Free Carbonyl compounds questions for AS Level Chemistry (9701), part of Organic chemistry. Each question has the answer and the marking points an examiner would look for.
Generate a full Carbonyl Compounds worksheet →All Organic chemistry questionsWhich statement describes 2,4-DNPH?
Answer: D
Which row shows the results of tests on propanone?
| 2,4-DNPH | Tollens’ reagent | alkaline aqueous iodine | |
|---|---|---|---|
| A | orange precipitate | silver mirror | yellow precipitate |
| B | orange precipitate | no change | no yellow precipitate |
| C | no change | no change | yellow precipitate |
| D | orange precipitate | no change | yellow precipitate |
Answer: D
Compound W, C₄H₈O, gives an orange precipitate with 2,4-DNPH, does not react with Tollens’ reagent and gives a yellow precipitate with alkaline aqueous iodine.
State what each test shows about W.
Identify W.
W is reduced by NaBH₄. Name the product and explain why it shows optical isomerism.
Explain why the product of reducing W is formed as a racemic mixture.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | carbonyl; not aldehyde; CH₃CO group 2,4-DNPH: carbonyl group B1 Tollens’: ketone not aldehyde B1 iodine: CH₃CO– group B1 | 3 |
| (b) | butanone butanone, CH₃COCH₂CH₃ B1 | 1 |
| (c) | butan-2-ol; chiral centre butan-2-ol B1 C2 is bonded to four different groups (a chiral centre) B1 | 2 |
| (d) | planar C=O attacked from either side the C=O group is planar M1 the nucleophile H⁻ attacks equally from above and below M1 | 2 |
Carbonyl compounds undergo nucleophilic addition with HCN.
Describe the mechanism for the reaction of ethanal with HCN.
Explain why the product from propanal is a racemic mixture.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | CN⁻ attacks δ+ C; intermediate; protonated CN⁻ attacks δ+ carbon B1 curly arrow C=O to O B1 anion intermediate B1 protonated by H⁺/HCN B1 | 4 |
| (b) | planar C=O attacked from either side carbonyl group is planar B1 equal chance of attack from either side giving both enantiomers B1 | 2 |
Aldehydes and ketones can be distinguished by chemical tests.
Describe how Tollens’ reagent distinguishes propanal from propanone.
Describe the reagent used to show that both contain a carbonyl group, and the result.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | silver mirror with aldehyde warm with Tollens’ reagent B1 silver mirror with propanal, no change with propanone B1 | 2 |
| (b) | 2,4-DNPH orange precipitate 2,4-dinitrophenylhydrazine B1 orange/yellow precipitate B1 | 2 |
This question is about compounds with the molecular formula C₃H₆O and their reactions.
Draw the structures of propanal and propanone, and state the type of isomerism shown.
Describe a test that gives a positive result with both compounds, and a test that distinguishes them. Give observations.
Describe the mechanism of the reaction of propanone with HCN in the presence of KCN, and explain whether the product is chiral.
Propan-1-ol can be oxidised to propanal or to propanoic acid. State the reagent and how the conditions differ for each product.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | CH₃CH₂CHO; CH₃COCH₃; functional group (structural) isomerism propanal structure B1 propanone structure B1 structural (functional group) isomerism B1 | 3 |
| (b) | 2,4-DNPH orange ppt both; Tollens silver mirror / Fehling red ppt with propanal only 2,4-DNPH gives an orange / yellow precipitate with both B1 Tollens’ reagent (warm) gives a silver mirror with propanal B1 or Fehling’s gives a red precipitate with propanal B1 no change with propanone B1 | 4 |
| (c) | nucleophilic addition; CN⁻ attacks δ+ C; alkoxide intermediate protonated; product not chiral for propanone (two methyl groups) nucleophilic addition: CN⁻ lone pair attacks δ+ carbonyl carbon B1 curly arrow from C=O π bond to O B1 the anion is protonated by HCN / H⁺ regenerating CN⁻ B1 the product (CH₃)₂C(OH)CN has no chiral centre since the carbon has two identical CH₃ groups B1 | 4 |
| (d) | acidified K₂Cr₂O₇; distil immediately for aldehyde; reflux with excess oxidant for acid acidified potassium dichromate(VI) B1 distil off the aldehyde as it forms B1 heat under reflux (with excess oxidant) for the carboxylic acid B1 | 3 |
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