OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify which formula does not represent 3-methylbut-1-ene among the given options.

Practise this question

Question

Multiple choice question 15 asking which formula does not represent 3-methylbut-1-ene, with options A to D in a table. Option A shows CH3CHCH3CHCH2, option B shows CH2CHCH(CH3)2, option C shows a skeletal formula of 3-methylbut-1-ene, and option D shows a skeletal formula of 2-methylbut-1-ene.
Question text

15 Which formula does not represent 3-methylbut-1-ene?

A CH3CHCH3CHCH2

B CH2CHCH(CH3)2

C

D

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 15 is D, worth 1 mark.

15 D 1 AO1.2

How to answer it

Interpreting Organic Formulae and Isomers

What this question tests

This question assesses your ability to interpret different representations of organic molecules—specifically structural, displayed, skeletal, and molecular formulae. You must translate condensed and skeletal structures into IUPAC names to identify which option represents a different isomer (a structural isomer) rather than 3-methylbut-1-ene.

Question 15: Detailed Breakdown

Identifying 3-methylbut-1-ene and Finding the Odd One Out

✅ Correct Answer: D

Option D shows 2-methylbut-1-ene, not 3-methylbut-1-ene. The double bond is situated between carbon-1 and carbon-2 of a 4-carbon principal chain, with a methyl group also on carbon-2.

Marks: 1 / 1 (AO1.2)

💡 Key Knowledge

  • IUPAC Naming Rules: Find the longest carbon chain containing the functional group (alkene = ends in -ene). Number the chain from the end nearest the functional group.
  • Skeletal Formulae: Lines represent carbon-carbon bonds. Each vertex/end represents a carbon atom. Hydrogen atoms attached to carbons are implied (assuming carbon forms 4 bonds).
  • Condensed Formulae: Groupings like CH(CH₃)₂ mean two methyl groups are branched off the adjacent carbon.

🧠 Exam Technique

Do not just guess based on overall molecular formula (all options might share the same formula C₅H₁₀). Instead, systematically name each option or trace the carbon skeleton to check connectivity.

❌ Common Errors

Students often misinterpret condensed bracketed groups like CH(CH₃)₂ as straight-chain extensions, or miscount the main chain length in skeletal drawings by missing the terminal methyl ends.

Analysing All Options for 3-methylbut-1-ene (C₅H₁₀):

  • Target Structure: CH₂=CH-CH(CH₃)-CH₃ (A 4-carbon chain with a double bond at position 1 and a methyl branch at position 3).
  • Option A: CH₃CHCH₃CHCH₂ (Condensed representation showing the correct connectivity for 3-methylbut-1-ene).
  • Option B: CH₂CHCH(CH₃)₂ ( CH₂=CH- attached to a carbon with two methyl branches, exactly matching 3-methylbut-1-ene).
  • Option C: Skeletal formula showing a 4-carbon chain with a double bond at the end (position 1) and a branch at carbon 3 (= 3-methylbut-1-ene).
  • Option D: Skeletal formula showing a 4-carbon chain with a double bond at carbon-1, but the methyl branch is on carbon-2 instead of carbon-3. This gives 2-methylbut-1-ene, making it the correct choice for "does not represent".

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.