OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 16

1 mark · Easy difficulty · Multiple Choice

Identify the correct reagents needed to convert butan-2-ol into 2-bromobutane.

Practise this question

Question

Multiple choice question 16 asking for the correct reagents for the conversion of CH3CH2CHOHCH3 into CH3CH2CHBrCH3. Four options are given: A (Br2 and H2SO4), B (Br2 and NaOH), C (NaBr and H2SO4), and D (NaBr and NaOH), along with a box for the answer and a mark allocation of [1].
Question text

16 What are the correct reagents for the conversion below?

CH3CH2CHOHCH3 → CH3CH2CHBrCH3

A Br2 and H2SO4

B Br2 and NaOH

C NaBr and H2SO4

D NaBr and NaOH

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing question number 16 with the correct answer as C, worth 1 mark.

16 C 1 1.1

How to answer it

Reagents for Alcohol to Haloalkane Conversion

OCR AS Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your knowledge of synthetic organic chemistry pathways, specifically the substitution reaction used to convert secondary alcohols into haloalkanes, and the in-situ generation of hydrogen halide reagents.

Question 16 Analysis

Conversion: CH₃CH₂CHOHCH₃ → CH₃CH₂CHBrCH₃

✅ Correct Answer: C

NaBr and H₂SO₄

The sodium bromide reacts with concentrated sulfuric acid in situ to produce hydrogen bromide ( HBr ), which then substitutes the -OH group on the secondary alcohol to form the bromoalkane.

💡 Key Knowledge

  • Alcohols react with hydrogen halides to form haloalkanes (nucleophilic substitution).
  • HBr is often generated in situ by reacting a sodium halide (like NaBr ) with an acid (like H₂SO₄ ).
  • The functional group change is -OH (alcohol) to -Br (haloalkane).

🧠 Exam Technique

Break down the transformation: you are replacing an -OH group with a -Br atom. Look for options that supply a source of bromide ions ( Br⁻ ) and an acid catalyst/proton donor.

❌ Common Errors

  • Choosing A ( Br₂ and H₂SO₄ ): Bromine water/liquid is used for electrophilic addition across C=C double bonds, not substitution in alcohols.
  • Choosing B or D: NaOH introduces alkaline conditions which promote substitution back from haloalkanes to alcohols, or elimination to form alkenes.
Mark Scheme Allocation: 1 mark for selecting option C. (Specification reference: 2.1.3 Alcohols).

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.