AQA A-Level Chemistry Paper 3, 2017: Question 14

1 mark · Easy difficulty · Multiple Choice

Identify which reaction step in a multi-stage synthesis occurs by nucleophilic substitution.

Practise this question

Question

Question 14 displays a four-step organic reaction scheme: ethane (CH3CH3) reacts with Cl2 in Step A to form chloroethane (CH3CH2Cl); chloroethane reacts with NaOH in Step B to form ethanol (CH3CH2OH); ethanol reacts with H2SO4 in Step C to form ethene (CH2=CH2); ethene reacts with Br2 in Step D to form 1,2-dibromoethane (BrCH2CH2Br). The question asks: 'Which step occurs by nucleophilic substitution?' with multiple-choice options A: Step A, B: Step B, C: Step C, and D: Step D.
Question text

14 The reaction sequence shows how CH3CH3 can be converted into BrCH2CH2Br

Which step occurs by nucleophilic substitution?

[1 mark]

A Step A

B Step B

C Step C

D Step D

Mark scheme

Show the mark scheme Mark scheme table row showing question number 14 with the correct answer B.

14 B

How to answer it

Organic Reaction Mechanisms in a Synthetic Pathway

📌 What this question tests

This question assesses your ability to identify fundamental organic reaction types and mechanisms across multi-step synthetic pathways. Specifically, it tests your understanding of halogenoalkane hydrolysis (nucleophilic substitution), alkane halogenation (free-radical substitution), alcohol dehydration (acid-catalysed elimination), and alkene halogenation (electrophilic addition).

Question 14 Analysis

Identifying Reaction Types Across a Multi-Step Organic Route

✅ Correct Answer

B — Step B

Mark Scheme: 1 mark for selecting option B.

In Step B, chloroethane ( CH₃CH₂Cl ) reacts with hydroxide ions from aqueous sodium hydroxide ( NaOH ) to form ethanol ( CH₃CH₂OH ). The hydroxide ion ( :OH⁻ ) acts as a nucleophile, attacking the electron-deficient δ+ carbon and substituting the chloride ion.

💡 Complete Step Breakdown

  • Step A: CH₃CH₃ + Cl₂ → CH₃CH₂Cl
    Mechanism: Free-radical substitution (initiated by UV light).
  • Step B: CH₃CH₂Cl + NaOH(aq) → CH₃CH₂OH
    Mechanism: Nucleophilic substitution (hydrolysis).
  • Step C: CH₃CH₂OH → CH₂=CH₂ (conc. H₂SO₄ )
    Mechanism: Acid-catalysed elimination (dehydration).
  • Step D: CH₂=CH₂ + Br₂ → BrCH₂CH₂Br
    Mechanism: Electrophilic addition across the C=C double bond.

🧠 Exam Technique & Strategy

  • Classify the functional group change: Look at the functional group in the reactant and product for each step before reading the multiple-choice options.
  • Identify attacking species: In Step B, :OH⁻ has a lone pair of electrons to donate to a δ+ carbon atom, defining it as a nucleophile.
  • Use elimination: Step A starts with an alkane (saturated hydrocarbon), which only undergoes free-radical substitution. Steps C and D involve making or breaking a double bond (elimination and addition respectively), ruling them out immediately.

❌ Common Errors & Misconceptions

  • Confusing Substitution with Elimination: Reacting a haloalkane with NaOH can lead to elimination (forming an alkene) if ethanolic NaOH and high heat are used. Here, the product is clearly shown as the alcohol, confirming nucleophilic substitution.
  • Confusing Nucleophilic and Electrophilic Addition: In Step D, the bromine molecule is polarised by the high electron density of the C=C double bond, making the attacking species an electrophile, not a nucleophile.
  • Misidentifying Step A: Step A replaces a hydrogen with chlorine, which is a substitution reaction, but it is free-radical substitution, not nucleophilic.

📐 Summary of Core Mechanism Terminology (AQA A-Level)

  • Nucleophilic Substitution: Typical of halogenoalkanes reacting with aqueous OH⁻ , CN⁻ , or excess ethanolic NH₃ .
  • Electrophilic Addition: Typical of alkenes reacting with halogens ( Br₂ ), hydrogen halides ( HBr ), or conc. H₂SO₄ .
  • Elimination: Typical of halogenoalkanes with hot ethanolic KOH , or alcohols with hot concentrated H₂SO₄ or H₃PO₄ .
  • Free-Radical Substitution: Typical of alkanes reacting with halogens in the presence of ultraviolet (UV) radiation.

Topics

Organic Chemistry · 3.3.3 Halogenoalkanes

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.