Edexcel A-Level Chemistry Paper 2, June 2024: Question 5

12 marks · Medium difficulty · Calculations

Analyze cyclic compounds through reactions including reforming, energy density calculations, free-radical substitution of cyclopentane, and elimination of bromocyclopentane.

Practise this question

Question

Question text

5 This question is about cyclic compounds.

(a) The hydrocarbon cyclopentane is present in some fuels and is used in the

manufacture of insulation for freezers.

cyclopentane

Cyclopentane can be synthesised by passing 2‑methylbutane over a hot

platinum catalyst.

+ H2

heat

2‑methylbutane cyclopentane

Name the type of reaction that takes place in this synthesis.

(1)

(b) The energy density of a fuel is defined as the energy released per dm3 of the

liquid fuel burned.

A sample of cyclopentane with a mass of 30.0g releases 1.41MJ of energy.

Calculate the energy density of cyclopentane.

Include units in your answer.

[Density of cyclopentane = 0.751 g cm–3]

(2)

form bromocyclopentane.

+ Br2 + HBr

cyclopentane bromocyclopentane

(i) State the condition needed to initiate the reaction.

(1)

(ii) Complete the table showing the steps of the mechanism of this reaction.

Curly arrows are not required.

(3)

Step *P76896A01432*Equation(s)

Initiation

+ Br + HBr

Propagation

Termination

(iii) Draw the structure of the organic product of an alternative termination step.

(1)

(d) Bromocyclopentane forms cyclopentene when heated under reflux with a

concentrated solution of potassium hydroxide in ethanol.

+ HBr

(i) Explain why the solvent ethanol is treated with anhydrous sodium sulfate*P76896A01532*

before use in this reaction.

(2)

(ii) Give the name of the reaction shown in the equation.

(1)

(iii) Predict the mechanism for the reaction by adding two curly arrows.

(1)

+ Br– + H O

:OH–

H H

(Total for Question 5 = 12 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

5(a) An answer that makes reference to the following point: (1)

• reforming/reformation Allow dehydrogenation / cyclisation

Ignore oxidation / catalytic

Do not award cracking / dehydration

Question

Answer Additional Guidance Mark

Number

5(b) Example of calculation (2)

• calculate volume of cyclopentane (1) Volume = 30.0 ÷ 0.751 / 39.947 (cm3)

Ignore units in M1

(1) 1.41 ÷ (39.947 ÷ 1000) = 35.297 MJ dm–3

• calculate energy density of cyclopentane

and

appropriate units Ignore SF except 1 SF

Allow TE from M1

Allow 3.5297 × 104 kJ dm–3 or 3.5297 × 107 J dm–3

3.5297 × 104 J cm–3

Question

Answer Additional Guidance Mark

Number

5(c)(i) An answer that makes reference to the following point: (1)

• UV (light) / ultraviolet (light) Allow sun(light) / temperature over 250°C

Ignore heat under reflux

Do not award high pressure

Question

Answer Additional Guidance Mark

Number

5(c)(ii) An answer that makes reference to the following points: NOTE – penalise use of hexagons once (3)

only in 5(c)(ii) and 5(c)(iii)

• Initiation Br → 2Br• (1)

Penalise omission of unpaired electrons

once only

Curly-half arrows if shown must be in

correct direction

Do not award double-headed arrow(s) in

• Propagation Penalise use of chlorine once only

(1)

• Termination

(1)

Ignore position of bromine relative to dot

on free radical

Question

Answer Additional Guidance Mark

Number

5(c)(iii) Allow displayed formulae (1)

•

Do not award

Question

Answer Additional Guidance Mark

Number

5(d)(i) An explanation that makes reference to the following points: (2)

• to dry the ethanol / solvent (1) Allow remove water from the ethanol / solvent

Allow to react with the ethanol to remove the water

Allow to ensure the reaction conditions are not

aqueous

Ignore just ‘it’s a drying agent’

Ignore to dry the reactants / removes water that

forms in the reaction

Do not allow ‘to dehydrate’

• as (presence of) water would result in formation of (1)

(some) cyclopentanol Allow alcohol / hydroxycyclopentane for

cyclopentanol

Allow as (presence of) water would result in (some)

(nucleophilic) substitution (by the hydroxide ion)

Ignore hydrolysis

Question

Answer Additional Guidance Mark

Number

5(d)(ii) An answer that makes reference to the following point: Comment: Allow dehydrohalogenation (1)

Do not award nucleophilic / electrophilic / free-radical

• elimination Ignore oxidation / reduction / basic

Question

Answer Additional Guidance Mark

Number

5(d)(iii) (1)

• double-headed arrow from C-H bond to C-C

bond

and

double-headed arrow from C-Br bond to Br

Ignore any dipoles, even if incorrect

(Total for Question 5 = 12 marks)

How to answer it

Cyclic Compounds & Organic Reaction Mechanisms Study Guide

What this question tests

This question assesses core organic chemistry topics including type of synthesis (reforming), multi-step calculations involving density and energy density units, free radical substitution mechanisms (initiation, propagation, termination), and elimination reactions of halogenoalkanes using ethanolic potassium hydroxide.

Question 5 (a)

Type of Synthesis Reaction

✅ Correct Answer

Reforming (or reformation)

Marks: 1

💡 Key Knowledge

Converting a straight-chain or branched alkane into a cyclic alkane (cycloalkane) and hydrogen gas over a platinum catalyst is classified as reforming or cyclisation.

❌ Common Errors

Writing 'cracking' or 'dehydration'. Cracking breaks carbon-carbon chains rather than forming rings, and dehydration involves the loss of water.

Question 5 (b)

Energy Density Calculation

📐 Step-by-Step Calculation

Find the volume of cyclopentane:
Volume = Mass ÷ Density = 30.0 g ÷ 0.751 g cm⁻³ = 39.947 cm³
Convert volume to dm³:
Volume in dm³ = 39.947 ÷ 1000 = 0.039947 dm³
Calculate energy density (Energy per dm³):
Energy Density = 1.41 MJ ÷ 0.039947 dm³ = 35.3 MJ dm⁻³ (or 35297 J cm⁻³)

Marks: 2

🧠 Exam Technique & Units

Always check volume unit conversions (cm³ to dm³ divide by 1000).
Include appropriate units in your final answer (e.g., MJ dm⁻³ or kJ dm⁻³ ).
Allow error-carried-forward (TE) if step 1 calculation was incorrect.

Question 5 (c) (i)

Initiation Condition for Halogenation

✅ Correct Answer

UV light (or ultraviolet light / sunlight / temperature over 250°C)

Marks: 1

❌ Common Errors

Writing 'heat under reflux' or 'high pressure'. UV light or high temperature is specifically required for homolytic fission of the halogen bond.

Question 5 (c) (ii)

Free Radical Substitution Mechanism Table

✅ Correct Answer Equations

Initiation: Br₂ → 2Br•
Propagation step 2: Cyclopentyl radical + Br₂ → bromocyclopentane + Br•
Termination: Cyclopentyl radical + Br• → bromocyclopentane

Marks: 3

❌ Common Errors

Omitting unpaired radical dots ( • ), using double-headed arrows instead of single-headed/half-arrows (if shown), or referencing chlorine instead of bromine.

Question 5 (c) (iii)

Alternative Termination Product

✅ Correct Answer

Two cyclopentyl radicals combining together: 1,1'-bicyclopentyl (drawn as two cyclopentane rings joined by a single C-C bond, with a dot removed and a bond connecting them).

Marks: 1

💡 Key Knowledge

Termination steps involve two free radicals colliding and reacting together to form a single, stable molecule, effectively doubling the carbon chain length or joining rings.

Question 5 (d) (i)

Role of Anhydrous Sodium Sulfate

✅ Correct Answers

To dry the ethanol / solvent (remove water).
To prevent the formation of cyclopentanol (prevent substitution competing with elimination).

Marks: 2

❌ Common Errors

Saying it 'dehydrates' the organic molecule or stating it removes water from the reactants. Anhydrous salts dry the *solvent/ethanol* to prevent aqueous substitution conditions.

Question 5 (d) (ii)

Name of Reaction

✅ Correct Answer

Elimination (or dehydrohalogenation)

Marks: 1

💡 Key Knowledge

Halogenoalkanes heated with potassium hydroxide dissolved in *ethanol* (under reflux) undergo elimination to form alkenes.

Question 5 (d) (iii)

Curly Arrow Mechanism for Elimination

✅ Correct Answer

First curly arrow starts from the lone pair on oxygen in :OH⁻ pointing to the adjacent C-H hydrogen.
Second curly arrow starts from the C-H single bond and goes to the adjacent C-C bond to form a double bond.
Third curly arrow starts from the C-Br bond and goes entirely onto the bromine atom (forming Br⁻).

Marks: 1

🧠 Top-Level Exam Technique

Ensure your curly arrows originate precisely from bonds or lone pairs and point accurately towards atoms or bond spaces. Vague arrowheads lose credit instantly.

Topics

Organic Chemistry · Physical Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.