OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 14

1 mark · Medium difficulty · Multiple Choice

Identify which pair of alcohols from the given skeletal formulae are structural isomers of each other.

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Question

Multiple-choice question showing the skeletal formulae of four alcohols labeled W, X, Y, and Z. W is cyclopentanol (a five-membered carbon ring with an attached OH group). X is 2-methylbutan-2-ol. Y is hexan-3-ol. Z is 2-methylbut-3-en-1-ol with a terminal C=C double bond, a branched methyl group, and a primary alcohol group. The question asks which pair of alcohols are structural isomers of each other, offering options A (W and X), B (W and Y), C (W and Z), and D (X and Y).
Question text

14 The skeletal formulae of four alcohols, W, X, Y, Z, are shown below.

OH

W

OH

X

OH

Y

Z OH

Which pair of alcohols are structural isomers of each other?

A W and X

B W and Y

C W and Z

D X and Y

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme excerpt showing the correct answer for question 14 is option C, worth 1 mark.

14 C 1

How to answer it

OCR AS Chemistry • Module 4: Core Organic Chemistry

Structural Isomers of Alcohols

What this question tests

  • Definition of structural isomers: Compounds with the same molecular formula but different structural formulae.
  • Interpreting skeletal formulae: Accurately counting carbon and hydrogen atoms from vertices, line ends, and implicit bonds.
  • Degrees of unsaturation: Understanding that a cyclic ring or a C=C double bond each reduces the hydrogen count by 2 relative to an open-chain saturated alcohol.

Question 14 Breakdown & Analysis

Identify which pair of alcohols are structural isomers

✅ Correct Answer

C — W and Z

Award [1 mark] for selecting option C.

Both compound W (cyclopentanol) and compound Z (2-methylbut-3-en-1-ol) have the identical molecular formula: C₅H₁₀O .

💡 Key Knowledge

  • Structural isomers: Must have identical molecular formulae. If the number of carbons, hydrogens, or oxygens differs, they cannot be isomers.
  • Saturated aliphatic alcohols: General formula CnH2n+2O .
  • Unsaturation: Each ring or C=C double bond subtracts 2 H atoms, giving a general formula of CnH2nO .

📐 Step-by-Step Determination of Molecular Formulae

Convert each skeletal structure into its molecular formula by counting all carbon, hydrogen, and oxygen atoms:

Compound Structure Description Carbon Count Features / Unsaturation Molecular Formula
W Cyclopentanol 5 carbons (5-membered ring) 1 ring (−2 H compared to alkane) C₅H₁₀O
X 2-methylbutan-2-ol 5 carbons (4 in main chain + 1 branch) Fully saturated, open-chain C₅H₁₂O
Y Hexan-3-ol 6 carbons (6-carbon straight chain) Fully saturated, open-chain C₆H₁₄O
Z 2-methylbut-3-en-1-ol 5 carbons (4 in chain + 1 methyl) 1 C=C double bond (−2 H) C₅H₁₀O

Comparing formulae shows that only W and Z share the exact same molecular formula ( C₅H₁₀O ) while having different connectivity of atoms.

🧠 Exam Technique: Fast Elimination

  1. Count Carbons First: Compound Y has 6 carbons, while W, X, and Z have 5 carbons. Immediately eliminate any option containing Y (rules out B and D).
  2. Spot Degrees of Unsaturation:
    • W has a ring → formula is C₅H₁₀O.
    • X is fully saturated → formula is C₅H₁₂O.
    • Z has an alkene double bond → formula is C₅H₁₀O.
  3. Match identical counts → W and Z match. Select C in seconds!

❌ Common Examiner Traps & Misconceptions

  • Assuming different functional classes cannot be isomers: Students frequently disregard Z because it has a C=C double bond (an alkenol) while W is a cycloalkanol. Remember: structural isomers can have completely different functional groups!
  • Miscounting chain length in Y: Candidates often miscount vertices in zigzag skeletal chains and think Y has 5 carbons. Hexan-3-ol has 6 carbons.
  • Forgetting ring hydrogens: In W, four of the ring CH₂ groups contribute 8 H's, and the CH(OH) carbon contributes 1 H + 1 OH H = 10 H's in total. Students incorrectly assume cyclic compounds follow CnH2n+2O .

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.