Edexcel International A Level Chemistry: Organic synthesis (YCH11) – 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.

This set covers Topic 20: Organic Synthesis, outcomes 20.1 to 20.5 with Core Practical 16, as set out in Issue 1 (September 2017) of Pearson Edexcel’s International Advanced Subsidiary/Advanced Level in Chemistry specification, YCH11. It is International A2 (IA2) content in Unit 5 (Transition Metals and Organic Nitrogen Chemistry). Start with the study guide and revision notes if needed. The course hub and checklist list every topic; the free diagnostic quizzes show weak areas.

Use these values: C 12.0, H 1.0, N 14.0, O 16.0. Proton NMR shifts: alkyl CH about δ 0.9–1.7; CH next to C=O about δ 2.0–2.6; CH on C–O about δ 3.3–4.2; aromatic H about δ 6.5–8.0.

Questions

1. Ethylmagnesium bromide is made in the laboratory.

(a) State the reagent and solvent used to make it from bromoethane. [2] (b) Explain why all apparatus must be dry. [1]

2. Complete combustion of 0.348 g of compound Q, which contains only C, H and O, gives 0.792 g of CO₂ and 0.324 g of H₂O. M⁺ in its mass spectrum is at m/z = 116. Calculate the empirical and molecular formulae of Q. [4]

3. Methylmagnesium iodide reacts with each compound below in dry ether, followed by dilute acid. Name each organic product.

(a) carbon dioxide [1] (b) methanal [1] (c) butanal [1] (d) propanone [1]

4. Amine R contains C 61.0%, H 15.3% and N 23.7% by mass. M⁺ is at m/z = 59.

(a) Show that the molecular formula is C₃H₉N. [2] (b) The proton NMR spectrum of R has three peaks with relative areas 6 : 1 : 2. Identify R, giving a reason. [2]

5. Compound T, C₅H₁₀O, gives an orange precipitate with 2,4-DNPH. It does not change Fehling’s solution and gives no precipitate with iodine and sodium hydroxide. Its IR spectrum has a strong band at 1715 cm⁻¹ and no broad O–H band. Its ¹³C NMR spectrum has three peaks. Its proton NMR spectrum has a quartet and a triplet, areas 2 : 3. Deduce the structure of T, explaining each step. [6]

6. 5-Hydroxyhexan-2-one is CH₃COCH₂CH₂CH(OH)CH₃.

(a) Predict what you would see when it is treated with each reagent: (i) 2,4-DNPH; (ii) Tollens’ reagent, warmed; (iii) acidified potassium dichromate(VI), warmed; (iv) iodine and sodium hydroxide, warmed; (v) sodium carbonate solution. [5] (b) Predict, with a reason, whether it is soluble in water. [1]

7. Methyl 2-methylpropanoate is made by heating 2-methylpropanoic acid with methanol and a few drops of concentrated sulfuric acid.

(a) Explain why the mixture is heated under reflux. [1] (b) Next, the student washes the crude ester with aqueous sodium carbonate. Give the purpose of this step and one precaution. [2] (c) Suggest a suitable drying agent and the sign that drying is complete. [1] (d) State how the pure ester is finally obtained. [1]

8. A student plans a Grignard reaction. Use the hazard data to suggest one control measure for each substance.

Substance	Hazard
ethoxyethane	extremely flammable liquid
1-bromobutane	harmful vapour
ethanoic anhydride	corrosive; reacts with water
magnesium turnings	flammable solid

[4]

9. In Core Practical 16, 4.83 g of 2-hydroxybenzoic acid (Mr 138.0) reacts with excess ethanoic anhydride. Aspirin has Mr 180.0.

(a) Write the equation, using molecular formulae. [1] (b) The dry crude product weighs 5.11 g and the recrystallised product 3.78 g. Calculate both percentage yields. [3] (c) Suggest why the crude yield overstates how much aspirin was made. [1] (d) Describe how the crude aspirin is recrystallised. [2] (e) The recrystallised sample melts over the range 128–132 °C. Pure aspirin melts at 136 °C. State what this shows. [1]

10. Pentan-1-amine is to be made from 1-bromobutane.

(a) Give reagents and conditions for a four-step route that uses a Grignard reagent and methanal, naming each intermediate. [4] (b) Give a two-step route with reagents and conditions. [2] (c) The four steps have yields of 85%, 70%, 75% and 60%. The two steps have yields of 80% and 70%. Calculate both overall yields. [2]

11. Compound W contains C 70.6%, H 5.9% and O 23.5%. M⁺ is at m/z = 136. Its IR spectrum has a strong band at 1765 cm⁻¹ but no broad O–H band. Its proton NMR spectrum shows a peak of area 5 at δ 7.0–7.4 and a singlet of area 3 at δ 2.3. Its ¹³C NMR spectrum has six peaks.

(a) Find the molecular formula. [2] (b) Deduce the structure of W. Explain why it is not methyl benzoate. [5]

12. A fragrant oil is isolated from crushed cloves. The oil is volatile, does not mix with water and breaks down near its own boiling temperature.

(a) Explain why steam distillation suits this oil. [1] (b) The distillate is cloudy, with the oil spread through the water. Describe how to obtain a dry sample of the oil. [2] (c) Explain why three small portions of extracting solvent are better than one large portion. [1]

Answers

1. (a) Magnesium (turnings) [1]; in dry ether (ethoxyethane) [1] (b) Water reacts with the Grignard reagent, forming ethane and destroying it [1] Examiner insight: “Ether” without “dry” is often not enough for the solvent mark.

2. n(C) = 0.792 ÷ 44.0 = 0.0180 mol; n(H) = 2 × 0.324 ÷ 18.0 = 0.0360 mol [1]; mass O = 0.348 − 0.216 − 0.036 = 0.096 g, so 0.00600 mol [1]; ratio 3 : 6 : 1, empirical formula C₃H₆O [1]; 116 ÷ 58.0 = 2, so C₆H₁₂O₂ [1] Examiner insight: Show the oxygen found by difference; a ratio with no oxygen step loses method credit.

3. (a) Ethanoic acid [1] (b) Ethanol [1] (c) Pentan-2-ol [1] (d) 2-Methylpropan-2-ol [1] Examiner insight: Count carbons: the Grignard adds one, the carbonyl compound supplies the rest.

4. (a) C 61.0 ÷ 12.0 = 5.08; H 15.3 ÷ 1.0 = 15.3; N 23.7 ÷ 14.0 = 1.69 [1]; ratio 3 : 9 : 1, C₃H₉N has Mr 59.0, matching M⁺ [1] (b) Propan-2-amine, (CH₃)₂CHNH₂ [1]: two equivalent CH₃ groups (6), one CH (1) and NH₂ (2) [1] Examiner insight: Link each area to a group; the other C₃H₉N amines give one or four peaks.

5. Orange precipitate and the 1715 cm⁻¹ band show C=O [1]; no Fehling’s change, so a ketone [1]; no iodoform, so no CH₃CO– group [1]; three ¹³C peaks for five carbons means a symmetrical molecule [1]; quartet and triplet in a 2 : 3 ratio show equivalent CH₃CH₂– groups next to C=O [1]; T is pentan-3-one, CH₃CH₂COCH₂CH₃ [1] Examiner insight: Each piece of evidence earns its own mark only when you state what it shows.

6. (a) (i) orange precipitate [1]; (ii) no silver mirror, as it has no aldehyde group [1]; (iii) orange to green, as the secondary alcohol is oxidised [1]; (iv) pale yellow precipitate [1]; (v) no effervescence, as there is no COOH [1] (b) Soluble: the OH and C=O groups form hydrogen bonds with water [1] Examiner insight: A colour change needs both colours; “turns green” alone may not score.

7. (a) Allows long heating without losing volatile methanol or ester [1] (b) Removes acid impurities (sulfuric acid and unreacted 2-methylpropanoic acid) [1]; invert and open the tap often to release CO₂ pressure [1] (c) Anhydrous magnesium sulfate; the liquid turns from cloudy to clear [1] (d) Distillation, collecting the fraction near the ester’s boiling temperature [1] Examiner insight: Name which impurity each wash removes; “to purify it” is too vague.

8. Ethoxyethane: no naked flames; heat with a water bath or electric heater [1]. 1-Bromobutane: use a fume cupboard [1]. Ethanoic anhydride: gloves and eye protection; keep it dry [1]. Magnesium: keep away from flames [1]. Examiner insight: Each control must answer that substance’s own hazard; repeated “wear goggles” scores once at most.

9. (a) C₇H₆O₃ + C₄H₆O₃ → C₉H₈O₄ + C₂H₄O₂ [1] (b) n = 4.83 ÷ 138.0 = 0.0350 mol; theoretical mass = 0.0350 × 180.0 = 6.30 g [1]; crude = 5.11 ÷ 6.30 × 100 = 81.1% [1]; recrystallised = 3.78 ÷ 6.30 × 100 = 60.0% [1] (c) The crude solid also contains impurities such as unreacted 2-hydroxybenzoic acid [1] (d) Dissolve in the minimum volume of hot solvent and filter hot [1]; cool to crystallise, collect by suction filtration, rinse with ice-cold solvent, then dry [1] (e) The sample is still impure: it melts below 136 °C and over a range [1] Examiner insight: Use the theoretical mass for both yields; a yield based on the crude mass mixes two different quantities.

10. (a) Mg in dry ether gives butylmagnesium bromide [1]; add methanal in dry ether, then dilute acid, giving pentan-1-ol [1]; KBr with 50% H₂SO₄ (or PCl₅), giving 1-bromopentane (or 1-chloropentane) [1]; excess ammonia in ethanol, heat in a sealed tube, giving pentan-1-amine [1] (b) KCN in ethanol, heat under reflux, giving pentanenitrile [1]; LiAlH₄ in dry ether [1] (c) 0.85 × 0.70 × 0.75 × 0.60 = 26.8% [1]; 0.80 × 0.70 = 56.0% [1] Examiner insight: Conditions count: “ammonia” without excess, ethanol and sealed tube may lose the mark.

11. (a) C 5.88, H 5.9, O 1.47, giving 4 : 4 : 1, so C₄H₄O (68.0) [1]; 136 ÷ 68.0 = 2, C₈H₈O₂ [1] (b) 1765 cm⁻¹ with no O–H shows an ester C=O [1]; area 5 at δ 7.0–7.4 is a C₆H₅ ring [1]; a CH₃ singlet with no adjacent H, at δ 2.3, is next to C=O [1]; W is phenyl ethanoate, CH₃COOC₆H₅ [1]; in methyl benzoate the CH₃ is on oxygen, so its singlet would be near δ 3.7–4.0 [1] Examiner insight: Quote the given shift ranges to place the CH₃ group.

12. (a) The oil distils with the steam below 100 °C, so it never reaches the temperature at which it decomposes [1] (b) Shake the distillate with an immiscible organic solvent in a separating funnel and keep the organic layer [1]; dry it with anhydrous magnesium sulfate, filter, then evaporate or distil off the solvent [1] (c) Each portion removes a fraction of the remaining oil, so repeated extractions remove more in total [1] Examiner insight: Link steam distillation to the low distillation temperature; naming the technique alone does not explain the choice.

Where marks are usually lost

• Leaving out oxygen by difference in combustion data.
• “Ether” without “dry” for Grignard reactions.
• Grignard products with the wrong number of carbons.
• Colour changes with only one colour given.
• Reagents with no conditions in route questions.
• Control measures that do not match the hazard given.
• A melting temperature described as “higher” when the sample is impure.

Next steps

• All free 10-minute diagnostics, to find weak topics
• Organic synthesis revision notes for a final recap
• Organic synthesis study guide to relearn anything you missed
• Printable chemistry checklist to tick off 20.1 to 20.5
• Edexcel IAL Chemistry course hub for every other topic
• Want live teaching? Book a free trial class

Official syllabus

Source: Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry, specification XCH11/YCH11, Issue 1, September 2017 (Pearson Education Limited). This set covers Topic 20: Organic Synthesis, outcomes 20.1 to 20.5, including Core Practical 16, in Unit 5.