AQA A-Level Chemistry Paper 3, 2019: Question 20

Nitration of Arenes: Reaction Mechanism

AQA A-Level Chemistry • Paper 2 • Organic Chemistry

What this question tests

This question tests your fundamental understanding of aromatic chemistry (arenes) and mechanism classification. Specifically, it assesses whether you can recognise that arenes (such as methylbenzene) undergo substitution rather than addition to preserve aromatic stability, and that the reactive species generated by concentrated nitric and sulfuric acids acts as an electrophile.

Question 20 (Multiple Choice)

Identifying the mechanism for nitration of methylbenzene [1 mark]

✅ Correct Answer

B: Electrophilic substitution

Mark Scheme Breakdown:
• Selecting option B awards 1 mark.
• Any other choice or multiple selections award 0 marks.

💡 Key Knowledge

The Electrophile: Concentrated H₂SO₄ protonates concentrated HNO₃ to generate the nitronium ion ( NO₂⁺ ):
HNO₃ + 2H₂SO₄ ➔ NO₂⁺ + H₃O⁺ + 2HSO₄⁻
Why Substitution? The benzene ring in methylbenzene has a delocalised π-system of high electron density. An addition reaction would destroy this extra stability (delocalisation energy of ~152 kJ mol⁻¹). Thus, a ring hydrogen is substituted to reform the intact aromatic ring.
Role of the Methyl Group: The -CH₃ group is electron-donating by positive inductive effect, making methylbenzene even more reactive towards electrophiles than unsubstituted benzene (directing to 2- and 4-positions).

🧠 Exam Technique: Two-Step Elimination

For any A-Level mechanism classification question, break the name into two halves:

Step 1: Electrophile or Nucleophile?
The arene ring is an area of high electron density (rich in π-electrons). It attracts electron-deficient species (electron pair acceptors). Therefore, the attacking species must be an electrophile. Eliminate C and D immediately.
Step 2: Addition or Substitution?
Addition would break delocalisation permanently, which is energetically unfavourable. The ring temporarily loses its delocalisation in the intermediate carbocation, then expels H⁺ to restore aromaticity. Therefore, it is substitution. Eliminate A.

❌ Common Misconceptions & Traps

Confusing Arenes with Alkenes (Choosing A): Alkenes undergo electrophilic addition because breaking a localized C=C bond is energetically easy. Arenes have delocalisation stability and will NOT undergo addition under standard conditions.
Mistaking HNO₃ for a Nucleophile (Choosing C/D): Students see lone pairs on oxygen in nitric acid and assume it acts as a nucleophile. In this mixture, sulfuric acid acts as a stronger acid and protonates nitric acid, ultimately forming the powerful electrophile NO₂⁺ .
Ignoring the Catalyst Role: Remember that H₂SO₄ is a catalyst regenerated at the end: H⁺ + HSO₄⁻ ➔ H₂SO₄ .

📐 Mental Walkthrough of the Mechanism

If asked to draw this mechanism in a structured question (worth up to 3–4 marks), examiners look for:

Curly arrow 1: From the delocalised π-ring of methylbenzene pointing directly to the positively charged nitrogen atom of NO₂⁺ .
Intermediate structure: A horseshoe-shaped partially delocalised ring spanning 5 carbon atoms, with the opening facing the tetrahedral carbon bonded to both -H and -NO₂ . The positive charge must reside inside the horseshoe.
Curly arrow 2: From the C–H bond pointing back into the horseshoe ring to restore the complete circular delocalised π-system.
Products: Nitromethylbenzene (primarily 2- or 4-isomer) and a regenerated H⁺ ion.