AQA A-Level Chemistry Paper 3, June 2022: Question 23

1 mark · Easy difficulty · Multiple Choice

Identify which compound is formed from bromoethane via a nucleophilic substitution reaction.

Practise this question

Question

Question 23 asks: 'Which compound is formed from bromoethane in a nucleophilic substitution reaction?' with four multiple choice options: A CH3CN, B CH3CH2NH2, C CH2=CH2, and D CH3CH2OSO2OH.
Question text

23 Which compound is formed from bromoethane in a nucleophilic substitution reaction?

[1 mark]

A CH3CN

B CH3CH2NH2

C CH2=CH2

D CH3CH2OSO2OH

Mark scheme

Show the mark scheme Mark scheme for question 23 shows the correct answer is B (AO2), worth 1 mark, corresponding to CH3CH2NH2.

23 B (AO2) 1 CH3CH2NH2

How to answer it

Nucleophilic Substitution of Halogenoalkanes

📌 What this question tests

This question assesses your understanding of organic reaction mechanisms involving haloalkanes, specifically:

  • Identifying products formed via nucleophilic substitution of bromoethane ( CH₃CH₂Br ).
  • Tracking the number of carbon atoms in reactants and products (especially during nitrile formation).
  • Distinguishing between nucleophilic substitution, elimination, and electrophilic addition reactions.

Question 23

Multiple Choice: Halogenoalkane Reaction Pathways [1 Mark]

✅ Correct Answer

B: CH₃CH₂NH₂ (ethylamine / aminoethane)

Award 1 mark for selecting option B.

💡 Key Knowledge

Haloalkanes react with excess ammonia under pressure via nucleophilic substitution:

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

  • The nucleophile is :NH₃ (acting as an electron-pair donor).
  • The carbon skeleton remains 2 carbons long ( CH₃CH₂– ).

🧠 Option-by-Option Breakdown

Option Compound Why it is correct or incorrect
A CH₃CN Incorrect: Reaction of bromoethane with KCN extends the chain by one carbon to give propanenitrile ( CH₃CH₂CN , 3 carbons). CH₃CN (ethanenitrile) has only 2 carbons in total and would be formed from bromomethane ( CH₃Br ).
B CH₃CH₂NH₂ Correct: Bromoethane ( CH₃CH₂Br ) reacts with excess ethanolic ammonia via nucleophilic substitution to form ethylamine ( CH₃CH₂NH₂ ).
C CH₂=CH₂ Incorrect: Ethene is formed when bromoethane reacts with hot ethanolic hydroxide ions, but this is an elimination reaction, not a substitution.
D CH₃CH₂OSO₂OH Incorrect: Ethyl hydrogen sulfate is formed by the electrophilic addition of concentrated sulfuric acid to ethene, not from bromoethane.

❌ Common Errors & Examiner Insights

  • The "Nitrile Trap" (Option A): Many students remember that cyanide ( :CN⁻ ) undergoes nucleophilic substitution with haloalkanes, so they immediately jump to option A without counting carbons. Always check that the product has the correct number of carbons ( CH₃CH₂– + –CN = 3 carbons).
  • Confusing Mechanism Types (Option C): Forgetting that OH⁻ acts as a base in ethanolic conditions (causing elimination to make an alkene), but as a nucleophile in aqueous conditions (making an alcohol). Ethene is an elimination product.

Topics

Organic Chemistry · 3.3.3 Halogenoalkanes · 3.3.11 Amines

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.