A student carries out oxidation reactions on two unknown organic compounds, X and Y. Compound X is oxidised by acidified potassium dichromate(VI) solution, producing a compound that gives a positive result with Tollens' reagent. Compound Y is also oxidised by acidified potassium dichromate(VI) solution, but the product does not react with Tollens' reagent. Identify the functional group present in each compound and explain what these observations tell us about the structure of each compound and the products formed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Acidified potassium dichromate(VI) solution is an oxidising agent commonly used in organic chemistry. Tollens' reagent produces a silver mirror when reduced by an aldehyde.
Model answer (5 marks)
Compound X contains an aldehyde group (–CHO). It is oxidised by acidified potassium dichromate(VI) to a carboxylic acid. The product still reacts with Tollens’ reagent because the aldehyde (or its oxidation product) can be further oxidised, reducing Ag⁺ to metallic silver.
Compound Y contains a ketone group (C=O flanked by two carbon groups). Ketones are resistant to oxidation under these conditions, so the product is a ketone that does not react with Tollens’ reagent.
The structural difference is that aldehydes have a hydrogen attached to the carbonyl carbon, allowing further oxidation, whereas ketones have two alkyl groups attached and cannot be oxidised further, explaining the different Tollens’ test results.
Compound Y contains a ketone group (C=O flanked by two carbon groups). Ketones are resistant to oxidation under these conditions, so the product is a ketone that does not react with Tollens’ reagent.
The structural difference is that aldehydes have a hydrogen attached to the carbonyl carbon, allowing further oxidation, whereas ketones have two alkyl groups attached and cannot be oxidised further, explaining the different Tollens’ test results.
Examiner tips
- State the functional group for each compound first. Explain the oxidation products and why they give/ do not give a Tollens’ test. Mention the key structural difference (H vs two R groups).
Common mistakes
- Confusing the product of aldehyde oxidation with a ketone. Forgetting that Tollens’ reagent reacts with aldehydes but not ketones. Not recognising that the oxidation of the aldehyde gives a carboxylic acid that still reacts with Tollens’ reagent.
Mark scheme (5 marks)
- Compound X contains an aldehyde group (–CHO) / is an aldehyde
- Compound X is oxidised to a carboxylic acid, which gives a positive result with Tollens' reagent because aldehydes (and their oxidation products) can be further oxidised / the aldehyde intermediate reacts with Tollens' reagent reducing silver ions to silver
- Compound Y contains a ketone group (C=O flanked by two carbon groups) / is a ketone
- Ketones cannot be oxidised to produce an aldehyde or carboxylic acid under these conditions (they resist further oxidation), so the product does not react with Tollens' reagent
- The key structural difference is that aldehydes have a hydrogen atom bonded to the carbonyl carbon (–CHO), whereas ketones have two carbon groups bonded to the carbonyl carbon, which explains the difference in reactivity with Tollens' reagent
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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