OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 10

1 mark · Medium difficulty · Multiple Choice

Determine the number of stereoisomers for the compound CH3CH(Br)CH=CHCH(Cl)CH3.

Practise this question

Question

Multiple-choice question 10 asks: 'How many stereoisomers does the compound CH3CH(Br)CH=CHCH(Cl)CH3 have?' The options are A: 2, B: 4, C: 6, and D: 8, with an answer box provided and marked with [1].
Question text

10 How many stereoisomers does the compound CH3CH(Br)CH=CHCH(Cl)CH3 have?

A 2

B 4

C 6

D 8

Your answer

[1]

Mark scheme

Show the mark scheme A mark scheme table showing an entry with row number 11, answer A, and 1 mark.

10 D 1 ALLOW 8

How to answer it

Stereoisomerism in Multifunctional Alkenes

📋 What this question tests

This multiple-choice question assesses your ability to identify different forms of stereoisomerism within a single organic molecule and determine the total number of stereoisomers:

  • Identifying chiral centres (optical isomerism) in a carbon chain.
  • Identifying carbon–carbon double bonds ( C=C ) exhibiting E/Z isomerism (geometric isomerism).
  • Recognising molecular asymmetry (distinct functional ends prevent meso-forms).
  • Applying the mathematical rule 2ⁿ to calculate the total number of stereoisomers.

Question 10 Breakdown

Compound: CH₃CH(Br)CH=CHCH(Cl)CH₃

✅ Correct Answer

Option D: 8 stereoisomers

Mark Scheme Note: 1 mark is awarded for selecting D. (Note: The attached mark scheme snippet shows Question 11 [A]; for this question, Question 10, the correct answer is D).

📐 Step-by-Step Analysis

  1. Chiral centre 1 (C2):
    Attached to -H , -CH₃ , -Br , and -CH=CHCH(Cl)CH₃ (4 different groups → 2 optical isomers).
  2. Alkene unit (C3=C4):
    Both C3 and C4 have two distinct groups attached ( -H and an alkyl halide group) → 2 geometric isomers (E and Z).
  3. Chiral centre 2 (C5):
    Attached to -H , -CH₃ , -Cl , and -CH=CHCH(Br)CH₃ (4 different groups → 2 optical isomers).
  4. Symmetry check:
    One terminal group has bromine ( -Br ) and the other has chlorine ( -Cl ). The molecule is unsymmetrical, meaning no meso forms exist.
  5. Total stereoisomers:
    2 × 2 × 2 = 2³ = 8 .

💡 Key Knowledge

  • Stereoisomers have the same structural formula but a different arrangement of atoms in 3D space.
  • They include both optical isomers (enantiomers around chiral carbons) and geometric isomers (E/Z isomers across restricted rotation double bonds).
  • When an unsymmetrical molecule contains n independent stereocentres/stereogenic units, the maximum number of stereoisomers is given by:
    Total Isomers = 2ⁿ
    Here, n = 3 (two chiral carbons + one E/Z double bond).

🧠 Exam Technique

  • Draw out the skeletal or displayed formula: Condensed formulas hide geometric isomerism. Drawing the C=C double bond flat with trigonal planar angles (120°) immediately reveals whether E/Z isomerism is possible.
  • Check both ends of the chain: If both ends had the same halogen (e.g., both -Br ), meso stereoisomers would reduce the total count. Because one is -Br and the other is -Cl , all 8 combinations are unique.
  • Tree Diagram Approach:
    • (E) alkene → 4 optical combinations (RR, RS, SR, SS)
    • (Z) alkene → 4 optical combinations (RR, RS, SR, SS)
    Total = 4 + 4 = 8.

❌ Common Errors

  • Forgetting the double bond (choosing B: 4): Many students spot the two chiral carbons and calculate 2² = 4 , completely forgetting that "stereoisomers" includes E/Z isomerism.
  • Only counting optical or geometric isomerism: Assuming stereoisomerism only means E/Z (choosing A: 2).
  • Incorrectly assuming internal symmetry: Assuming the two chiral centres are identical and cancelling out enantiomers as meso forms without checking the halogen substituents.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.1 Basic concepts and hydrocarbons · 6.2 Nitrogen compounds, polymers and synthesis

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.