Edexcel A-Level Chemistry Paper 3, June 2022: Question 5

Mark scheme

Show the mark scheme

Question

Acceptable Answers Additional Guidance Mark

Number

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

logically structured answer with linkages and fully-sustained applied:

reasoning. The mark for indicative content should be added to

the mark for lines of reasoning. For example, an

Marks are awarded for indicative content and for how the answer answer with five indicative marking points that is

is structured and shows lines of reasoning. partially structured with some linkages and lines of

reasoning scores 5 marks (3 marks for indicative

The following table shows how the marks should be awarded for content and 2 marks for partial structure and some

indicative content. linkages and lines of reasoning).

If there are no linkages between points, the same

Number of Number of marks five indicative marking points would yield an

indicative awarded for overall score of 3 marks (3 marks for indicative

marking points indicative content and no marks for linkages).

seen in answer marking points

How to answer it

Comparing the Reactivity of Benzene and Phenol with Bromine

Edexcel A-Level Chemistry • Organic Chemistry (Arenes & Phenols)

What this question tests

This 6-mark extended-response question assesses your understanding of relative ring activation in aromatic systems. You must compare reaction types, required conditions, and organic products for benzene versus phenol, explaining why phenol reacts much more readily due to electron donation from an oxygen lone pair into the delocalised pi system.

Question 5* (6 Marks)

Explain the difference in the reactivity of bromine with benzene and with phenol. Include the type of reaction, the products that form, and any conditions required. Mechanisms are not required.

✅ Model Indicative Content

Benzene reaction: Electrophilic substitution. Requires a halogen carrier catalyst (e.g., FeBr₃ or AlBr₃ ) or liquid bromine under harsher conditions.
Benzene products: Bromobenzene ( C₆H₅Br ) and hydrogen bromide ( HBr ).
Phenol reaction: Electrophilic substitution (specifically ring halogenation/bromination). Occurs readily at room temperature with aqueous bromine (bromine water) without a catalyst.
Phenol products: 2,4,6-tribromophenol (a white precipitate) and hydrogen bromide ( HBr ).
Reactivity comparison / Explanation: Phenol is more reactive because the lone pair of electrons on the oxygen atom in the -OH group is partially delocalised into the π -system of the benzene ring.
This increases electron density around the ring, which polarises approaching electrophiles ( Br₂ ) more strongly, facilitating attack compared to delocalised π -electrons in unsubstituted benzene.

💡 Key Knowledge

Delocalisation: Benzene has a stable ring of delocalised π -electrons spread over all 6 carbon atoms, making it resistant to addition reactions that would destroy ring stability.
Activating groups: The -OH group is strongly activating and 2,4,6-directing due to p-π overlap.
Polarisation: Because electron density is so high in phenol, the non-polar Br-Br bond is induced to become sufficiently polarised to react without a Lewis acid catalyst.

🧠 Exam Technique & Level of Response

This is a Levels of Response question. To achieve Level 3 (maximum marks):

You must cover all chemical aspects: conditions, reactions, and products for both substances.
You must provide fully coherent scientific reasoning linking the oxygen lone pair to increased ring electron density.
Keep comparisons direct (e.g., state what benzene needs vs. what phenol can do at room temperature).

Mark Breakdown:
• Up to 4 marks for Indicative Chemical Content (conditions, products, mechanisms/types, electron effect).
• Up to 2 additional marks awarded based on the quality of written communication, logical structure, and seamless linkages between structure and reactivity.

❌ Common Student Errors

Wrong reaction type: Stating that either molecule undergoes an addition reaction with bromine (benzene never does under normal conditions; phenol substitutes, it does not add across a double bond like alkenes do).
Missing catalysts: Forgetting that benzene requires a halogen carrier catalyst ( FeBr₃ ) and describing it as reacting at room temperature like phenol.
Vague explanations: Saying "phenol has an OH group so it attracts bromine better" without explicitly mentioning the lone pair on oxygen delocalising into the ring and increasing electron density.

Topics

Organic Chemistry · Topic 18: Organic Chemistry III