OCR A Level Chemistry A: Carbonyl compounds (H432) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

Here you can practise section 6.1.2, Carbonyl compounds, in the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. Each outcome from 6.1.2(a) to 6.1.2(e)(ii) appears at least once, as do both mechanisms and the PAG7 tests. H432 is the full A level. Mole calculations from Module 2 come back in questions 8, 10 and 11.

Read the study guide and the revision notes first if the reactions are new to you. The OCR chemistry hub links every H432 section, the printable checklist lets you mark off carbonyl outcomes, and the free diagnostics show where else to revise.

Relative atomic masses for the calculations: Ag 107.9, H 1.0, O 16.0, C 12.0, N 14.0.

Questions

1. Name the organic product when each compound reacts with NaBH₄.

(a) hexanal [1] (b) cyclopentanone [1] (c) State the type of reaction in which NaBH₄ adds to the C=O group. [1]

2. 2-Phenylethanal, C₆H₅CH₂CHO, is converted into a carboxylic acid.

(a) Give the reagents. [1] (b) State the conditions. [1] (c) Describe the colour change. [1] (d) Write an equation for the reaction, using [O]. [1]

3. Cyclopentanone reacts with NaCN(aq)/H⁺(aq).

(a) Show the structure of the hydroxynitrile formed. [1] (b) Suggest why the reagent is NaCN(aq)/H⁺(aq) rather than hydrogen cyanide itself. [1] (c) State why this reaction is useful in organic synthesis. [1] (d) Name the type of reaction. [1]

4. Describe the mechanism for the reaction of propanal, CH₃CH₂CHO, with NaBH₄ in water. Show the dipole, all curly arrows, the intermediate and the products. [4]

5. 3-Methylbutanal, (CH₃)₂CHCH₂CHO, reacts with HCN.

(a) Describe the mechanism, using H⁺ for the protonation step. [4] (b) Name the organic product. [1]

6. Three unlabelled bottles contain heptanal, heptan-2-one and heptan-2-ol. Describe two chemical tests that identify all three. Give the observations and explain the result for the aldehyde. [6]

7. Tollens’ reagent is used to tell aldehydes from ketones.

(a) State what Tollens’ reagent is. [1] (b) Describe how the test is carried out safely and the positive result. [2] (c) Explain the different results for an aldehyde and a ketone in terms of oxidation and reduction. Include a half-equation for silver. [3]

8. 0.648 g of an aldehyde, CₙH₂ₙO, reacts with excess Tollens’ reagent. 1.94 g of silver is deposited.

(a) Calculate the Mᵣ of the aldehyde. [3] (b) Give the structures of the two aldehydes that fit. [2]

9. An unknown ketone (molecular formula C₆H₁₂O) is mixed with 2,4-DNP solution, and an orange solid separates out.

(a) Describe how the precipitate is turned into a sample whose melting point can be used. [3] (b) The prepared sample melts sharply at 129–130 °C. Use these values for the derivatives to identify the ketone: hexan-2-one 107 °C; hexan-3-one 130 °C; 4-methylpentan-2-one 95 °C; 3,3-dimethylbutan-2-one 125 °C. [1] (c) Explain why measuring the melting point of the unpurified precipitate could have led to the wrong answer. [1]

10. 3.42 g of heptan-2-one, CH₃CO(CH₂)₄CH₃, is reduced with NaBH₄. 2.61 g of heptan-2-ol is collected. Write an equation using [H] and calculate the percentage yield. [4]

11. Compound R contains 72.0% carbon, 12.0% hydrogen and 16.0% oxygen by mass. Its Mᵣ is 100.

(a) Show that the molecular formula of R is C₆H₁₂O. [3] (b) R reacts with NaCN(aq)/H⁺(aq) to give 2-hydroxy-3-methylhexanenitrile, CH₃CH₂CH₂CH(CH₃)CH(OH)CN. Give the structure and name of R. [2] (c) State what is seen when R is warmed with Tollens’ reagent, and write an equation for the organic change using [O]. [2] (d) Name the product of reducing R with NaBH₄ and write the equation using [H]. [2] (e) Predict the observation when R is warmed with acidified potassium dichromate. [1]

Answers

1. (a) Hexan-1-ol [1] (b) Cyclopentanol [1] (c) Nucleophilic addition (also a reduction) [1] Examiner insight: For (c), “reduction” describes the overall change but “nucleophilic addition” names the reaction type the question asks for; give the mechanism type.

2. (a) K₂Cr₂O₇ (potassium dichromate) acidified with dilute H₂SO₄ [1] (b) Heat under reflux (with excess oxidising agent) [1] (c) Orange to green [1] (d) C₆H₅CH₂CHO + [O] → C₆H₅CH₂COOH [1] Examiner insight: Give “heat under reflux” as the condition; “distil” is how a primary alcohol is stopped at the aldehyde, which is the opposite of what is wanted here.

3. (a) The ring carbon that was C=O now carries OH and CN: C₅H₈(OH)CN, a cyclopentane ring with –OH and –C≡N on the same carbon (1-hydroxycyclopentane-1-carbonitrile) [1] (b) HCN is a very toxic, volatile compound; NaCN(aq) with acid supplies CN⁻ and H⁺ in solution instead [1] (c) It forms a new C–C bond, increasing the carbon chain length by one [1] (d) Nucleophilic addition [1] Examiner insight: In (a), put both groups on the former carbonyl carbon; placing OH and CN on different carbons shows a different compound.

4. Dipole Cδ+=Oδ− shown; H⁻ lone pair attacks the carbonyl carbon (curly arrow) [1]. π-bond pair of C=O moves onto O (arrow starts on the double bond) [1]. Alkoxide intermediate CH₃CH₂CH₂O⁻, charge on O [1]. O⁻ lone pair takes H from H₂O, the O–H pair returning to the water’s O, giving propan-1-ol and OH⁻ [1] Examiner insight: Curly arrows begin on electrons, so draw the lone pair on H⁻ and start there, and make the arrow end clearly on the carbonyl carbon, not on the oxygen.

5. (a) CN⁻ attacks the δ+ carbonyl carbon, the arrow beginning at the lone pair on its carbon atom [1]. The C=O π pair moves onto O [1]. Intermediate (CH₃)₂CHCH₂CH(CN)O⁻ [1]. O⁻ lone pair bonds to H⁺, giving (CH₃)₂CHCH₂CH(OH)CN [1] (b) 2-Hydroxy-4-methylpentanenitrile [1] Examiner insight: An arrow from the nitrogen of CN⁻ implies a C–N–C link; check that the arrow starts where the new bond forms.

6. Add 2,4-DNP to each [1]. Orange precipitate with heptanal and heptan-2-one; no precipitate with heptan-2-ol, so that bottle is identified [1]. Warm the other two with Tollens’ reagent in a water bath [1]. Silver mirror with heptanal [1]; no change with heptan-2-one [1]. Heptanal is oxidised to heptanoic acid, and Ag⁺ is reduced to Ag [1] Examiner insight: Give the observation for every compound in each test, including “no change”; a test described for only one bottle does not show that the other two are distinguished.

7. (a) Ammoniacal silver nitrate (silver nitrate solution with aqueous ammonia) [1] (b) Heat gently using a water bath; a naked flame could ignite the organic compound [1]; a silver mirror (or grey-black precipitate) forms with an aldehyde [1] (c) The aldehyde is oxidised to a carboxylic acid, RCHO + [O] → RCOOH [1]. The silver ions are reduced: Ag⁺ + e⁻ → Ag [1]. A ketone is not oxidised by Tollens’ reagent, so Ag⁺ is not reduced and there is no change [1] Examiner insight: In (c), say which species is oxidised and which is reduced; “the aldehyde reacts with silver” gives neither process.

8. (a) n(Ag) = 1.94 ÷ 107.9 = 0.0180 mol [1]; one aldehyde reduces two Ag⁺, so n(aldehyde) = 0.00899 mol [1]; Mᵣ = 0.648 ÷ 0.00899 = 72 [1] (b) 14n + 16 = 72 gives n = 4, so C₄H₈O: CH₃CH₂CH₂CHO (butanal) [1] and (CH₃)₂CHCHO (2-methylpropanal) [1] Examiner insight: Using a 1 : 1 ratio of aldehyde to silver halves the Mᵣ to 36; write the half-equation first so the ratio is clear.

9. (a) Filter off the precipitate [1]; recrystallise it from the minimum volume of hot solvent, cool, filter and wash with cold solvent [1]; dry it before measuring the melting point [1] (b) Hexan-3-one [1] (c) A crude solid melts below its true value and over a range, so it could seem to match a lower entry such as 125 °C (3,3-dimethylbutan-2-one) [1] Examiner insight: In (a), “purify it” alone is too vague; name recrystallisation and the key steps.

10. CH₃CO(CH₂)₄CH₃ + 2[H] → CH₃CH(OH)(CH₂)₄CH₃ [1]; n(heptan-2-one) = 3.42 ÷ 114.0 = 0.0300 mol [1]; theoretical mass of heptan-2-ol = 0.0300 × 116.0 = 3.48 g [1]; yield = 2.61 ÷ 3.48 × 100 = 75.0% [1] Examiner insight: The product Mᵣ (116.0) differs from the reactant’s (114.0); dividing 2.61 by 3.42 gives 76.3% and compares two different substances.

11. (a) C 72.0 ÷ 12.0 = 6.00; H 12.0 ÷ 1.0 = 12.0; O 16.0 ÷ 16.0 = 1.00 [1]; ratio 6 : 12 : 1, empirical formula C₆H₁₂O [1]; empirical mass 6(12.0) + 12(1.0) + 16.0 = 100, equal to Mᵣ, so R is C₆H₁₂O [1] (b) Remove the CN carbon and turn CH(OH) back into C=O: CH₃CH₂CH₂CH(CH₃)CHO [1], 2-methylpentanal [1] (c) A silver mirror [1]; CH₃CH₂CH₂CH(CH₃)CHO + [O] → CH₃CH₂CH₂CH(CH₃)COOH [1] (d) 2-Methylpentan-1-ol [1]; CH₃CH₂CH₂CH(CH₃)CHO + 2[H] → CH₃CH₂CH₂CH(CH₃)CH₂OH [1] (e) Orange to green [1] Examiner insight: In (a) a “show that” needs the comparison of empirical mass with Mᵣ written out; quoting C₆H₁₂O without it leaves the last step unsupported.

Where marks are usually lost

• Starting the cyanide arrow at nitrogen instead of at the carbon lone pair.
• Leaving out the arrow from the C=O bond to oxygen, or the protonation step.
• Writing H₂ instead of 2[H], or O₂ instead of [O], in organic redox equations.
• Using a 1 : 1 aldehyde : silver ratio in silver-mirror calculations.
• Giving a 2,4-DNP result as a colour change rather than a precipitate.
• Describing a test without the “no change” result for the compounds that do not react.
• Heating Tollens’ reagent with a flame.
• Naming hydroxynitriles with the OH carbon as C1.
• Dividing product mass by reactant mass in yield questions.

Next steps

• Carbonyl tables and mechanism steps in brief: revision notes
• The full explanation of each outcome: study guide
• More mole and yield questions: amount of substance practice
• More half-equation work: redox practice
• Find gaps across H432: all free 10-minute diagnostics
• Every OCR chemistry section: OCR A Level Chemistry hub
• Mark off what you have done: H432 checklist
• Want a teacher to go through it with you? Book a free trial class

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 6.1.2 Carbonyl compounds, learning outcomes (a) to (e). Published by OCR.