Edexcel A-Level Chemistry Paper 2, June 2023: Question 8

14 marks · Hard difficulty · Extended Writing

Discuss isomerism in organic compounds involving structural isomers, electrophilic addition mechanisms, regioselectivity, and stereoisomerism including E/Z and optical isomers.

Practise this question

Question

Question text

8 This question is about isomerism in organic compounds.

(a) How many structural isomers are there with the formula C5H12?

(1)

A 2

B 3

C 4

D 5

(b) Propene reacts with bromine to form 1,2-dibromopropane as the only product.

Draw the mechanism for the reaction between propene and bromine.

Include curly arrows and any relevant lone pairs and dipoles.

(3)

1,2-dibromopropane, 1-bromo-2-chloropropane and 2-bromo-1-chloropropane

but no 1,2-dichloropropane.

(i) Explain, by reference to your mechanism in (b), why no

1,2-dichloropropane forms.

(2)

(ii) Explain why far more 1-bromo-2-chloropropane forms than

2-bromo-1-chloropropane.

(2)

… *P71913A02028*

*(d) Discuss the different types of stereoisomerism that occur in organic compounds.

Use only molecules A and B as examples.

CHCl C(CH3)Br CH3CH(OH)COOH

A B

Include in your answer:*P71913A02128*

• how the different types of isomerism arise

• the naming of alkenes with the formula A

• the properties of isomers with the formula B

• diagrams of the different isomers.

(6)

… 22

… *P71913A02228*

(Total for Question 8 = 14 marks)

Mark scheme

Show the mark scheme

Question Answer Mark

number

8(a) (1)

The only correct answer is B (3)

A is incorrect because pentane, 2-methylbutane and 2,2-dimethylpropane are the isomers of C5H12

C is incorrect because pentane, 2-methylbutane and 2,2-dimethylpropane are the isomers of C5H12

D is incorrect because pentane, 2-methylbutane and 2,2-dimethylpropane are the isomers of C5H12

Question

Answer Additional Guidance Mark

Number

8(b) An answer that makes reference to the following points: Allow skeletal formulae throughout. (3)

• curly arrow from C=C towards top Br

• dipole on Br2

• curly arrow from Br−Br bond to, or just beyond, lower Br Allow + on carbon 1 and Br on carbon 2

• intermediate

• lone pair on Br−

Do not award Brδ−

• curly arrow from lone pair towards C+

6 points awarded 3 marks

4 or 5 awarded 2 marks

2 or 3 awarded 1 mark

1 or 0 awarded 0 marks

Example of mechanism

Question

Answer Additional Guidance Mark

Number

8(c)(i) An explanation that makes reference to the following points: Allow these points shown on a labelled (2)

mechanism

• because sodium chloride (only) contains Cl− ions and the initial attack is by Allow (the first step involves an electrophile

an electrophile (1) but) Cl− is a nucleophile / not an electrophile /

there is no Cl+ / Cl +

• Brδ+ / bromine must attack first (so no dichloro-product can form) Allow Cl− only reacts when a carbocation has

(1) been formed

Do not award Br−

Do not award Br+ must attack first.

Question

Answer Additional Guidance Mark

Number

8(c)(ii) An explanation that makes reference to the following points: Allow answers referring to the mechanism (2)

• because a secondary carbocation (intermediate) is formed (1) Allow a correct diagram or description of the

secondary carbocation

• more stable than a primary (carbocation intermediate) (1)

Allow a correct diagram or description of the

primary and/or the secondary carbocation with

a comment about it being less stable /

secondary being more stable for 2 marks

Question

Acceptable Answers Additional Guidance Mark

Number

8(d)* This question assesses a student’s ability to show a coherent and logically Guidance on how the mark scheme should be (6)

structured answer with linkages and fully-sustained reasoning. applied:

The mark for indicative content should be added

Marks are awarded for indicative content and for how the answer is structured to the mark for lines of reasoning. For example,

and shows lines of reasoning. an answer with five indicative marking points that

is partially structured with some linkages and

The following table shows how the marks should be awarded for indicative lines of reasoning scores 4 marks (3 marks for

content. indicative content and 1 mark for partial structure

and some linkages and lines of reasoning).

Number of Number of marks If there are no linkages between points, the same

indicative marking awarded for five indicative marking points would yield an

points seen in indicative marking overall score of 3 marks (3 marks for indicative

answer points content and no marks for linkages).

5–4 3

3–2 2

The following table shows how the marks should be awarded for structure and

lines of reasoning.

Number of marks In general it would be expected that 5 or 6

awarded for structure indicative points would get 2 reasoning marks,

of answer and and 3 or 4 indicative points would get 1 mark for

sustained line of reasoning, and 0, 1 or 2 indicative points would

reasoning score zero marks for reasoning.

Answer shows a coherent and logical 2

structure with linkages and fully

sustained lines of reasoning

demonstrated throughout.

Answer is partially structured with some 1

linkages and lines of reasoning.

Answer has no linkages between points 0 General points to note

and is unstructured. If there is any incorrect chemistry, deduct mark(s)

Comment: from the reasoning. If no reasoning mark(s)

Look for the indicative marking points first, then consider the mark for awarded do not deduct mark(s).

structure of answer and sustained line of reasoning e.g.

Indicative content

• IP1 – C=C Ignore cis-trans

restricted rotation around C=C / double bond / pi bond Allow ‘no rotation about C=C’

• IP2 – Attached atoms

the higher the atomic number of the attached atom, the higher the priority

of the group

and

in the E isomer the highest priority atom on each carbon are on opposite Allow atomic mass / mass of group / heaviest

sides of the double bond /

in the Z isomer the highest priority atom on each carbon atom are on the

same side of the double bond

• IP3 – Example of E/Z

Example of E isomer and Z isomer as labelled diagram

E isomer Z isomer

Ignore chemical names even if incorrect

Allow diagrams without labels but clearly

identified in their explanation

• IP4 – Optical isomerism

a non-superimposable mirror image / cannot be superimposed / contain a Allow have four different atoms or groups

chiral centre / do not have a plane of symmetry attached to a carbon atom

• IP5 – Example of optical isomers

Two 3-dimensional structures that are mirror images

Do not award images that are not in 3D

• IP6 – Property

(optical isomers) rotate the plane of (plane-) polarised light (by equal

amounts) in opposite directions

How to answer it

Organic Isomerism, Electrophilic Addition, and Stereoisomerism

What this question tests

This comprehensive organic chemistry question assesses structural isomerism counting, electrophilic addition mechanisms involving curly arrows and dipoles, carbocation stability intermediates, and an extended-writing 6-mark evaluation of stereoisomerism (both E/Z geometric isomerism and optical isomerism).

Part (a) — Structural Isomers of C₅H₁₂

Question 8(a)

✅ Correct Answer

B (3)

Marks: 1

💡 Key Knowledge

Alkanes with formula C₅H₁₂ have 3 chain isomers: pentane, 2-methylbutane, and 2,2-dimethylpropane.

❌ Common Errors

Confusing structural isomers with stereoisomers, or failing to systematically check for branched-chain variations.

Part (b) — Mechanism of Electrophilic Addition

Question 8(b)

✅ Correct Answer & Mark Breakdown (3 Marks)

Mark 1: Curly arrow from the C=C pi bond pointing towards the slightly positive (d+) bromine atom, plus the Br-Br dipole label (d+ and d-).
Mark 2: Formation of the correct carbocation intermediate and a curly arrow pointing from the Br-Br bond (or just beyond) towards the lower bromine atom.
Mark 3: Bromide ion (Br⁻) with a clearly shown lone pair, and a curly arrow from that lone pair pointing to the positively charged carbon (C⁺).

🧠 Exam Technique

Always draw curly arrows starting precisely from a bond or a lone pair. Make sure dipoles are explicitly labelled on symmetrical molecules like Br₂ when they approach a high electron-density region like a double bond.

❌ Common Errors

Drawing curly arrows ending randomly in space rather than touching the target atom/bond. Omitting lone pairs on the attacking nucleophile (Br⁻).

Part (c)(i) — Absence of 1,2-dichloropropane

Question 8(c)(i)

✅ Correct Answer (2 Marks)

Sodium chloride only provides chloride ions (Cl⁻), whereas the initial attack on the alkene must be carried out by an electrophile (the polarized bromine molecule, Br₂).
Therefore, bromine must act as the electrophile first to initiate the reaction; chloride ions cannot initiate the electrophilic addition.

💡 Key Knowledge

Electrophilic addition to alkenes requires an electrophile to create a carbocation intermediate before a nucleophile can attack.

Part (c)(ii) — Regioselectivity and Carbocation Stability

Question 8(c)(ii)

✅ Correct Answer (2 Marks)

The reaction pathway forms a secondary carbocation intermediate.
Secondary carbocations are more stable than primary carbocations due to the positive inductive effect of more alkyl groups.

🧠 Exam Technique

Always explicitly state the *type* of carbocation formed (secondary vs primary) and link it directly to stability to secure both marks.

Part (d) — Extended Response: Stereoisomerism in Molecules A and B

Question 8(d)* (6 Marks)

✅ Indicative Content Points (IP1 - IP6)

IP1 (E/Z Isomerism): Restricted rotation around the C=C double bond (due to the pi bond).
IP2 (Assigning Priority): Higher atomic number of the attached atom gives higher priority. In the *E* isomer, high-priority groups are on opposite sides; in the *Z* isomer, they are on the same side.
IP3 (Diagrams for A): Clear 2D structural or skeletal diagrams displaying both *E* and *Z* stereoisomers for CHCl=C(CH₃)Br .
IP4 (Optical Isomerism): Arises due to a chiral centre (4 different groups attached to a carbon atom), creating non-superimposable mirror images (enantiomers) with no plane of symmetry.
IP5 (Diagrams for B): Clear 3D tetrahedral representation of the two optical enantiomers for CH₃CH(OH)COOH using dashed/wedged bonds.
IP6 (Physical/Chemical Property): Enantiomers rotate the plane of plane-polarised light in opposite directions.

🧠 Level of Response Strategy

To achieve top-tier marks (Level 3, 5-6 marks), your answer must contain at least 5-6 valid indicative points combined with a logical structure, clear scientific terminology, and well-drawn 3D/2D stereochemical diagrams.

❌ Common Errors

Using outdated terms like "cis-trans" instead of "*E*/*Z*" when describing alkene stereoisomerism, or failing to draw 3D tetrahedral wedges/dashes for optical isomers.

Topics

Organic Chemistry · Topic 6: Organic Chemistry I · Topic 17: Organic Chemistry II

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.