The generally accepted mechanism for nucleophilic aromatic substitution
in nitro-substituted aryl halides is shown by example below:
Attack of the strong nucleophile on the halogen substituted aromatic carbon
forming an anionic intermediate.
Loss of the leaving group, the halide ion restores the aromaticity.
Kinetics of the reaction are observed to be second order.
The addition step is the rate determining step (loss of aromaticity).
Nucleophilic substitution, and therefore reaction rate, is facilitated
by the presence of a strong electron withdrawing group (esp. NO2)
ortho or para to the site of substitution, which stabilise
the cyclohexadienyl anion through resonance.
Aryl halide reactivity : -F > -Cl > -Br > -I (note
the contrast to simple nucleophilic substitution)
The more electronegative the group the greater the ability to attract electrons
which increases the rate of formation of the cyclohexadienyl anion.
