AQA A-Level Chemistry Paper 3, 2017: Question 15

1 mark · Easy difficulty · Multiple Choice

Identify which square planar platinum complex represents a stereoisomer of the anti-cancer drug cisplatin.

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Question

Question 15 asks: 'Cisplatin is an anti-cancer drug. Which structure represents a stereoisomer of cisplatin?' Four square planar structures labelled A to D are shown: Structure A has Pt bonded to adjacent NH3 ligands (top and left) and adjacent Cl ligands (right and bottom); Structure B has Pt bonded to opposite NH3 ligands (top and bottom) and opposite Cl ligands (left and right); Structure C has Pt bonded to two NH2 ligands and two Cl ligands; Structure D has Pt bonded to two NH2 ligands opposite each other and two Cl ligands opposite each other. Four lozenges are provided to select from A, B, C, or D.
Question text

15 Cisplatin is an anti-cancer drug.

Which structure represents a stereoisomer of cisplatin?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme table showing question number 15 with the correct answer B.

15 B

How to answer it

Stereoisomerism in Transition Metal Complexes: Cisplatin

📌 What this question tests

This question assesses your understanding of transition metal complex shapes, geometric isomerism (cis/trans stereoisomerism in 4-coordinate square planar complexes), and the specific chemical formula and structure of the anti-cancer drug cisplatin, [Pt(NH₃)₂Cl₂].

Question 15 Analysis

Identifying the Stereoisomer of Cisplatin

Total: 1 Mark (Multiple Choice)

✅ Correct Answer: B

Option B represents transplatin, the trans-stereoisomer of cisplatin.

  • The formula remains identical: [Pt(NH₃)₂Cl₂] .
  • Identical ligands are opposite each other (bond angle 180°): the two neutral ammine ( NH₃ ) ligands are trans, and the two chloride ( Cl ) ligands are trans.
  • This makes it a true geometric stereoisomer of cisplatin.

💡 Key Knowledge

  • Complex Shape: Pt(II) complexes with four ligands are square planar with bond angles of 90° and 180°.
  • Cisplatin Formula: Neutral complex Pt(NH₃)₂Cl₂ containing two uncharged NH₃ ligands and two anionic chloride ( Cl⁻ ) ligands.
  • Stereoisomers: Species with the same molecular formula and structural connectivity, but a different spatial arrangement of atoms.
  • Cis vs Trans:
    • Cis-isomer: Identical groups adjacent (90° apart).
    • Trans-isomer: Identical groups opposite (180° apart).

📐 Step-by-Step Option Elimination

  1. Check Option A: Has two NH₃ ligands at 90° and two Cl ligands at 90°. This is cisplatin itself, not another stereoisomer.
  2. Check Option B: Has two NH₃ ligands at 180° and two Cl ligands at 180°. This is transplatin. It has the same formula as cisplatin but a different 3D arrangement → Correct Stereoisomer.
  3. Check Options C & D: Contain NH₂ groups (amido ligands) instead of neutral NH₃ (ammine) ligands. They have different chemical formulas ( Pt(NH₂)₂Cl₂ ) and different oxidation states/charges, so they cannot be isomers.

❌ Common Errors & Pitfalls

  • Overlooking the ligand formula: Glancing quickly and missing that C and D have NH₂ rather than NH₃ . Ammine ligands must always be written as NH₃ .
  • Selecting the drug itself (Option A): The question asks for a stereoisomer of cisplatin. Option A is cisplatin itself, so it cannot be an isomer of itself.
  • Confusing square planar with tetrahedral: Tetrahedral complexes of the form [Ma₂b₂] do not show cis/trans isomerism. Cis/trans is possible here exclusively because the complex is square planar.

🧠 Exam Technique & Examiner Insight

Multiple-choice questions on cisplatin frequently test your attention to detail regarding ligand formulas:

  • Rule of elimination: Always verify the ligands first. Cisplatin consists of ammonia ( NH₃ ) and chlorine ( Cl ). Seeing NH₂ allows you to immediately eliminate options C and D without further thought.
  • Check the stem carefully: Did the question ask for cisplatin or a stereoisomer? A is cisplatin; B is its stereoisomer (transplatin). Misreading the stem leads directly into the distractor trap (Option A).

Topics

Inorganic Chemistry · Organic Chemistry · 3.2.5 Transition Metals · 3.3.13 Amino Acids, Proteins and DNA

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.