Edexcel A-Level Chemistry Paper 2, June 2022: Question 3

5 marks · Medium difficulty · Short Open Response

Complete the electrophilic addition mechanism for the reaction of 2-methylbut-1-ene with chlorine, name the product, and classify a given dihalogenoalkane structure.

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Question

Question text

3 This is a question about dihalogenoalkanes.

(a) Dihalogenoalkanes are formed when alkenes react with halogens.

(i) Complete the mechanism for the production of a dihalogenoalkane

from 2-methylbut-1-ene and chlorine.

Include curly arrows and any relevant lone pairs.

(3)

(ii) Give the name of the dihalogenoalkane produced.

(1)

(b) What is the classification of the dihalogenoalkane shown?

(1)

A primary

B secondary

C tertiary

D primary and secondary

(Total for Question 3 = 5 marks)

Mark scheme

Show the mark scheme

Question Answer

Additional Guidance Mark

Number

3(a)(i) Example of mechanism: (3)

• curly arrow from C=C to chlorine

and

curly arrow from Cl−Cl to ‘bottom’ chlorine atom(1) Ignore dipoles even if incorrect

• structure of carbocation intermediate Allow correct structural/displayed formulae for

and intermediate and/or product

structure of final product (1)

• chloride ion with lone pair Allow TE on incorrect primary carbocation

and

curly arrow from lone pair to C+ of carbocation (1)

Additional Guidance Mark

Number

3(a)(ii) (1)

• 1,2-dichloro-2-methylbutane Allow name shown on mechanism

Ignore missing hyphens and commas

Do not allow 2-methyl-1,2-dichlorobutane

TE on structure in (a)(i)

Allow correct name even if incorrect structure in (i)

Question

Answer Mark

Number

3(b) The only correct answer is A (primary) (1)

B is not correct because there is no chlorine atom bonded to a carbon atom which is bonded to two other

carbon atoms

C is not correct because there is no chlorine atom bonded to a carbon atom which is bonded to three other

carbon atoms

D is not correct because both chlorine atoms are bonded to carbon atoms which are bonded to only one

carbon atom

(Total Question 3 = 5 marks)

How to answer it

Reactions of Alkenes and Dihalogenoalkane Classification

📚 What this question tests

This question assesses your understanding of electrophilic addition mechanisms in alkenes, including carbocation stability, curly arrow notation, IUPAC naming conventions for halogenoalkanes, and the structural classification of halogenated organic compounds.

Question 3 (a)(i)

Electrophilic Addition Mechanism

✅ Correct Answer

Arrow 1: Curly arrow starting from the C=C double bond pointing to the Cl atom (electrophile).
Arrow 2: Curly arrow from the Cl–Cl bond pointing to the bottom Cl atom, showing heterolytic fission.
Intermediate: A tertiary carbocation structure: (CH₃)₂C⁺–CH(Cl)–CH₂–CH₃ (or displayed formula equivalent).
Product: 1,2-dichloro-2-methylbutane.
Nucleophile step: Chloride ion :Cl⁻ with a visible lone pair, and a curly arrow originating from the lone pair pointing to the positively charged carbon ( C⁺ ).

💡 Key Knowledge

Halogens are non-polar, but approach of the electron-dense π-bond induces an instantaneous dipole, making the near Cl atom δ+ .
The reaction proceeds via the most stable carbocation intermediate (tertiary over primary), which dictates the major product.

🧠 Exam Technique

Always start curly arrows precisely: the first must begin at the double bond, and the second must start on the bond line between the two halogen atoms.
Make sure lone pairs and full negative charges are clearly drawn on attacking nucleophiles like :Cl⁻ .

❌ Common Errors

Starting the first curly arrow from a carbon atom instead of directly from the π-bond (double bond).
Forgetting to show the lone pair on the chloride ion during the second step of the mechanism.
Forming an incorrect primary carbocation instead of the stable tertiary carbocation.

🎯 Mark Breakdown (3 marks): Mark 1 for both initial curly arrows. Mark 2 for the correct tertiary carbocation intermediate and final structure. Mark 3 for the chloride ion with a lone pair and the final curly arrow to the C⁺.

Question 3 (a)(ii)

IUPAC Naming of Dihalogenoalkanes

✅ Correct Answer

1,2-dichloro-2-methylbutane

Minor spelling variations like missing hyphens or commas are tolerated by examiners, but correct prefix ordering and numbering are mandatory.

🧠 Exam Technique

Number the longest carbon chain to give substituents the lowest possible locants (1,2-dichloro-2-methyl is lower than 3,4-dichloro-3-methyl).
List substituents alphabetically ( chloro before methyl ), ignoring numerical prefixes like 'di-'.

❌ Common Errors

Numbering from the wrong end of the carbon chain, resulting in higher locants (e.g., 3,4-dichloro...).
Placing substituents in order of appearance rather than strictly alphabetical order.

🎯 Mark Breakdown (1 mark): Awarded for the correct IUPAC name matching the product from part (a)(i) (Error Carried Forward applies if consistent).

Question 3 (b)

Classification of Dihalogenoalkanes

✅ Correct Answer

A — primary

Both chlorine atoms are bonded to carbon atoms that are attached directly to only one other carbon atom, making both halogen-bearing carbons primary sites.

💡 Key Knowledge

Halogenoalkane classification depends on the number of carbon atoms attached to the carbon bearing the halogen atom:
• Primary (1°): attached to 1 other carbon.
• Secondary (2°): attached to 2 other carbons.
• Tertiary (3°): attached to 3 other carbons.

❌ Common Errors

Confusing the classification of the central carbon backbone (which is secondary) with the classification of the specific carbon atoms bonded to the chlorine atoms.
Selecting option D ("primary and secondary") by misinterpreting the overall structure's branching.

🎯 Mark Breakdown (1 mark): 1 mark for selecting option A.

Topics

Organic Chemistry · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.