A-LevelChemistryOrganic synthesisFeb/Mar 2021Paper 1 Q271 Mark
In this question you can assume that ¹H and ³H have the same chemical properties. A sample of ethanal contains only one isotope of hydrogen, ¹H. It is reduced to compound Z, C₂H₆O, in a nucleophilic addition reaction using NaBH₄. All the hydrogen atoms in the NaBH₄ are the ³H isotope. NaBH₄ CH₃CHO → C₂H₆O compound Z Compound Z is then oxidised back to ethanal and water. C₂H₆O + [O] → CH₃CHO + H₂O Which statement about the final mixture of products is correct?
ABoth ethanal and water contain ³H atoms.
BEthanal is the only product containing ³H atoms.
CNeither ethanal nor water contain ³H atoms.
DWater is the only product containing ³H atoms.
✓ Correct Answer
The correct answer is A: Both ethanal and water contain ³H atoms.
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
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