Nucleophilic aromatic substitution 161Picryl chloride (87) reacts with hydroxylamine hydrochloride to yield 2,4,6-trinitroaniline
(53) (picramide) and not the expectedN-hydroxy-2,4,6-trinitroaniline.^35 In contrast, the same
reaction in the presence of sodium ethoxide is reported to yield 4,6-dinitrobenzofuroxan (94)
via substitution of the halogen by hydroxylamine, followed by an internal redox reaction
between the hydroxyamino group and one of the adjacento-nitro groups.^219
Picryl chloride has been used successfully in a number of copper-mediated Ullmann cou-
pling reactions. 2,2′,4,4′,6,6′-Hexanitrobiphenyl has been synthesized by heating picryl chlo-
ride with copper powder.^220 ,^221 The same reaction in the presence of a hydride source (hot
aqueous alcohol) yields 1,3,5-trinitrobenzene (TNB).^221 ,^222 The Ullmann reactions between
picryl chloride and isomeric iodonitrobenzenes with copper bronze in DMF has been used to
synthesize 2,2′,4,6-, 2,3′,4,6-, and 2,4,4′,6-tetranitrobiphenyls.^223
4.8.1.3 The chemistry of 2,4-dinitrochlorobenzene
2,4-Dinitrochlorobenzene is an industrially important chemical synthesized from the nitration
of chlorobenzene with mixed acid. The halogen atom of 2,4-dinitrochlorobenzene is activated
by twoo/p-nitro groups and is particularly reactive. Consequently, 2,4-dinitrochlorobenzene
is used as a cheap and readily available starting material for the synthesis of many
explosives.
Cl
NO 2NO 2
95NH 2O 2 NNO 2NHO 2 NNO 2O 2 N NHNO 2NH NO 2O 2 NNO 2 O 2 NNO 2+12 9796O 2 Nreflux
95–98 %HNO 3H 2 SO 4 , HNO 3CaCO 3 , H 2 O
Nekal (emulsifier)86 % (2 steps)Figure 4.36The standard industrial and laboratory method for the synthesis of the high explo-
sive known as hexyl (12) (2,2′,4,4′,6,6′-hexanitrodiphenylamine) involves treating 2,4-
dinitrochlorobenzene (95) with aniline to produce 2,4-dinitrodiphenylamine (96), followed
by a two-stage nitration.^41 ,^224
The direct nitration of phenol and other substrates containing electron-donating groups
is often very vigorous and low yielding due to the formation of excessive by-products from
oxidative degradation. A much safer and more convenient route involves nitrating a derivative
of the substrate which already contained some nitro groups on the aromatic nucleus. The effect
of these nitro groups would be to moderate the reaction and protect the substrate from oxidation.
Picric acid (4) is formed in high yield from the mixed acid nitration of 2,4-dinitrophenol (98),