134 Synthetic Routes to AromaticC-Nitro Compounds
Nitration of m-nitroaniline (28) with fuming nitric acid and oleum yields 2,3,4,6-
tetranitroaniline (29), a powerful but chemically unstable explosive.^38 2,3,4,6-Tetranitroaniline
readily reacts with a range of nucleophiles, including water to yield 3-amino-2,4,6-
trinitrophenol.
NH 2O 2 N NO 2NH 2O 2 N NO 2O 2 N NO 2NO 2
3130100 % H 2 SO 4 ,
100 % HNO 370 –75 °C, 52 %Figure 4.9The remarkable ability of the amino group to promote electrophilic substitution is illustrated
by the nitration of 3,5-dinitroaniline (30) to 2,3,4,5,6-pentanitroaniline (31) in 52 % yield when
treated with anhydrous mixed acid.^39 The pre-existing nitro groups are advantageous by making
the aromatic ring less prone to oxidation.
Nitrations of aromatic amines often involve the intermediate formation ofN-nitramines,
although these are rarely seen under the strongly acidic conditions of mixed acid nitration
(Section 4.5).N,2,4,6-Tetranitro-N-methylaniline (tetryl) is an important secondary high
explosive usually synthesized from the nitration ofN,N-dimethylaniline or 2,4-dinitro-N-
methylaniline.^40 The synthesis of tetryl is discussed in Section 5.14.
The high explosive known as hexyl (2,2′,4,4′,6,6′-hexanitrodiphenylamine) is synthesized
in two steps from the nitration of 2,4-dinitrodiphenylamine. The first nitration uses 55 % nitric
acid at elevated temperature for conversion to 2,2′,4,4′-tetranitrodiphenylamine. Introduction
of two more nitro groups to yield hexyl requires a mixture of concentrated sulfuric and nitric
acids, although the reaction can be conducted at room temperature (Section 4.8.1.3).^41
4.3.2.3 Toluene
The methyl group of toluene makes nitration a relatively facile process. However, as more
nitro groups are introduced, the aromatic ring becomes more electron deficient and deactivated
towards electrophilic attack, and so requires more vigorous conditions for further nitration.
The direct nitration of toluene to TNT with mixed acid is not industrially feasible. First, a very
large excess of strong mixed acid or oleum would be required to compensate for the water
formed during the mono- and di-nitration, and secondly, such a strong nitrating agent would
lead to many by-products. Consequently, the nitration of toluene to 2,4,6-trinitrotoluene (TNT)
is usually conducted in two or three steps.13aThis is a common strategy for the polynitration of
many aromatic substrates. Substituents which activate towards electrophilic substitution also
make side-reactions like oxidation more facile. Mixed acid, particularly at high temperatures,
is a strong oxidizing agent, and oleum, if present, can sulfonate activated substrates. Conse-
quently, while a strong mixed acid may be needed for the introduction of the final nitro group
into a polynitroarylene, this reagent is probably not suitable for initial nitration.
Industrial TNT production produces both atmospheric and water pollution. The spent acid
from the three stages of mono-, di- and tri-nitration pose considerable disposal problems. On
an industrial scale the mixed acid from previous di- and tri-nitrations is usually refortified
with nitric acid and used for mono- and di-nitration respectively. Diluted sulfuric acid is often