urbaf2

480 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

(3) oxidation of nitrosophenols;

(4) nitration of benzene or other aromatic hydrocarbons with nitric acid in the

presence of mercury salts (Wolffenstein and Bötters’s [23] method,

pp. 110, 520)

In industrial practice methods (1) to (3) are applied, the choice depending on the

product which is to be prepared, on economic considerations, etc.

Nitration of phenol. The process is effected in two stages. The first is the sulph-

onation of the phenol, followed by acting with nitric acid on a solution of phenolsul-

phonic acid in sulphuric acid. In the latter process the sulpho groups are replaced

by nitro groups (p. 501). This indirect procedure has to be adopted because the

direct action of nitric acid on phenol yields undesirable by-products, sometimes
brightly coloured, which contaminate the principal product.

The reaction of substitution of the sulpho groups by nitro groups should be

absolutely complete. If not, the nitrophenolsulphonic acids which, due to the presence

of one or more sulpho groups, are water soluble, will remainin the spent acid and

in the washings from nitrophenol purification.

The direct action of nitric acid on phenol has been the subject of investigation

by a number of workers. Thus, Wesselsky [34] succeeded in isolating a dark coloured

substance by treating resorcinol in ether solution with fuming nitric acid. Brunner

and Kramer [35] assumed that the following products, having a phenoxazine ring,

were formed: resazurin (I) and resorufin (II) along with nitro derivatives of resor-

cinol :

I

The reaction of nitric acid with phenols (including resorcinol) has been fully

investigated by K. H. Meyer and Elbers [36]. They isolated a reddish-brown

product to which they assigned the indophenol N-oxide structure (III). The compound

can be, formed directly from phenol and nitric acid, according to the scheme: