Nitration 135reconcentrated, but at considerable expense. Another source of pollution is from ‘red water’,
the aqueous washing from sulphite treatment of crude TNT and resulting from the presence of
unstable unsymmetrical isomers of TNT.
Hill and co-workers^42 studied the two stage nitration of toluene to TNT and showed that the
production ofm-isomers was mainly from the initial mono-nitration step. Hill and co-workers
showed that the nitration of toluene to dinitrotoluene at temperatures above 40◦C always
produces 3.5 % or more ofm-isomers. In contrast, the nitration of pureo-orp-nitrotoluenes
produces TNT with less than 0.1 % ofm-isomers. Initial nitration of toluene to dinitrotoluene
(mainly 2,4- and 2,6-isomers) is highly exothermic and an increase in temperature is known
to increase the amount ofm-isomers in the nitration product. Hill and co-workers showed
that mixed acid containing up to 7 % water was as effective as oleum–nitric acid mixtures for
nitrating toluene to dinitrotoluene, yields being>99 % after 1 hour of reaction time. Conducting
these nitrations between 5 and− 35 ◦C resulted in a reduction inm-isomer ratio from 2.06 % to
1.40 %. It was further shown that conducting the initial nitration of toluene at− 10 ◦C, followed
by further nitration under standard conditions, resulted in crude TNT containing only 1.8 %
ofm-isomers as compared to the usual 3.5 %. Hill and co-workers also nitrated toluene to
dinitrotoluene with potassium nitrate in sulfuric acid at− 10 ◦C (97 %) and with nitric acid in
triflic acid at lower temperatures. The Olin Corporation^43 reports that this di-nitration can be
achieved with nitric acid alone.
The nitration of dinitrotoluene to TNT requires the use of mixed acid fortified with oleum,
and the use of elevated temperature, which produces its own pollutants and impurities, including
tetranitromethane and nitrogen oxides. An excellent discussion of industrial TNT production
has been given by Urba ́nski.13a
4.3.2.4 Other alkylbenzenes
The presence of two methyl groups inm-xylene makes it a more reactive substrate than toluene,
but as a consequence, this substrate is more susceptible to oxidation. Therefore, the nitration of
m-xylene requires lower temperatures and the use of less concentrated mixed acid. In fact, the
nitration of either 2,4- or 2,6-dinitro-m-xylenes to 2,4,6-trinitroxylene (TNX) can be achieved
with mixed acid containing up to 10 % water.^44 TNX is a less powerful explosive than TNT
and has a poor oxygen balance (−78.4 %).
4.3.2.5 Benzene
The synthesis of 1,3,5-trinitrobenzene (TNB) from the direct nitration ofm-dinitrobenzene is
very difficult. Desvergnes^45 reported a 71 % yield of TNB (2) on treatment ofm-dinitrobenzene
(32) with a large excess of mixed acid composed of anhydrous nitric acid and 60 % oleum at
a reaction temperature of 110◦C for several days. Similar results are also reported from other
sources.^46
NO 2NO 2NO 2O 2 N NO 2
32 230 % oleum,
fuming HNO 3
110 °C, 71 %
or
NO 2 BF 4 , FSO 3 H,
150 °C, 50 %Figure 4.10