Edexcel A-Level Chemistry Paper 3, June 2024: Question 5
6 marks · Medium difficulty · Extended Writing
Compare and contrast the reactions of bromine with benzene and of bromine with cyclohexene, considering reaction types, products, special conditions, and justification.
Practise this questionQuestion
Question text
5 Compare and contrast the reactions of bromine with benzene and
of bromine with cyclohexene.
Consider the types of reaction that occur, the products that form, and any special
conditions that may be required.
Justify your answers.
Detailed mechanisms for the reactions are not required.
(6)
… *P74455A01032*
(Total for Question 5 = 6 marks)
Mark scheme
Show the mark scheme
Question
Answer Additional Guidance Mark
Number
*5 This question assesses the 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 reasoning. applied:
The mark for indicative content should be added
Marks are awarded for indicative content and for how the answer is to the mark for lines of reasoning. For example, a
structured and shows lines of reasoning. response with four indicative marking points that
is partially structured with some linkages and lines
The following table shows how the marks should be awarded for of reasoning scores 4 marks (3 marks for
indicative content. indicative content and 1 mark for partial structure
Number of indicative marking Number of marks awarded for and some linkages and lines of reasoning).
points seen in answer indicative marking points If there were no linkages between the points, then
64 the same indicative marking points would yield an
5-4 3 overall score of 3 marks (3 marks for indicative
3-2 2 content and zero marks for linkages).
The following table shows how the marks should be awarded for
structure and lines of reasoning
Number of marks awarded for
structure of answer and
sustained lines of reasoning
Answer shows a coherent logical 2
structure with linkages and fully
sustained lines of reasoning
demonstrated throughout
Answer is partially structured with 1
some linkages and lines of
reasoning
Answer has no linkages between 0
points and is unstructured
Indicative content
Similarity
• IP1 both reactions involve attack by an electrophile
Differences
• IP2 benzene reaction is a substitution, cyclohexene reaction is an
addition
• IP3 benzene forms 2 products, cyclohexene forms 1 product Benzene forms bromobenzene + HBr and
cyclohexene forms just 1,2-dibromocyclohexane
Allow equations for evidence of IP3 but not IP2
Ignore names if correct structures drawn
• IP4 so benzene reaction requires a catalyst of FeBr3 Allow Fe/AlBr3 as catalyst/halogen carrier
(to induce dipole in bromine) Allow AlCl3
Allow this shown by equation
e.g. FeBr + Br → FeBr – + Br+
32 4
Ignore reference to heat/reflux/room temperature
• IP5 cyclohexene reacts at room temperature / Accept cyclohexene is more reactive due to high
with bromine water electron density of double bond / localised pi
electrons
Ignore just ‘does not need a catalyst’
Ignore just ‘no special conditions’
• IP6 benzene is less reactive / more stable as (pi) electrons are
delocalised (across the ring)
(Total for Question 5 = 6 marks)
How to answer it
Comparing Reactions: Bromine with Benzene vs. Cyclohexene
What this question tests
This 6-mark extended-response question assesses your understanding of the contrasting chemical reactivities of aromatic compounds (benzene) versus unsaturated aliphatic alkenes (cyclohexene). You must compare reaction types, specific products, necessary catalysts/conditions, and link these back to underlying bonding structures (delocalised pi electrons vs. localised pi electrons).
Question Breakdown & Mark Scheme Analysis
To achieve full marks (4 marks for indicative content + 2 marks for structure and reasoning), your answer must draw explicit comparisons using key scientific terminology.
✅ Key Indicative Points (IPs)
- IP1 (Similarity): Both reactions involve attack by an electrophile.
- IP2 (Reaction Type): Benzene undergoes substitution (electrophilic substitution), whereas cyclohexene undergoes addition (electrophilic addition).
- IP3 (Products): Benzene forms 2 products (bromobenzene and HBr). Cyclohexene forms 1 product (1,2-dibromocyclohexane).
- IP4 (Catalyst): Benzene requires a halogen carrier catalyst (e.g., FeBr₃ or AlBr₃ ) to polarise the bromine molecule.
- IP5 (Conditions/Reactivity): Cyclohexene reacts readily at room temperature / with bromine water without a catalyst.
- IP6 (Bonding/Stability): Benzene is more stable due to delocalised pi electrons across the ring, whereas cyclohexene has localised electrons in a high-density double bond.
💡 Core Chemical Principles
- Delocalisation: The ring of electrons in benzene lowers its overall thermodynamic stability (higher resonance energy), making it resistant to reactions that would destroy the stable ring system.
- Electron Density: The localised pi bond in cyclohexene has a high electron density that readily polarises approaching non-polar Br₂ molecules, inducing a dipole automatically.
🧠 Exam Technique & Structure
- The 4+2 Mark Split: 4 marks are awarded for chemistry content (IP points), and 2 marks are awarded for clear, logical structure and coherent lines of reasoning.
- Use comparative link words (e.g., "whereas", "in contrast", "similarly") to secure the 2 structural reasoning marks.
❌ Common Student Errors
- Stating benzene does addition reactions (it preserves its ring system via substitution).
- Forgetting to mention the halogen carrier catalyst for benzene, or claiming cyclohexene needs harsh conditions.
- Failing to explicitly compare both molecules point-by-point, resulting in unstructured bullet points that lose the 2 quality-of-reasoning marks.
Topics
Organic Chemistry · Topic 6: Organic Chemistry I · Topic 18: Organic Chemistry III
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.