AQA A-Level Chemistry Paper 3, 2019: Question 26

1 mark · Medium difficulty · Multiple Choice

Identify the correct representation of part of the mechanism for the Friedel–Crafts acylation of benzene with propanoyl chloride.

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Question

Question 26 asks which diagram correctly represents part of the mechanism for the acylation reaction between propanoyl chloride and benzene. Option A shows propanoyl chloride with a curly arrow from chlorine to the carbonyl carbon. Option B shows an arrow pointing from the aluminum atom of aluminium chloride to the chlorine of propanoyl chloride. Option C shows a benzene ring with a curly arrow from the delocalised pi system attacking the carbon cation of an acylium ion. Option D shows the positively charged intermediate with a curly arrow pointing from the C–H bond towards the central positive charge.
Question text

26 The reaction between propanoyl chloride and benzene is an example of acylation.

Which is a correct representation of part of the mechanism of this reaction?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme table indicating that for question 26, the correct answer is C, carrying 1 mark.

26 C 1

How to answer it

AQA A-Level Chemistry • Aromatic Chemistry

Friedel–Crafts Acylation Mechanism

What this question tests

This question assesses your understanding of the mechanism of electrophilic substitution (specifically Friedel–Crafts acylation of benzene), including:

  • The precise movement of electron pairs indicated by curly arrows.
  • Formation of the acylium electrophile ( CH₃CH₂C⁺=O ) using an AlCl₃ catalyst.
  • Attack of the benzene delocalised π-system onto the carbocation/electrophilic carbon centre.
  • Elimination of a proton (H⁺) to re-establish the stable aromatic ring.

Question 26 Analysis

Multiple Choice [1 Mark]

✅ Correct Answer: C

Option C correctly displays the electrophilic attack step:

  • The curly arrow starts inside the delocalised π-electron ring of benzene (the electron-rich donor).
  • The curly arrow points directly to the positively charged carbonyl carbon atom, C⁺, of the acylium ion ( CH₃CH₂CO⁺ ).
  • This accurately reflects a pair of π-electrons forming a new C–C covalent bond with the electrophile.

💡 Key Knowledge

Friedel–Crafts Acylation Steps:

  1. Generation of electrophile:
    CH₃CH₂COCl + AlCl₃ → CH₃CH₂CO⁺ + AlCl₄⁻
    A lone pair from chlorine attacks aluminium (an electron-deficient Lewis acid).
  2. Electrophilic attack:
    A pair of electrons from the benzene π system attacks C⁺ , forming a positively charged intermediate with a broken delocalised ring.
  3. Regeneration of aromaticity:
    The pair of electrons from the C–H bond moves into the ring system to restore full aromatic delocalisation, releasing H⁺ .

❌ Why Options A, B, and D Are Incorrect

  • A is incorrect: The arrow points from the chlorine atom / empty space towards the carbonyl carbon. In reality, heterolytic fission of the C–Cl bond sends the electron pair from the bond onto the chlorine atom to form a chloride ion (or AlCl₄⁻ complex).
  • B is incorrect: The arrow starts at the aluminium atom of AlCl₃ and points to the Cl atom of propanoyl chloride. This is completely backwards! Curly arrows must always start at an electron pair (a lone pair on Cl) and point to the electron-deficient aluminium centre (Lewis acid).
  • D is incorrect: The arrow restoring the ring starts from the ring-to-acyl carbon bond instead of the C–H bond. Breaking the C–C bond would simply kick off the acyl group rather than losing H⁺ to regenerate the stable aromatic benzene ring.

🧠 Exam Technique: Checking Curly Arrows

  • Origin: Always ensure the curly arrow begins precisely at an electron pair (either a covalent bond line or a lone pair of electrons). Never start an arrow from a positive charge or an atom's label!
  • Destination: The arrow head must point to the specific atom forming the new bond or the atom accepting the lone pair.
  • In benzene intermediates: The horseshoe representing the partially delocalised ring must span across 5 ring carbons (facing the sp³ carbon), with the + charge inside the horseshoe. The restoring arrow must go from the C–H bond into the ring system.
Mark Scheme Allocation: 1 mark for selecting C.

Topics

Organic Chemistry · 3.3.10 Aromatic Chemistry · 3.3.9 Carboxylic Acids and Derivatives · 3.3.1 Introduction to Organic Chemistry

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.