OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 7

1 mark · Medium difficulty · Multiple Choice

Identify the organic product formed when menthol is heated with an acid catalyst.

Practise this question

Question

The question shows the chemical structure of menthol, a substituted cyclohexane ring with a methyl group, an isopropyl group, and an alcohol group attached. Below it, four alternative structures labeled A, B, C, and D are given, each showing different alkene products resulting from dehydration, and the user is asked to select the correct structure.
Question text

7 Menthol, shown below, is heated with an acid catalyst.

OH

menthol

Which structure could be formed?

A B C D

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer is option A.

7 A 1 AO1.2

How to answer it

Acid-Catalysed Dehydration of Menthol

OCR A-Level Chemistry — Multiple Choice Question

What this question tests

This question assesses your understanding of alcohol chemistry, specifically the elimination (dehydration) reaction of secondary alcohols when heated with an acid catalyst (such as concentrated H₂SO₄ or H₃PO₄). You need to trace carbon numbering, identify the position of the hydroxyl group, and visualise how removing an OH group alongside an adjacent hydrogen creates a carbon-carbon double bond (alkene).

Question 7 Breakdown

Dehydration Mechanism & Isomer Identification

✅ Correct Answer: A

Structure A correctly depicts the elimination of water from the secondary alcohol carbon (bearing the OH) and its adjacent carbon, forming an alkene function within the ring at the correct relative position to the methyl and isopropyl groups.

Awarded 1 mark for selecting A (AO1.2).

💡 Key Knowledge

  • Dehydration: Alcohols lose a water molecule (H₂O) in the presence of an acid catalyst to form alkenes.
  • Regioselectivity: The OH group leaves from carbon-1 (relative to ring numbering where OH is attached), and an adjacent hydrogen is removed from carbon-2 or carbon-6 to form the double bond.
  • Skeleton tracking: Always track fixed anchor points like the methyl group (CH₃) at the top and the isopropyl group at the bottom to avoid getting tricked by rotated ring structures.

🧠 Exam Technique

When dealing with cyclic structures and multiple choice options, locate reference substituents (like the methyl and isopropyl groups) first. Use them to "orientate" each option in your head so you can directly compare where the double bond ends up relative to the substituents.

❌ Common Errors

  • Choosing C because students misinterpret the side-chain or mistake a substitution for an elimination reaction.
  • Confusing the numbering of the ring carbons, leading to selection of structures B or D where the double bond is incorrectly placed adjacent to the methyl-bearing carbon.

Topics

Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.