OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 25

8 marks · Medium difficulty · Structured Questions

Explain the boiling points of butane and pentane, write propagation equations for the radical substitution of butane with bromine, and complete an organic reaction flowchart for alkenes.

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Question

Question text

25 This question is about hydrocarbons.

(a) The boiling points of 2 hydrocarbons are shown below.

Hydrocarbon Boiling point / °C

butane 0

pentane 36

Explain the difference in the boiling points of butane and pentane.

… [2]

(b) Butane reacts with bromine by radical substitution to form a mixture of organic products.

The reaction needs UV radiation for the initiation stage.

Write equations for the propagation stage that follows to form 2-bromobutane.

Use skeletal formulae and ‘dots’ (•) to show the position of any radicals.

Propagation

[2]

Three reactions of an alkene are shown in the flowchart.

Complete the flowchart to show the missing reagents, catalysts and the structures of organic

products.

H H

H3C C C CH3

H H

alkane

Reagent …

Catalyst …

H3C CH3

HBr

C C

H H

bromoalkane

Reagent …

Catalyst …

alcohol

[4]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

25 (a) 2 AO1.1 Comparisons needed throughout

Each marking point is independent 2 ORA throughout

Chain length: interaction between molecules

Chain length (in pentane) is longer Assume the following for longer chain

AND ▪ larger/bigger molecule

more (surface) contact ▪ more C (and H)

OR greater surface area/SA✓ ▪ more atoms

▪ more electrons

BUT ‘branching’ is a CON

IGNORE comments about packing

London forces: strength and energy

ALLOW induced dipole(–dipole)

Stronger / more London forces

interactions for London forces

OR more energy to break London forces ✓

IGNORE van der Waals’/vdw forces

(b) 2 AO3.1 ALLOW 1 mark (ECF) for 2 ‘correct’

Skeletal formulae required 2 equations with dot omitted or incorrectly

positioned

+ Br• + HBr

✓ ALLOW 1 mark for forming 1-bromobutane

with dots correct for 1-bromobutane

e.g.

+ Br2 + Br•

Br ✓

No credit for responses using molecular

formulae for organic structures

20 AO

element

structural OR displayed formula as long as

unambiguous

✔

H2 AND Ni (catalyst) AO1.2

ALLOW Pt OR Pd for Ni

H H

ALLOW H2O(g) for steam

H3C C C CH3 AO2.5 (g) OR temperature >100ºC required

✔

H Br

bromoalkane For acid,

ALLOW H+/H SO /H PO

24 3 4

✔

steam AND acid (catalyst) ALLOW small slip in acid formula

AO1.2 e.g. phosphoric acid as H2PO3, etc

H H

ALLOW vertical bond to any part of OH,

H3C C C CH3 i.e. OH OH OH

✔

| | |

H OH

alcohol AO2.5

BUT DO NOT ALLOW –HO OR OH–

How to answer it

AS Level Chemistry Study Guide: Hydrocarbons

What this question tests

This assessment covers fundamental organic chemistry topics: explaining intermolecular forces (London forces) in alkanes based on chain length, writing radical substitution mechanisms using skeletal formulae and radical dots, and recalling/applying alkene addition reactions (hydrogenation, electrophilic addition of hydrogen bromide, and hydration to form alcohols).

Part (a): Explaining Boiling Points of Alkanes

Boiling point differences between butane and pentane

✅ Correct Answer

Pentane has a longer carbon chain / larger molecule / more surface area.
Therefore, pentane has stronger London forces (or van der Waals' forces) requiring more energy to overcome.

💡 Key Knowledge

Boiling points in simple molecular substances depend directly on the strength of intermolecular forces. As chain length increases, molecular contact area increases, resulting in greater temporary dipole-induced dipole interactions.

🧠 Exam Technique

Always use comparative language (e.g., "pentane is larger", "London forces are stronger"). Examiners strictly penalize answers that talk about breaking covalent bonds instead of intermolecular forces.

❌ Common Errors

Students frequently lose marks by claiming that molecular bonds break during boiling, or by mentioning "branching" incorrectly. Mentioning "molecular weight" or "molecular mass" without linking it to surface area and contact will also lose credit.

Marks available: 2 marks (Independent marking points: 1 for chain length / surface area, 1 for stronger London forces / more energy required).

Part (b): Radical Substitution Propagation Equations

Forming 2-bromobutane via radical substitution

✅ Correct Answer

Propagation step 1:
CH₃CH₂CH₂CH₃ + Br• → CH₃CH(•)CH₂CH₃ + HBr

Propagation step 2:
CH₃CH(•)CH₂CH₃ + Br₂ → CH₃CH(Br)CH₂CH₃ + Br•

(Note: Skeletal formulae are required in the final mark scheme format).

💡 Key Knowledge

Propagation steps always feature a radical on both the reactant side and the product side. The dot (•) indicates an unpaired electron on a carbon or bromine atom.

🧠 Exam Technique

Draw clear skeletal lines. Make sure the radical dot is positioned precisely on the carbon atom holding the unpaired electron (specifically carbon-2 for 2-bromobutane).

❌ Common Errors

Using molecular formulae (like C₄H₁₀ ) instead of skeletal formulae results in zero credit for the organic structures. Missing the dot or placing it on the wrong carbon also drops marks.

Marks available: 2 marks (1 mark per correct propagation equation using skeletal formulae and correct radical positioning).

Part (c): Alkene Reaction Flowchart

Reactions of alkenes: Hydrogenation, Hydration, and Electrophilic Addition

✅ Correct Answer

Top branch (Hydrogenation to alkane): Reagent: H₂ | Catalyst: Ni (or Pt / Pd)

Middle branch: Given HBr forms 2-bromobutane.

Bottom branch (Hydration to alcohol): Reagent: Steam ( H₂O(g) ) | Catalyst: Acid ( H₃PO₄ or H₂SO₄ )

💡 Key Knowledge

Alkenes undergo addition reactions easily due to the electron-rich double bond (pi bond). Changing reaction conditions dictates whether you form an alkane, a halogenoalkane, or an alcohol.

🧠 Exam Technique

When specifying reagents and catalysts, be precise. Write "steam" or " H₂O(g) " for hydration—writing just " H₂O " implies liquid water and requires high temperatures/specific conditions to be accepted.

❌ Common Errors

Forgetting to specify the acid catalyst for hydration, or writing ambiguous chemical formulae for reagents (e.g., incorrect formulas for phosphoric acid).

Marks available: 4 marks (1 mark for top reagents/catalyst, 1 mark for bromoalkane structure, 1 mark for bottom reagents/catalyst, 1 mark for alcohol structure).

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.