Electrophilic Substitution

Electrophilic substitution occurs in two phases:

  1. Generation of the electrophile; and

  2. Substitution.

Generation of the electrophile follows one of two pathways, depending upon whether a Bronsted acid or a Lewis acid is present:

The mechanism of the actual attack and re-aromatization is consistent regardless of electrophile:

In each case, the "filled-empty" interaction is between the p HOMO of the aromatic and the s* or empty p (LUMO) of the electrophile.

The number of possible reagents from which to generate the electrophile is quite large. The Table presents a few examples:

E B Typical Catalyst
O2N OH2 H2SO4
Cl, Br Cl-, Br- AlX3 or BX3
RC=O X- X = Cl, AlCl3
X = OH, H2SO4
Tl(OCOCF3) CF3CO2- None
HOSO2 OH2 SO3
D OD2 D2SO4


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