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4.

Aromatic Nucleophilic Substitution

4.1.

The SN2ArMechanism

•

this mechanism requires strongly electron withdrawing
and/or para position in order to operate
it is typically a replacement of a halide with a nucleophile

substituents

in

ortho

•

Examples:

Mechanism
Free
Energy

two-step process

Electron withdrawing groups stabilise the formation of the Meisenheimer complex and
increase the rate of reaction. Electron donating groups retard the rate of reaction.

meta substituents do notenable stabilisation of charge.

Effect of leaving group

Fluorine is strongly electron withdrawing and able to stabilise Meisenheimer
intermediates. The rate is not determined by leaving group ability but rather the ability
to form Meisenheimer complex.

4.2.

Benzyne Mechanism

“Elimination - Addition” mechanism

General reaction scheme

Mechanism:

sideways overlap of two sp2orbitals form a π bond orthogonal to the plane of the
aromatic π cloud.

Cl

OH

Cl

O2N

NO2

NO2

OH

O2N

NO2
picric acid

NO2

NaOH, 350oC
4500 lb/in2

H2O, warm

phenol

Nu

X

Nu

X

+X-

Reaction coordinate

Nu-

T.S.

T.S.

"Meisenheimer complex"

slow

fast