Nucleophilic aromatic substitution 157fromN-hydroxy-2,4,6-trinitroaniline. The synthesis of 1,2,4,5-trinitrobenzene from the ni-
tric acid oxidation of 2,4-bis(hydroxyamino)-1,5-dinitrobenzene has also been reported.^195 ,^196
The harsh conditions employed for the nitric acid oxidation of arylhydroxylamines limits the
scope of the reaction. Borsche and Feske^197 reported the synthesis of 2,3,4,6-tetranitrotoluene
from the nitric acid oxidation of 3-(hydroxylamino)-2,4,6-trinitrotoluene, although a yield for
this reaction was not given. Considering the susceptibility of the methyl group to oxidation the
yield is expected to be low.
NO 2NO 2NO 2O 2 NO 2 NO 2 NO 2 NNHOHNHOHHOHNNHOHNHOHHOHNNO 2NO 2NO 2NO 2NO 2HNO 32858655
partial
reductionHNO 3 ,
CrO 3Figure 4.28A synthesis of hexanitrobenzene (55) has been reported which involves hydroxyamino
oxidation. Although no experimental details of this procedure are given the synthesis is reported
to start from partial reduction of 1,3,5-trinitrobenzene (2) to 1,3,5-tris(hydroxyamino)benzene
(85), followed by ring nitration and subsequent oxidation of the three hydroxyamino groups of
(86) with nitric acid in the presence of chromium trioxide.^198 Nielsen and co-workers attempted
to replicate this synthetic route but were unsuccessful.^153
4.8 Nucleophilic aromatic substitution
The nitro group is one of the most potent substituents for withdrawing electron density from
the aromatic nucleus. While such groups make electrophilic substitution more difficult they
facilitate nucleophilic attack on the aromatic ring. Nitro groups introduce centres of low elec-
tron density at theo- andp-positions on the benzene ring, and consequently, halogens and
other leaving groups in these positions are especially susceptible to nucleophilic attack. The
effect of leaving group activation byo/p-nitro groups is seen in the case of 1,4-dichloro-2,6-
dinitrobenzene, a substrate readily obtained from the nitration ofp-dichlorobenzene.^199 In this
substrate the 1-chloro group is flanked by twoo-nitro groups and is readily displaced by a
range of nucleophiles; the 4-chloro group is positionedm- to these nitro groups and is much
less reactive towards nucleophilic attack.^200
In general, the more nitro groups present on the aromatic ring the easier the leaving group
displacement. Nucleophilic aromatic substitution is therefore a very important reaction in the
chemistry of polynitroarylenes. While the use of such reactions has been extensive in the
synthesis of explosives, the reaction also has important implications for the chemical stability
of many polynitroarylenes (discussed in Section 4.8.2).