OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 3

1 mark · Medium difficulty · Multiple Choice

Determine the number of hydrogen atoms in the molecular formula of the given medicinal drug from its skeletal structure.

Practise this question

Question

Question 3 asks: 'The skeletal structure of a medicinal drug is shown below. How many hydrogen atoms are in its molecular formula?' The skeletal formula shows a tetralin core (a benzene ring fused to a saturated six-membered ring). Attached to one carbon of the saturated ring is a methylamino group (-NHCH3). Attached to another carbon across the ring is a 3,4-dichlorophenyl ring. Four multiple-choice options are provided: A: 14, B: 15, C: 17, D: 19, followed by an answer box.
Question text

3 The skeletal structure of a medicinal drug is shown below.

How many hydrogen atoms are in its molecular formula?

H

N

Cl

Cl

A 14

B 15

C 17

D 19

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme row for question 3 showing the correct answer is C, with an additional guidance note 'ALLOW 17', worth 1 mark.

3 C 1 ALLOW 17

How to answer it

Counting Hydrogen Atoms in Skeletal Structures

📌 What this question tests

This question assesses your ability to accurately interpret skeletal formulas of complex organic molecules (in this case, the antidepressant drug sertraline). It tests your understanding of carbon valency (4 bonds), explicit vs. implicit hydrogen atoms, aromatic rings, and terminal methyl/amine groups.

Question 3 • Multiple Choice [1 Mark]

Deducing the Number of Hydrogen Atoms

Molecular Structure: Fused ring system with an amine side-chain and a dichlorophenyl group

✅ Correct Answer

C (17)

Mark allocation:
• 1 mark for selecting C (or writing 17).

💡 Key Knowledge

  • Carbon forms 4 bonds: Any bond not explicitly shown to a heteroatom or adjacent carbon is a C–H bond.
  • Terminal lines: A bare line end represents a methyl group ( –CH₃ ).
  • Heteroatoms: Hydrogens on nitrogen/oxygen must be explicitly drawn; check both the N–H and attached carbons.
  • Aromatic rings: Each vertex in a benzene ring already has 3 bonds (delocalised system); any substituted vertex has 0 hydrogens.

📐 Step-by-Step Hydrogen Count Breakdown

Divide the complex molecule into 4 distinct regions to avoid double-counting or missing atoms:

Molecular Region Structural Details H Count
1. Amine Side-Chain • Terminal methyl group ( –CH₃ ): 3 H
• Secondary amine proton ( –NH– ): 1 H 		4
2. Alicyclic (Central 6-membered) Ring • Carbon bonded to –NH– : 3 C/N bonds shown → 1 H
• Top methylene ( –CH₂– ) groups: 2 carbons with 2 bonds each → 2 × 2 = 4 H
• Carbon bonded to dichlorophenyl ring: 3 C–C bonds shown → 1 H
• Ring-junction carbons (fused to benzene ring): 4 C–C bonds each → 0 H 		6
3. Fused Benzene Ring • 2 ring-junction carbons: substituted → 0 H
• 4 remaining aromatic CH vertices: 4 × 1 = 4 H 		4
4. 3,4-Dichlorophenyl Ring • 1 carbon attached to central ring: substituted → 0 H
• 2 carbons bonded to chlorine atoms ( –Cl ): substituted → 0 H
• 3 remaining unsubstituted aromatic CH vertices: 3 × 1 = 3 H 		3
Total Number of Hydrogen Atoms 4 + 6 + 4 + 3 = 17

❌ Common Errors

  • Forgetting the terminal methyl group: Treating the stick end as just a bond or counting only 1 H instead of 3 ( –CH₃ ).
  • Overlooking the N–H hydrogen: Skipping the explicit H drawn on nitrogen.
  • Adding hydrogens to ring junctions: Forgetting that fused carbons already share 4 carbon-carbon bonds and have zero hydrogens attached.
  • Miscounting the dichlorophenyl ring: Forgetting that each attached chlorine atom replaces an aromatic hydrogen atom.

🧠 Exam Technique

  • Draw directly on the paper: Dot or sketch in every implicit hydrogen atom onto the exam paper using your pencil before tallying.
  • Cross them off as you count: Tick off each carbon vertex as you account for its full complement of 4 bonds.
  • Sum by fragments: Tallying in small groups (side-chain, central ring, aromatic rings) makes arithmetic verification fast and foolproof under time pressure.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.