NITRO DERIVATIVES OF TOLUENE 317ponding to the maximum rate of nitration when the total concentration is higher
than in the homogeneous system.
This results from the rule already observed that nitration of aromatic com-
pounds occurs mainly in the mineral acid phase.
When a large excess of nitric acid is used there is very little difference between
the rate of nitration in heterogeneous and homogeneous systems. This is due to the
larger excess of nitric acid present in the mineral acid phase.
Orlova has also investigated the influence on the rate of nitration of the presence
of trinitrotoluene in dinitrotoluene. Table 73 gives the corresponding figures obtained
in sulphuric acid of various concentrations at 100°C. The nitrating mixtures con-tained 10% HNO 3 :
TABLE 73
YIELD OF TRINlTROTOLUENE WHEN MIXTURES OF DINITROTOLUENE
AND TRINITROTOLUENE ARE NITRATEDContent of TNT in the original mixture % of dinitrotoluene nitrated within 30 min--
of DNT and TNT at concentration of sulphuric acid (in % H^2 SO^4 )
% 870
24
45
64
84
927.8
7.1
2.0
0
09121.0
19.5
18.2
17.5
26.09522.0
22.0
22.0
27.5
48.3
47.098
74.0
81.1
101 104_ i -- -.
- 75.0 78.4
82.7 83.9
- 75.0 78.4
Figure 72 shows the influence of trinitrotoluene on the rate of nitration of
dinitrotoluene in heterogeneous systems at 90°C. It is interesting to note that the
addition of 66-70% of trinitrotoluene to dinitrotoluene more than halves the
rate of the nitration, Further increase in the content of trinitrotoluene promotes
nitration of dinitrotoluene. When the content of trinitro compound reaches 91%
the yield of trinitration is almost the same as that of the pure dinitrotoluene.
On the basis of her own experiments and those reported in the literature Orlova
[2a] came to the conclusion that the mechanism of nitration of toluene to TNT
in heterogeneous conditions can be depicted in the following terms.
Nitration of toluene to mononitrotoluene and of the latter to dinitrotoluene
in heterogeneous conditions should be considered mainly a “surface” reaction.
This is deduced from the fact that the rate of the reaction depends on the intensity
of stirring. The reaction of nitration of dinitrotoluene to trinitrotoluene in het-
erogeneous system is not limited to the surface dividing the two phases. This
is proved by the fact that the rate of nitration depends very little on the rate of
stirring. The reactants -dinitrotoluene and nitric acid - are distributed between
two phases according to their coefficients of distribution and to the ratio between