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AROMATIC NITRO COMPOUNDS 205

The oxidation of nitrobenzene to o- nitrophenol (with traces of the p- isomer)

when mixed with dry sodium hydroxide, and slightly heated, may be another

example of the oxidation of nitro compounds by nucleophilic substitution

(Wohl [46]):

(25)

The yield of o- nitrophenol in this reactions amounts up to 50% of the theoretical

value.

A specific feature of these reactions is that the substitution occurs in the ortho

and para positions. This is typical of nucleophilic reactions whereas the meta-

orienting influence of the nitro group (i.e. the conventional substitution rule)

can be observed when electrophilic substitution occurs.

Another example of nucleophilic hydroxylation of nitro compounds is the

formation of nitrophenols by the action on nitrobenzene with aqueous ferrous

sulphate in the presence of hydrogen peroxide in a yield of ca. 3% [46a] (Fen-

ton’s reagent) [46b] ; Weiss et al. [46]) and by the action of ionizing radiations on

aqueous solution (Weiss and Stein [46c]) :

Aromatic hydroxylation is known to take place in animal and human organisms

and therefore it is of great importance to know the metabolism of various aro-

matic compounds including drugs (D. Robinson, J. N. Smith, R.T. Williams

[47]). The presence of the nitro group in a molecule, resulting in its activation,

may sometimes lead to a rather unusual course of reaction. The Richter [48] re-

action might be taken as an example, in which m- bromobenzoic acid may be obtained

by reacting potassium cyanide with p- nitrobromobenzene. Likewise, when react-

ing potassium cyanide with m- nitrobromobenzene, a mixture of o- and p- bromo-

benzoic acids are formed.

According to Bunnett and his co-workers [49,50,50a] the reaction is of the

nucleophilic substitution type and may be represented by the following scheme: