OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 10

1 mark · Medium difficulty · Multiple Choice

Identify the number of alicyclic structural isomers of C5H10.

Practise this question

Question

Multiple choice question 10 asking for the number of alicyclic structural isomers of C5H10, with four options: A (3), B (4), C (5), and D (6). Below the options is a box labelled 'Your answer' and a mark allocation of [1] at the right.
Question text

10 What is the number of alicyclic structural isomers of C5H10?

A 3

B 4

C 5

D 6

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 10 is C, with 1 mark awarded.

10 C 1 2.5 ALLOW 5

How to answer it

Alicyclic Structural Isomers of C₅H₁₀

What this question tests

This question assesses your ability to understand structural isomerism, specifically applying the definition of alicyclic compounds (non-aromatic rings) to a given molecular formula (C₅H₁₀) and systematically drawing or deducing all possible ring structures.

Question 10

Determining the Number of Alicyclic Isomers

✅ Correct Answer

The correct option is C (5).

There are 5 distinct alicyclic structural isomers with the molecular formula C₅H₁₀.

💡 Key Knowledge

  • Alicyclic: Aliphatic cyclic compounds—rings of carbon atoms that do not contain a benzene ring system.
  • Degree of Unsaturation: C₅H₁₀ has a general formula of CₙH₂ₙ, which corresponds to 1 ring (or 1 double bond). Since we need alicyclic isomers, all must contain a single ring.

🧠 Exam Technique

To avoid missing any isomers or double-counting, build them systematically by varying ring size and alkyl substituent branches:

  1. Start with the largest possible ring (5-membered).
  2. Decrease the ring size by one carbon and place that carbon as a methyl side-chain (4-membered ring).
  3. Further decrease ring size and rearrange substituent groups (3-membered rings with methyl/ethyl or two methyl groups).

❌ Common Errors

  • Including alkenes: Students often forget that C₅H₁₀ also represents straight-chain and branched alkenes. The question specifically asks for alicyclic isomers.
  • Double counting positional isomers: For example, placing a methyl group on different carbons of a cyclobutane ring yields the same molecule due to symmetry.

📐 Step-by-Step Breakdown of the 5 Isomers

  1. Cyclopentane: A 5-membered carbon ring. ( C₅H₁₀ )
  2. Methylcyclobutane: A 4-membered ring with one methyl branch. ( C₅H₁₀ )
  3. 1,1-dimethylcyclopropane: A 3-membered ring with two methyl branches on the same carbon. ( C₅H₁₀ )
  4. cis-1,2-dimethylcyclopropane: A 3-membered ring with methyl groups on adjacent carbons pointing on the same side. ( C₅H₁₀ )
  5. trans-1,2-dimethylcyclopropane: A 3-membered ring with methyl groups on adjacent carbons pointing on opposite sides. ( C₅H₁₀ ) *(Note: 1,2-dimethyl isomers also involve stereoisomerism, but both contribute to valid structural arrangements of carbon skeletons requested at this level).* Alternatively, counting **ethylcyclopropane** (3-membered ring with an ethyl group) completes the 5 structural carbon skeletons.
Mark Scheme Note: 1 mark awarded for selecting option C. Module reference: 2.5 (Isomerism).

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), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.