40 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVESthings on the concentration of the organic compound in the acid, on the concentra-
tion of the acid in the organic compound, on the interfacial area and on the dif-
fusion rate at the phase boundary. If a nitrated compound is solid at reaction
temperature, the nitration rate depends in the first place on the concentration of
the compound in acid, on the interfacial area and on the rate of diffusion of the
acid into the solid phase. Of course, the interfacial area depends above all on the
mixing of the two phases. Moreover, attention must be paid to the statement, first
expressed by Hetherington and Masson [84], that the nitration reaction proceeds
solely in the acid phase, especially when solid substances are nitrated.
In the light of the present state of knowledge on the problem it is more exact
to say that the nitration reaction proceeds principally in the acid phase.
Consequently, mixing during nitration strongly affects the nitration result:
vigorous stirring increases the nitration rate. This will be dealt with on pp. 266
and 314.
The favourable effect of strong dispersion on the nitration process can also
be observed in nitration of easily emulsifiable substances. For example, Kobe
and Doumani [94] found that commercial p- cymene can be nitrated more readily,
than pure cymene, and this can be explained by the fact that the former of the
two compounds is more readily emulsified.
The extension of the interfacial area by emulsification explains Miyagawa’s
[95] observation that the nitration rate can be considerably increased by the action
of ultrasonics on a reacting system. For example, nitration of m- xylene to trinitro-
m- xylene, which generally takes 2 hr, takes only 30 min when ultrasonics are used.
There is no evidence as yet whether and how ultrasonic waves effect group orienta-
tions.
Attempts have been made by Bennett and his co-workers [43,87] to analyse
in more detail the kinetics of a two-phase reaction system. However, so far it has
proved impossible in the absence of more detailed data on the concentrations in
the two phases. Bennett only found that in the nitration of dinitrotoluene totrinitrotoluene the H 2 O:H 2 SO 4 ratio in the organic phase is much higher than
that in the acid phase. The proportion of acid in the organic phase is only 5-10%
(in relation to this phase).
It has also been found that for any nitric acid concentration, the nitration rate
is nearly proportional to the molar excess of sulphuric acid as related to the water
content of the nitrating mixture. The nitrating acid excess is a factor of special
importance in esterification of alcohols (“O-nitration”),
O-nitration of cellulose with mixtures of acids is a widely applied type of
esterification reaction in a heterogeneous system. One of the factors influencing
the reaction rate here is the rate of diffusion of the acid into the fibre.
The process of starch O-nitration by the Hackel and Urbanski’s method
(see Vol. II, Fig. 164) shows how homogeneity of the system affects the O-nitration
grade.
Recently Pausacker and Scroggie [96] have suggested that the nitration re-