OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 18

1 mark · Easy difficulty · Multiple Choice

Identify the product formed from the reaction of pent-2-ene with bromine

Practise this question

Question

Multiple choice question 18 asks: A student reacts pent-2-ene with bromine in the laboratory. Which compound is formed? Options are A: 1,1-dibromopentane, B: 1,2-dibromopentane, C: 2,2-dibromopentane, D: 2,3-dibromopentane. There is an answer box provided and a mark allocation of [1].
Question text

18 A student reacts pent-2-ene with bromine in the laboratory.

Which compound is formed?

A 1,1-dibromopentane

B 1,2-dibromopentane

C 2,2-dibromopentane

D 2,3-dibromopentane

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 18 showing the correct answer as D, with 1 mark awarded.

18 D 1

How to answer it

Reaction of Pent-2-ene with Bromine

What this question tests

This question assesses your knowledge of the chemical reactions of alkenes, specifically electrophilic addition of halogens across the carbon-carbon double bond, along with IUPAC organic nomenclature and numbering rules.

Question 18

Halogenation of Alkenes and IUPAC Nomenclature

✅ Correct Answer: D

2,3-dibromopentane

When bromine reacts with pent-2-ene, the C=C double bond is located between carbon-2 and carbon-3. The bromine atoms add across this double bond, attaching specifically to carbons 2 and 3.

💡 Key Knowledge

  • Pent-2-ene structure: CH₃-CH=CH-CH₂-CH₃
  • Reaction type: Electrophilic addition
  • Reagent/Observation: Bromine (Br₂) liquid or water decolourises from orange/brown to colourless.

🧠 Exam Technique

Draw out the skeletal or displayed formula of the organic reactant before choosing your answer. Number the longest carbon chain from the end that gives the lowest locant numbers to functional groups or substituents.

❌ Common Errors

  • Choosing 1,2-dibromopentane (B) by assuming the double bond is always at the end of the chain (confusing pent-2-ene with pent-1-ene).
  • Failing to number the carbon chain correctly, leading to incorrect locant numbers like 2,2-dibromopentane.
Mark Scheme Allocation: 1 mark for selecting option D.

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 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.