83 Electrophilic Aromatic Substitution

Electrophilesare particles with a deficiency of electrons. Therefore they are likely to react
with substances that have excess electrons. Aromatic compounds have increased electron
density in the form of delocalizedπ-orbitals.

83.1 Step 1: Formation of aπ-complex

At first, the electrophile interacts with the delocalized orbitals of the aromatic ring and a
π-complex is formed.

Figure 172

No chemical bonds are formed at this stage. Evidence of the formation of aπ-complex as
an intermediate state has been found for some reactions, but not for all, since the chemical
interaction inπ-complexes is very weak.

83.2 Step 2: Formation of aσ-complex

After theπ-complex is formed, in the presence of an electron acceptor another complex is
formed - theσ-complex. It is a cationic species, an intermediate that lacks aromatic prop-
erties, but its fourπ-electrons are delocalized across the ring, which stabilizes the cation
somewhat, sometimes allowing its isolation. An example would be the salt mesityl fluorob-
orate, which is stable at low temperatures, and is prepared by the reaction of mesitylene
(1,3,5-trimethylbenzene) with fluoroboric acid (BF 3 /HF); the cation of this salt is proto-
nated mesitylene.σ-complexes are also known asWheland intermediates.

83.3 Step 3: Formation of a Substituted product

At the next stage theσ-complex decomposes, freeing a hydrogen cation and forming the
product of substitution.

Figure 173