Understanding Friedel-Crafts Acylation Reaction

Sep 12, 2024

Friedel-Crafts Acylation Reaction

Overview

  • Friedel-Crafts Acylation involves benzene reacting with an acyl group.
  • Uses an acid chloride and a catalyst like AlCl3 (a Lewis acid).
  • Results in the formation of an acyl group on the benzene ring.

Comparison with Friedel-Crafts Alkylation

  • Similar mechanism but differs with the presence of a carbonyl group in the product.
  • Recommended to understand Friedel-Crafts Alkylation first for better understanding.

Formation of Super Electrophile

  • The molecule adding to benzene is an acyl group from an acid chloride.
  • Acid chloride: a derivative of carboxylic acid where OH is replaced with chlorine.
  • Super electrophile activation is similar to aromatic halogenation and alkylation.

Mechanism Steps

  1. Formation of Complex
    • Chlorine from the acid chloride attacks aluminum (AlCl3) forming a complex.
    • Aluminum forms four bonds with eight electrons, acquiring a formal charge of -1.
    • Chlorine gains a positive charge.
  2. Formation of Acylium Ion
    • Chlorine’s positive charge leads it to take electrons from the carbonyl carbon.
    • Results in ALCl4- with neutral chlorine.
    • Acyl group now has a carbon with a positive charge and an incomplete octet.
  3. Resonance Structure
    • Carbon pulls on electrons from oxygen forming a resonance structure.
    • Carbon triple-bonded to oxygen; oxygen has a positive charge.
    • Acylium ion: more stable due to oxygen's complete octet.

Addition to Benzene

  • Electrophile Interaction
    • Positive oxygen in acylium ion doesn’t solely hold the charge due to resonance.
    • Carbon becomes partially positive, attracting benzene's electrons.
    • Electrons move from carbon to oxygen, stabilizing the acylium ion.

Final Reaction Steps

  1. Benzene Intermediate Formation
    • Carbon double-bonded to oxygen and single-bonded to an R group.
    • Intermediate has a positive charge on benzene where the pi bond broke.
  2. Reformation of Aromaticity
    • ALCL4- contributes: chlorine removes hydrogen from sp3 carbon.
    • Electrons from hydrogen return to benzene, reforming aromaticity.
    • Product: benzene with acyl group, HCl, and regenerated AlCl3.

Additional Information

  • Differences and similarities with Friedel-Crafts Alkylation will be discussed in upcoming videos.
  • Resources available on author's website (LayerforSicom/EAS) for further study.
  • Free ebook "10 Secrets to Acing Organic Chemistry" available at orgosecrets.com.
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