WJEC A-Level Chemistry AS Unit 2, June 2025: Question 4

2 marks · Easy difficulty · Short Answer

Complete the diagram to show the mechanism for the nucleophilic substitution of bromoethane to form ethanol, including relevant dipoles and curly arrows.

Practise this question

Question

The question asks to complete a diagram showing the nucleophilic substitution mechanism of bromoethane to form ethanol, including relevant dipoles in the reactants. The diagram displays a hydroxide ion with a lone pair, HO:⁻, above the fully displayed formula of bromoethane, an arrow indicating reaction, and products ethanol (fully displayed) plus a bromide ion, :Br⁻.
Question text

4. Complete the diagram below to show the mechanism for the nucleophilic substitution of

bromoethane to form ethanol. You should include any relevant dipoles in the reactants. [2]

–

HO

H H H H

–

H C C Br H C C OH + Br

H H H H

Mark scheme

Show the mark scheme The mark scheme illustrates the completed mechanism: delta-positive on the carbon bonded to bromine and delta-negative on the bromine atom. A curly arrow starts from the lone pair on the hydroxide ion pointing directly to the delta-positive carbon atom, and a second curly arrow starts from the C–Br bond pointing to the bromine atom. Marking points: C–Br dipole (1 mark) and curly arrows (1 mark). Notes state not to award the dipole mark if additional incorrect dipoles/charges are included, and not to award the curly arrows mark if additional incorrect arrows are included.

Marks available

Question Marking details

AO1 AO2 AO3 Total Maths Prac

C—Br dipole (1)

curly arrows (1)

do not award dipole mark if additional incorrect dipoles / charges

included

do not award curly arrows mark if additional incorrect arrows

included

How to answer it

Nucleophilic Substitution Mechanism of Bromoethane

SPECIFICATION FOCUS: HALOALKANES

What this question tests

This question assesses your direct recall and precise drawing skills for the nucleophilic substitution mechanism (SN2 pathway) between a primary halogenoalkane (bromoethane) and the hydroxide ion (OH⁻). Key competencies include:

  • Identifying and drawing permanent bond dipoles based on electronegativity differences (C–Br bond).
  • Correctly positioning curly arrows showing the movement of electron pairs from an electron-rich site to an electron-deficient carbon.
  • Representing heterolytic bond fission of the carbon-halogen bond.

Question 4 (2 Marks)

Mechanism & Dipole Representation

✅ Correct Answer & Required Elements

Two distinct features must be added to the starting materials on the left side of the equation:

  1. Dipoles on the C—Br bond [1 mark]:
    • δ+ placed on the carbon atom directly bonded to the bromine.
    • δ− placed on the bromine atom (Br).
  2. Two curly arrows [1 mark]:
    • Arrow 1: Starts exactly from the lone pair (or negative charge) on the oxygen of HO:⁻ and points directly to the δ+ C atom.
    • Arrow 2: Starts from the C—Br single bond and points directly onto the Br atom.

💡 Mechanism Diagrammatic Guide

How your annotations should look on the left-hand reactant side:

HO:⁻ │ (Arrow starts from lone pair on O) ▼ H H │ │ H ─ C ─ Cδ+ ──► Brδ− (Arrow starts on C─Br bond, │ │ points to Br atom) H H

Note: Both curly arrows must be standard double-headed arrows, indicating the movement of a pair of electrons.

🧠 Exam Technique & WJEC Marking Criteria

  • Precision of arrow origins: The first curly arrow must start precisely at the electron pair (lone pair or negative charge) on the OH⁻ ion. If it starts in empty space or on the hydrogen atom of HO⁻ , the mark is lost.
  • Precision of arrow termination: The arrow from HO:⁻ must point clearly to the electrophilic carbon atom, not to any attached hydrogens.
  • Bond-breaking arrow: The second arrow must originate directly on the line representing the C—Br bond and head straight to the Br atom, representing heterolytic fission.

❌ Common Traps & Penalties

  • Adding superfluous dipoles: The mark scheme specifically states: "do not award dipole mark if additional incorrect dipoles/charges included". Do not label the C–H bonds or the other methyl carbon with dipoles!
  • Inverted dipoles: Putting δ− on carbon and δ+ on bromine is an immediate zero for that mark. Bromine is more electronegative than carbon.
  • Extra curly arrows: Do not draw intermediate carbocations or additional arrows. Extra arrows disqualify the curly arrow mark.
  • Missing the lone pair: While the diagram gave HO:⁻ , always ensure your arrow stems directly from the pair of dots.
Mark Scheme Breakdown (AO1 - 2 Marks Total):
• [1 Mark] Correct partial charges Cδ+—Brδ− shown on the reactant molecule.
• [1 Mark] Both correct curly arrows: one from the lone pair of HO:⁻ to the C atom, and one from the C—Br bond to the Br atom.

Topics

Organic Chemistry · 2.4 Organic compounds · 2.6 Halogenoalkanes

Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.