Edexcel A-Level Chemistry Paper 2, June 2023: Question 1

4 marks · Medium difficulty · Short Open Response

Draw the skeletal formula of 1,3-dimethylcyclohexane, identify the general formula for a cycloalkene, and explain why reacting ethane with chlorine in the presence of UV light is a poor method to prepare 1,2-dichloroethane.

Practise this question

Question

Question text

1 This question is about some organic compounds.

(a) Draw the skeletal formula of 1,3-dimethylcyclohexane.

(1)

(b) What is the general formula for a cycloalkene?

(1)

A CnH2n–2

B CnH2n

C CnH2n+1

D CnH2n+2

The student suggests reacting ethane with chlorine in the presence of

ultraviolet radiation.

Give two reasons why this is not a good method to prepare 1,2-dichloroethane.

(2)

(Total for Question 1 = 4 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

1(a) Example of skeletal formula (1)

• skeletal formula

Do not award displayed or structural formulae

Question Answer Mark

number

1(b) (1)

The only correct answer is A (CnH2n−2)

B is incorrect because CnH2n is the general formula of an alkene or a cycloalkane

C is incorrect because CnH2n+1 is the general formula of an alkyl group

D is incorrect because CnH2n+2 is the general formula of a straight chain or branched chain alkane

Question

Answer Additional Guidance Mark

Number

1(c) An answer that makes reference to the following points: Allow names or formulae (2)

• you cannot control how many chlorine atoms will substitute

you cannot control the carbon atom on which the chlorine

will substitute

a mixture of / side / unwanted / products will form (1)

• need to separate the 1,2-dichlroethane from any other products Allow a specific example of a correct unwanted product

made (1) e.g chloroethane, 1,1,1-trichloroethane,

1,1-dichloroethane, 1-chlorobutane, butane

Do not award H2

Ignore

Low yield, waste products, damage to ozone, dangers of

UV, chain reaction, HCl

(Total for Question 1 = 4 marks)

How to answer it

Organic Compounds and Synthesis Study Guide

What this question tests

This question assesses fundamental organic chemistry knowledge, including interpreting and drawing skeletal formulae of cyclic alkanes, recognising general formulae for homologous series (specifically cycloalkenes), and evaluating the limitations of free-radical substitution as a laboratory synthesis method for halogenoalkanes.

Question 1 (a)

Drawing Skeletal Formulae

Draw the skeletal formula of 1,3-dimethylcyclohexane.

✅ Correct Answer

A hexagon ring representing the cyclohexane backbone, with two single bonds sticking outward at carbon positions 1 and 3 to represent the methyl groups ( -CH₃ ).

💡 Key Knowledge

In skeletal formulae, carbon and hydrogen atoms attached to carbon are omitted. Vertices represent CH₂ groups (unless substituted).
Numbering around the ring starts at one methyl group (position 1) and moves to the second methyl group at position 3.

❌ Common Errors

Drawing a displayed or structural formula instead of a skeletal formula (this immediately loses the mark!).
Placing the methyl groups at positions 1 and 4 (yielding 1,4-dimethylcyclohexane).

Mark Breakdown: (1 mark) Awarded for a fully correct skeletal formula of 1,3-dimethylcyclohexane. No credit is given for full structural or displayed formulae.

Question 1 (b)

General Formulae of Homologous Series

What is the general formula for a cycloalkene?

✅ Correct Answer

A: CₙH₂ₙ₋₂

💡 Key Knowledge

Straight/branched chain alkenes and cycloalkanes both share the general formula CₙH₂ₙ .
Adding a ring structure introduces one degree of unsaturation (reducing hydrogens by 2). Adding a double bond introduces a second degree of unsaturation. Therefore, a cycloalkene (one ring + one double bond) has the general formula CₙH₂ₙ₋₂ .

❌ Common Errors

Selecting CₙH₂ₙ (Option B), which describes alkenes with a single double bond or standard cycloalkanes.
Selecting CₙH₂ₙ₊₂ (Option D), which is the general formula for standard saturated alkanes.

Mark Breakdown: (1 mark) For identifying option A.

Question 1 (c)

Evaluation of Synthetic Routes

Give two reasons why reacting ethane with chlorine under UV light is not a good method to prepare 1,2-dichloroethane.

✅ Correct Answer

Any two of the following validation points:

Lack of control over substitution (substitution can occur at different carbon atoms or multiple times).
Formation of a complex mixture of side products (e.g., chloroethane, 1,1-dichloroethane, 1,1,1-trichloroethane).
Difficulty in separating the desired 1,2-dichloroethane from the various other unwanted organic products formed during the free-radical substitution reaction.

🧠 Exam Technique & Examiner Guidance

Keep answers focused on the reaction mechanism: free-radical substitution is notoriously unselective.
Acceptable alternative examples of unwanted products include chloroethane, 1,1-dichloroethane, or 1-chlorobutane.
Do not waste time mentioning low yield, waste products, ozone damage, or HCl as a reason—whilst true, the mark scheme specifically requires points addressing lack of positional/extent control and the need for difficult separation.

Mark Breakdown: (2 marks) 1 mark for identifying poor control over substitution / formation of a mixture of products, and 1 mark for highlighting the difficult separation process required to isolate 1,2-dichloroethane.

Topics

Organic Chemistry · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.