A Guidebook to Mechanism in Organic Chemistry

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Electrophilic and Nucleophilic Substitution in Aromatic Systems

which shows all the characteristics (in the way of effect of substi­
tuents, etc.) of a typical electrophilic substitution reaction, but such
displacements are not common and are usually of little preparative
importance.
In the face of the wholly polar viewpoint of aromatic substitution
that has so far been adopted, it should be emphasised that examples
of homolytic aromatic substitution by free radicals are also known
(p. 250).

THE EFFECT OF A SUBSTITUENT ALREADY PRESENT

The effect of a substituent already present in a benzene nucleus in

governing not only the reactivity of the nucleus towards further

electrophilic attack, but also in determining what position the in­

coming substituent shall enter, is well known. A number of empirical

rules have been devised to account for these effects but they can be

better explained on the basis of the electron-donating or -attracting

powers of the initial substituent.

(i) Inductive effect of substituents

Alkyl groups are ejectron-donating and so will increase electron-

availability over tfae^nucleus. The effect in toluene

probably arises in part from a contribution to the hybrid by. forms

such as (XIII), i.e. by hyperconjugation (p. 20). The inductive effect

of most other substituents, e.g. halogens, OH, OMe, NH 2 , S0 3 H, NOa

etc. will be in the opposite direction as the atom next to the nucleus

is more electronegative than the carbon to which it is attached, e.g.:

H H

etc.

(XIII)

CI