OCR A-Level Chemistry AS Depth in chemistry (02), June 2024: Question 5

10 marks · Medium difficulty · Structured Questions

Complete reaction pathways for the reactions of but-1-ene with bromine, hydrogen/nickel, and steam/acid, outline the electrophilic addition mechanism for the reaction with HBr, and explain why 2-bromobutane is formed as the major product.

Practise this question

Question

Question text

5 This question is about some reactions of alkenes.

(a) Complete the flowchart for the reactions of but-1-ene, by adding the structures of the organic

products in each box.

Br2 H2/Ni

H CH2CH3

C C

H H

but-1-ene

H O(g) / H+

Mixture of products

[4]

(b) HBr reacts readily with alkenes.

(i) Outline the mechanism for the reaction of but-1-ene with HBr to form 2-bromobutane.

Include curly arrows, relevant dipoles and the structure of the product.

H CH2CH3

C C

H H

[4]

(ii) During this reaction, a small amount of 1-bromobutane is also produced.

Explain why 2-bromobutane is the major product.

… [2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

5 (a) 4 ALLOW any combination of skeletal OR structural OR displayed

formula as long as unambiguous

DO NOT ALLOW structure if H(s) are missing from ONE

structural formula

……. BUT ALLOW any further omissions as ECF

Take care with numbers of carbons

IGNORE connectivity,

e.g.

ALLOW I I

OH CH2CH3

But DO NOT ALLOW –HO

one mark for each correct structure ✓✓✓✓

(b) (i) 4 Throughout,

20 ALLOW any combination of skeletal OR structural OR displayed

formula as long as unambiguous

For curly arrows,

ALLOW straight or snake-like arrows and small gaps

(see examples)

DO NOT ALLOW half headed or double headed arrows but allow

ECF if seen more than once

1st curly arrow must

1st curly arrow (from ANY alkene) • go to a H atom of H–Br

Curly arrow from double bond to H of H–Br ✓

partial charge on C=C • AND

DO NOT ALLOW

• start from, OR be traced back to any point across width of

C=C

2nd curly arrow 2nd curly arrow must

Correct dipole on H–Br + –

• start from, OR be traced back to, any part of H–Br bond

AND curly arrow for breaking of H–Br bond ✓ –

• AND go to Br

ALLOW ECF for 2nd and 3rd curly arrow marking points if used Br

instead of HBr

Question Answer 21 Marks Guidance

3rd curly arrow 3rd curly arrow must

Correct carbocation with + charge on C • go to the C+ of carbocation

AND curly arrow from Br– to C+ of carbocation ✓ AND

• start from, OR be traced back to any point across width of

DO NOT ALLOW δ+ on C of carbocation lone pair on :Br–

• OR start from – charge on Br– ion

(Lone pair NOT needed if curly arrow shown

from – charge on Br–)

IGNORE connectivity of alkyl groups in carbocation and product

IF drawn both intermediates and products with no labelling

ALLOW 3rd curly arrow mark BUT NOT product mark, unless

clearly labelled as’2-bromobutane’ or ‘major’ product

Correct product (independent mark) ✓ ALLOW ECF for product from incorrect carbocation. e.g. 1-

bromobutane:

(ii) (major product forms from) most/more stable 22 2 ALLOW carbonium ion for carbocation

intermediate/carbocation ✓

IGNORE descriptions of the major/minor product in terms of

(major product forms from a) secondary carbocation OR Markownikoff’s rule

carbocation bonded to more C atoms / more alkyl groups e.g. H atom joins to C with most H

OR carbocation bonded to fewer H atoms ✓

IGNORE references to stability of the product

ALLOW ORA

How to answer it

Reactions of Alkenes Study Guide

What this question tests

This question assesses your knowledge of alkene chemistry, specifically addition reactions (halogenation, hydrogenation, and hydration), electrophilic addition mechanisms involving curly arrows and dipoles, and explaining carbocation stability to determine major products.

Question Part (a)

Alkene Addition Reactions Flowchart

Completing organic product structures for but-1-ene reactions

✅ Correct Answers (Products)

With Br₂ (Halogenation): 1,2-dibromobutane ( CH₃CH₂CH(Br)CH₂Br )
With H₂ / Ni (Hydrogenation): butane ( CH₃CH₂CH₂CH₃ )
With H₂O(g) / H⁺ (Hydration - Mixture of products): butan-1-ol ( CH₃CH₂CH₂CH₂OH ) AND butan-2-ol ( CH₃CH₂CH(OH)CH₃ )

💡 Key Knowledge

Alkenes undergo electrophilic addition with halogens and steam/acid, and catalytic addition with hydrogen. As but-1-ene is unsymmetrical, hydration yields a mixture of positional isomers (butan-1-ol and butan-2-ol).

❌ Common Errors

Students often lose marks by drawing incorrect carbon chain lengths or missing hydrogen atoms on structural/displayed formulas. Ensure unambiguous connectivity (e.g., attach -OH properly via oxygen, not HO-).

🎯 Mark Allocation: 1 mark for each correct structure in the flowchart (Total: 4 marks).

Question Part (b)(i)

Electrophilic Addition Mechanism

Outlining the mechanism for but-1-ene reacting with HBr to form 2-bromobutane

✅ What Examiners Look For

Dipole: Partial positive charge on H ( δ⁺ ) and partial negative charge on Br ( δ⁻ ) on the H–Br bond.
First Curly Arrow: Starts from the C=C double bond and points directly to the δ⁺ hydrogen atom of H–Br.
Second Curly Arrow: Shows the heterolytic fission of the H–Br bond, starting from the bond and pointing to the Br atom.
Intermediate: A secondary carbocation structure showing a positive charge ( ⁺ ) on the correct secondary carbon atom.
Third Curly Arrow: Starts from the lone pair (or negative charge) on the bromide ion ( :Br⁻ ) and points to the positive carbon of the carbocation.
Product: Correct structure of 2-bromobutane.

🧠 Exam Technique & Precision

Arrow origins matter immensely! The first curly arrow can originate from any point along the C=C double bond width, but it must point to the H atom, not the space between atoms. Do not put a partial charge ( δ⁺ ) directly on a carbon of the carbocation—the positive charge is a full integer charge ( ⁺ ).

❌ Common Errors

Using half-headed (radical) fishhook arrows instead of standard double-headed curly arrows. Putting the first arrow originating from the H–Br bond instead of the alkene π-bond.

🎯 Mark Allocation: 4 marks available covering arrows, dipoles, intermediate carbocation, and final product.

Question Part (b)(ii)

Explaining Major vs. Minor Products

Why 2-bromobutane is formed as the major product

✅ Correct Answer / Mark Scheme Points

The major product forms from the most stable intermediate / carbocation.
2-bromobutane forms via a secondary carbocation (whereas 1-bromobutane forms via a primary carbocation).
Secondary carbocations are more stable because they have more alkyl groups / more carbon atoms bonded to the positive carbon, providing a greater inductive electron-donating effect.

💡 Examiner Guidance

Descriptions referencing Markownikoff's rule alone are insufficient for full credit at A-Level; examiners strictly reward explanations based on the stability of the intermediate carbocation.

🎯 Mark Allocation: 2 marks total (1 mark for mentioning stable intermediate/carbocation, 1 mark for specifying secondary carbocation / bonded to more alkyl groups).

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.