NITRO DERIVATIVES OF BENZENE^255In the Griesheim method the oxidation of trinitrotoluene is accomplished by
means of chromic acid in the presence of sulphuric acid, a temperature 40-50°C
being maintained. By diluting the solution with water, trinitrobenzoic acid is pre-
cipitated. Boiling the acid in water results in the evolution of CO 2 and the formation
of trinitrobenzene.
The Griesheim method is generally used for the laboratory preparation of
sym-trinitrobenzene. However, it is too expensive to be applied in industry. Tri-
nitrobenzene prepared in this way is much more costly than trinitrotoluene, where-
as the gain in explosive power obtained is relatively small.
A less expensive method which Gamer and Abernethy [50] attribute to Koehler,
consists in the oxidation of trinitrotoluene with 90-92% nitric acid in the presence
of 93% sulphuric acid at 190°C. However, it requires very prolonged heating-
about 40 hr. The method has not been adopted because of the considerable
risk of explosion. Indeed, an explosion occurred in a factory at St. Chamas when
experiments on a semi-industrial scale were carried out.
It should be mentioned that sym-trinitrobenzene is formed as a by-product
of the nitration of toluene to trinitrotoluene (p. 338).
Preparation from picryl chloride. According to Ullmann and Bielecki [51] and
also J. Meyer [52], trinitrobenzene may. be obtained by removing the chlorine
atom from picryl chloride in the presence of copper:(14)The reaction is carried out in about 85% alcohol solution at boiling point.
Difficulties in obtaining picryl chloride are a drawback for applying the method
industrially.
Preparation from benzyl chloride. This method has been proposed by Bron-
chard [53]. It consists in the nitration and oxidation of benzyl chloride:
(15)