OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 5

1 mark · Hard difficulty · Multiple Choice

Determine the number of chiral carbon atoms in the given steroid drug molecule

Practise this question

Question

Multiple choice question 5 showing the chemical structure of a complex steroid drug molecule containing multiple rings, a fluorine atom (F), hydroxyl groups (OH), carbonyl groups (C=O), and methyl groups. Below the structure, the question asks 'How many chiral carbon atoms are there in a molecule of the drug?' with four options: A 5, B 6, C 7, D 8. An answer box and mark allocation of [1] are at the bottom.
Question text

5 The structure of a drug is shown below:

O

HO OH

HO

F

O

How many chiral carbon atoms are there in a molecule of the drug?

A 5

B 6

C 7

D 8

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme showing question number 5 with correct answer D, 1 mark, and note allowing 8 as the correct number of chiral centres.

5 D 1 AO1.2 ALLOW 8 (correct number of chiral centres)

How to answer it

Identifying Chiral Centres in Complex Drug Molecules

What this question tests

This question assesses your ability to recognise and count chiral carbon atoms (chiral centres) within a complex skeletal formula, specifically a multi-ring steroid-like pharmaceutical structure. It tests your knowledge of stereoisomerism and your attention to detail regarding saturated versus unsaturated carbon environments.

Question 5

Answer: D (8)

✅ Correct Answer

The correct option is D (8 chiral carbon atoms).

💡 Key Knowledge

  • A chiral carbon (asymmetric carbon) is bonded to four different groups or atoms.
  • Carbon-carbon double bonds (C=C) and carbonyl carbons (C=O) cannot be chiral because they do not have four single bonds to four distinct groups.
  • CH₂ groups and CH₃ groups cannot be chiral as they have multiple identical hydrogen atoms attached.

🧠 Exam Technique

Systematically inspect every single saturated ring carbon in the steroid skeleton. Check junctions, substituted carbons, and side chains. Tick them off one-by-one on the exam paper to avoid double-counting or missing any.

❌ Common Errors

  • Counting carbons involved in double bonds (in the left-hand ring) or the carbonyl oxygen groups (C=O).
  • Missing chiral centres located on side chains or ring junctions due to the visual complexity of skeletal formulas.
Mark Scheme Note: 1 mark awarded for selecting D. AO1.2 (Demonstrating knowledge and understanding of scientific ideas).

Topics

Module 6: Organic chemistry and analysis · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.