Nucleophilic aromatic substitution 169o/p-positions to one another, making them considerably more reactive towards nucleophilic
attack.^255
The 2,4,5- (124), 2,3,4- (126), and 2,3,6- (128) isomers of trinitrotoluene are produced
during the nitration of toluene and are present in the crude product.^255 Such isomers reduce the
chemical stability ofα-TNT and need to be removed from the product. The increased reactivity
of these unsymmetrical isomers towards nucleophilic attack can be exploited in the purification
ofα-TNT. Thus, heating crude TNT with an aqueous solution of sodium sulfite will replace any
reactive nitro groups present in the unsymmetrical isomers with a sodium sulfonate group (125,
127, 129). These sulfonates are water-soluble and are washed out of the product in the aqueous
liquors.^256 It is important to remember that while polynitroarylenes like 2,4,6-trinitrotoluene
show sufficient chemical stability for wide use as explosives they are still reactive towards
nucleophilic attack, although much less so than their unsymmetrical isomers – nitro groups
also reduce electron density atm-positions, but to a lesser extent thano/p-positions. The
reaction ofα-TNT with sodium sulfite is much slower than with its unsymmetrical isomers,
but at elevated temperatures and with long reaction times the proportion ofα-TNT lost as
water-soluble sulfonate can be significant. The purification reaction also serves to remove
other impurities present in crude TNT, namely, tetranitromethane, which also forms a water-
soluble sulfonate on reaction with aqueous sodium sulfite.
In a similar manner, of the isomeric trinitrobenzenes, only the symmetrical 1,3,5-isomer
shows sufficient chemical stability for use as an explosive. Even so, the aromatic ring of
1,3,5-trinitrobenzene is highly electron deficient and reaction with alkali metal carbonates or
bicarbonates in aqueous boiling methanol yields 3,5-dinitroanisole.^257 Unsymmetrical isomers
of trinitrobenzene are much more reactive than the 1,3,5-isomer, with only relatively mild
conditions needed to effect the displacement of their nitro groups.^258
It is found that some nucleophiles will displace an activated nitro group in preference to a
halogen atom in a similarly activated position. Accordingly, 2,4,5-trinitrochlorobenzene reacts
with ammonia to form 5-chloro-2,4-dinitroaniline.^259
4.8.3 Displacement of other groups
The displacement of both halides and nitro groups from polynitroarylenes has been covered in
Sections 4.8.1 and 4.8.2. The centres of low electron density induced by electron-withdrawing
nitro groups also allow the displacement of many other groups, including hydrogen, alkoxy,
aryloxy, sulfonate ester etc.
4.8.3.1 Displacement of hydrogen
A reaction of growing importance in energetic materials synthesis is that of amination by
nucleophilic substitution of aromatic hydrogen. Such reactions usually involve treating aro-
matic nitro compounds with an aminating agent under basic conditions. The reaction of 1,3,5-
trinitrobenzene (TNB) with hydroxylamine under basic conditions yields 2,4,6-trinitroaniline
(picramide) and a small amount of 1,3-diamino-2,4,6-trinitrobenzene (DATB).^260 Mitchell
and co-workers^261 found at elevated temperatures and in the presence of sodium methox-
ide in DMSO, picramide reacts with excess hydroxylamine to yield 1,3,5-triamino-2,4,6-
trinitrobenzene (TATB). Such reactions involving hydrogen displacement come under the
general classification of vicarious nucleophilic substitutions (VNS).^262