acids. The colour changes from orange to green.
- Secondary and tertiary Secondary alcohols are oxidised to ketones. Tertiary alcohols are not easily oxidised, so the dichromate stays orange.
- Fehling's and Tollens' tests Aldehydes give a red precipitate with Fehling's solution and a silver mirror with Tollens' reagent. Ketones give no change.
- Elimination Alcohols are dehydrated to alkenes by heating with concentrated phosphoric or sulfuric acid. This is an acid-catalysed elimination, and some alcohols give more than one alkene.
- Practical techniques Distillation separates a volatile product as it forms. Reflux heats a mixture for a long time without losing volatile substances, allowing complete reaction.
Common mistakes in 3.3.5 Alcohols
- Saying oxidising a primary alcohol always gives a carboxylic acid. The product depends on whether you distil or reflux.
- Giving the dichromate colour change the wrong way round. It changes from orange to green.
- Saying tertiary alcohols cannot react at all. They resist oxidation but still undergo other reactions such as elimination.
- Using [O] in the wrong place or forgetting water. For example CH3CH2OH + [O] → CH3CHO + H2O.
Try three 3.3.5 Alcohols exam-style questions
1. Write an equation for the oxidation of propan-1-ol to propanoic acid using [O]. (1 mark)
Show the mark scheme answer
CH3CH2CH2OH + 2[O] → CH3CH2COOH + H2O.
2. Describe how you would show that propanal is an aldehyde, not a ketone. (2 marks)
Show the mark scheme answer
Warm with Tollens' reagent; silver mirror forms (or warm with Fehling's solution and a red precipitate forms).
3. Explain why heating under reflux is used to make a carboxylic acid from a primary alcohol. (2 marks)
Show the mark scheme answer
Reflux prevents volatile substances such as the aldehyde from escaping; so the aldehyde is further oxidised to the carboxylic acid.
3.3.5 Alcohols FAQs
What does OxfordAQA International A Level Chemistry 3.3.5 Alcohols cover?
It covers classifying alcohols, oxidation with acidified dichromate, distillation and reflux, aldehyde tests and elimination to alkenes.
Which unit is 3.3.5 Alcohols examined in?
It is examined in Unit 2, the second International AS paper, which covers organic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
What do Required practicals 4 and 5 involve?
Required practical 4 is distilling a product from a reaction, such as an aldehyde from a primary alcohol. Required practical 5 uses test-tube reactions to identify alcohols, aldehydes, alkenes and carboxylic acids.
