A Guidebook to Mechanism in Organic Chemistry

Electrophilic and Nucleophilic Substitution in Aromatic Systems

OH OH OH OH

(VI) (VII) (VIII)

The*^Stroso-phenol (VII) so obtained is known to be oxidised very
rapidly by nitric acid to yield the nitrophenol (VIII) and nitrous
acid; thus more nitrous acid is produced and the process is pro­
gressively speeded up. No nitrous acid need be present initially in the
nitric acid for a little of the latter attacks phenol oxidatively to yield
HNOg. The rate-determining step is again believed to be the forma­
tion of the intermediate (VI). Some direct nitration of such reactive
aromatic compounds by ®N0 2 also takes place simultaneously, the
relative amount by the two routes depending on the conditions.

HALOGENATION
Halogenation, e.g. bromination, with the halogen itself only takes
place in the presence of a catalyst such as ZnCl 2 , FeBr 3 , AlBr 3 , etc.
The nature of the catalyst is usually that of a Lewis acid and it acts
by inducing some degree of polarisation in the halbgen molecule,
thereby increasing its electrophilic character, so that its now more
positive end attacks the w electrons of the nucleus:

(V) is that the energy that becomes available from the formation of the

C—N0 2 linkage may be used to assist in the subsequent fission of the

strong C—H bond, which would otherwise be a relatively difficult

undertaking.

A further point of preparative significance still requires explana­

tion, however. Highly reactive aromatic compounds, such as phenol,

are found to undergo ready nitration even in dilute nitric acid and at

a far more rapid rate than can be explained on the basis of the con­

centration of eNG 2 that is present in the mixture. This has been

shown to be due to the presence of nitrous acid in the system which

nitrosates the reactive nucleus via the nitrosonium ion, eNO:

HN0 2 + 2HN0 3 - H 3 0®+2NOf + ®NO