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Chapter 12 : Reactions of Arenes. Electrophilic Aromatic Substitution
Ch 12 contents

Electrophilic Aromatic Substitution

Overall an electrophilic aromatic susbtitution (EArS) can be represented as follows:

Electrophilic aromatic substitution

There are three fundamental components to an electrophilic aromatic substitution mechanism:

  1. formation of the new σ bond from a C=C in the arene nucleophile
  2. removal of the proton by breaking the C-H σ bond
  3. reforming the C=C to restore the aromaticity
curly arrows for electrophilic aromatic substitution

The mechanism is represented by the following series of events:

cyclohexadienyl cation = arenium ion
The reaction of the electrophile E+ with the arene is the slow step since it results in the loss of aromaticity even though the resulting cation is still resonance stablised. This carbocation is also described as the cyclohexdienyl cation or arenium ion or as a sigma-complex.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary