NITRO DERIVATIVES OF TOLUENE 275According to the same author, nitrating with a more dilute nitric acid (e.g.
32%, sp. gr. 1.32) at 105°C leads to the formation of phenylnitromethane, i.e.
nitration takes place in the side chain.
When nitrating toluene with nitrogen dioxide in the liquid or gaseous phase,
nitrotoluene along with phenylnitromethane is obtained. As Topchiyev [19] reported.
the following yields expressed as per cent theoretical, were obtained, using a large
excess of N 2 O 4 (200 g for 40 g of toluene) (Table 52):
TABLE 52At temperature Phenylnitromethane Nitrotoluene Unreacted toluene
°C % % %14-15^9 46.2^40
105-110 5 17 3INDUSTRIAL METHODS OF NITRATION OF TOLUENEVarious methods of preparing nitrotoluenes for subsequent nitration to TNT are
described below (pp. 348, 357) where methods of manufacturing TNT are discussed.
Here only those methods are reviewed which are used industrially for preparing
the pure isomers : o-, m-, and p- nitrotoluene.
In industrial practice nitrating mixtures of the composition: 28-62% HNO 3 ,
56-21% H 2 SO 4 , 5-20% H 2 O are used for the nitration of toluene.
Usually more of the mixture is used for nitration than theory demands, e.g.
a 1-5% excess is applied so that the ratio of HNO 3 used to HNO 3 calculated varies
from 1.01 to 1.05. An excess of the nitrating mixture not only favours complete
nitration, but also inhibits the formation of undefined, dark coloured tarry by-
products, which are readily formed if an insufficient amount of nitrating acid
is used. Toluene may be added to the acid or vice versa.
The latter operation (adding acid to toluene) favours the formation of tarry
products (see p. 266) but at the same time the formation of higher nitrated products
is avoided, as the toluene is never in contact with an excess of acid.
A temperature within the range 25° to 40°C is maintained during mixing. Then
it is raised to 60°C and maintained thus for 30 min to 1 hr. After cooling the re-action mass to 25-30°C, nitrotoluene is separated from the waste acid in a separa-
tor, the nitrotoluene being the upper layer.
A commonly used method of obtaining TNT is the direct nitrating of a mixture
of the three isomers of nitrotoluene. Details are given later (Chapter IX) in the de-
scription of the manufacture of DNT and TNT. The purification of TNT from
unsymmetrical derivatives of m- nitrotoluene consists as a rule in the sulphitation
of di- or, more often, tri-nitrotoluenes or in crystallization of the trinitro-products.
However, in factories producing dye intermediates, the isomers of nitrotoluene