A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems

(XXX)

These are analogous to SN 2 reactions but with attack taking place

from the side rather than from the back of the carbon atomtlndergoing

nucleophilic attack; they differ also in that this atom never becomes

bonded to more than four other atoms at once (cf. p. 58). This

mechanism is probably sufficiently different from the normal SN 2 for

it to be designated specifically as S^2 (aromatic).

(ii) Substitution of atoms other than hydrogen

Aromatic nucleophilic substitution more commonly refers to the

replacement of atoms other than hydrogen and both SNl and SJV2

(aromatic) mechanisms are encountered. The only important examples

proceeding via the mechanism are the replacement reactions of

diazaijjujn salts m

ArOH + H®

Arl

in which the rate-determining step is the elimination of nitrogen
from the diazonium cation followed by rapid reaction of the aryl
cation with a nucleophile, the rates being first order in ArNa® and
independent of the concentration >f the nucleophile. A number of
the reactions of diazonium salts, particularly in less .polar solvents,
proceed by a radical mechanism, however (p. 255).
The most common example of an S^2 (aromatic) reaction is the
replacement of an activated halogen atom,

yield a-aminopyridine (XXX), a compound of value in the synthesis

of sulphapyridine: