About Carbonyl compounds in A Level Chemistry
Carbonyl compounds is one of the 18 main topics in our A Level Chemistry (Cambridge (CAIE) 9701) question bank, and about 5% of its real questions come from it. Tayyari Karo has 30 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2, Paper 4; 26 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 16 hard, 13 medium, 1 easy.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 6 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2026 · Paper 1 · Carbonyl compounds
Four isomers of C5H10O: CH3CH(OH)CH2CH=CH2; (CH3)3CCHO; CH3COCH2CH2CH3; (CH3)2CHCOCH3. Which row gives the number of these isomers that give a precipitate with 2,4-DNPH and the number that give a precipitate with alkaline I2(aq)?
- 2 with 2,4-DNPH; 2 with alkaline I2
- 2 with 2,4-DNPH; 3 with alkaline I2
- 3 with 2,4-DNPH; 2 with alkaline I2
- 3 with 2,4-DNPH; 3 with alkaline I2
Show answer and explanation
Answer: D. 3 with 2,4-DNPH; 3 with alkaline I2
2,4-DNPH: the aldehyde and two ketones (3). Alkaline I2: the CH3CH(OH)- alcohol and the two methyl ketones (CH3CO-): 3.
October/November 2025 · Paper 1 · Carbonyl compounds
Seven aldehydes and ketones each contain only one oxygen atom per molecule and have four or fewer carbon atoms. Alkaline I2(aq) is added to each separately. How many of these carbonyl compounds produce a yellow precipitate with alkaline I2(aq)?
- 1
- 2
- 3
- 4
Show answer and explanation
Answer: C. 3
The iodoform (triiodomethane) test gives a positive (yellow precipitate) result only for methyl ketones and acetaldehyde-type CH3CO– or CH3CH(OH)– structures; among the C4-or-fewer carbonyl compounds, three fit this CH3CO– pattern (e.g. ethanal, propanone, butan-2-one).
May/June 2024 · Paper 1 · Carbonyl compounds
The product of the reaction between propanone and hydrogen cyanide is hydrolysed under acidic conditions. What is the formula of the final product?
- CH3CH(OH)COOH
- CH3CH2CH2COOH
- (CH3)2CHCONH2
- (CH3)2C(OH)COOH
Show answer and explanation
Answer: D. (CH3)2C(OH)COOH
HCN adds to propanone to form the hydroxynitrile (CH3)2C(OH)CN, which on acid hydrolysis of the nitrile group gives the hydroxy-acid (CH3)2C(OH)COOH (2-hydroxy-2-methylpropanoic acid).
May/June 2023 · Paper 1 · Carbonyl compounds
Which compound gives a positive test with alkaline aqueous iodine and does not show optical isomerism?
- CH3COCH2CH2OH
- CH3CH2CH(OH)CHO
- CH3COCH(OH)CH3
- (CH3)2C(OH)CHO
Show answer and explanation
Answer: A. CH3COCH2CH2OH
CH3COCH2CH2OH has the CH3CO- group needed for a positive iodoform test, and has no chiral carbon (so no optical isomerism), unlike the other options which have a chiral centre.
May/June 2022 · Paper 1 · Carbonyl compounds
Which compound will react with LiAlH4 to form two optical isomers? A CH3CH2COCH3; B CH3CH2CH2CHO; C CH3CH2COCH2CH3; D CH3CH(CH3)CH2CO2H
- CH3CH2COCH3
- CH3CH2CH2CHO
- CH3CH2COCH2CH3
- CH3CH(CH3)CH2CO2H
Show answer and explanation
Answer: A. CH3CH2COCH3
LiAlH4 reduces the ketone CH3CH2COCH3 (butan-2-one) to butan-2-ol, CH3CH2CH(OH)CH3, which has a chiral carbon (4 different groups) giving two optical isomers; the other products (from an unbranched aldehyde, a symmetrical ketone, or reduction of a carboxylic acid to a primary alcohol) do not have a chiral centre.
May/June 2021 · Paper 1 · Carbonyl compounds
Which statement is correct for the reaction of carbonyl compounds with HCN?
- The reaction is catalysed by concentrated H2SO4.
- Pentan-2-one and HCN react to give a chiral product.
- The reaction is a condensation reaction.
- The reaction is nucleophilic substitution.
Show answer and explanation
Answer: B. Pentan-2-one and HCN react to give a chiral product.
Addition of HCN to pentan-2-one gives 2-hydroxy-2-methylpentanenitrile, whose central carbon bears four different groups (OH, CN, CH3, C3H7) — a chiral centre; the reaction is base-catalysed nucleophilic addition, not substitution or condensation.
