150 Synthetic Routes to AromaticC-Nitro Compounds
Peroxydisulfuric acid (H 2 S 2 O 8 ) is one of the most powerful peroxyacid oxidants available.
Peroxydisulfuric acid is usually preparedin situas a solution in sulfuric acid via the addition
of 90–98 % hydrogen peroxide to an excess of oleum (Equations 4.5 and 4.6). This preparation
now poses a problem because of the limited availability of high concentration hydrogen perox-
ide solutions to standard laboratories. Peroxydisulfuric acid is also generatedin situby passing
ozone–oxygen mixtures through oleum (Equation 4.7).^39 Solutions of peroxydisulfuric acid
are unstable and slowly decompose at room temperature. Water must be rigorously excluded
because of the rapid formation of the much weaker oxidant, peroxymonosulfuric acid.
H 2 SO 4 + SO 3 + O 3 H 2 S 2 O 8 + O 2 (Eq. 4.7)Figure 4.24The real potential of peroxydisulfuric acid for the synthesis of highly nitrated arylenes
was brought to light by research conducted at the Naval Air Warfare Center, Weapons
Division, China Lake.^39 ,^139 ,^143 ,^145 ,^153 ,^154 Some of the highly nitrated polynitroarylenes syn-
thesized during this study are illustrated in Table 4.1. Peroxydisulfuric acid prepared from
both hydrogen peroxide and ozone has been used to synthesize tetranitrobenzenes^39 ,^139
and pentanitrobenzene^139 ,^153 from isomeric trinitroanilines and tetranitroanilines respectively
(Table 4.1, Entries 2a and 2b). Tetranitrotoluenes^139 ,^145 and pentanitrotoluene^143 ,^39 have been
synthesized from isomeric trinitrotoluidines and tetranitrotoluidines respectively (Table 4.1,
Entries 3, 4a and 4b). Highly nitrated biphenyls, terphenyls and stilbenes have also been syn-
thesized from the corresponding amino derivatives (Table 4.1, Entry 7).^139 ,^154
Reactions are usually conducted by adding concentrated hydrogen peroxide to a solution of
the arylamine in oleum. Reactions are often slow at room temperature but can be increased by
using sulfuric acid solutions containing a high concentration of hydrogen peroxide, and hence,
peroxydisulfuric acid. This is usually achieved with>95 % hydrogen peroxide solution and
oleum containing 30 % dissolved sulfur trioxide.
Some substrates show limited solubility in sulfuric acid solutions and this can affect the rate
of oxidation. However, the main factor for slow amine oxidation is due to the high concentration
of protonated amine under these highly acidic conditions. Under these conditions only weakly
basic amines have a high enough concentration of unprotonated form to permit oxidation
to occur. As a result, sulfuric acid solutions of peroxydisulfuric acid are only useful for the
oxidation of very weakly basic amines. Peroxydisulfuric acid oxidizes trinitrotoluidines to
tetranitrotoluenes (Table 4.1, Entry 3) but leaves the more basic dinitrotoluidines unaffected.^145
The opposite is true of peroxyacids like peroxytrifluoroacetic acid and so the reagents are very
much complementary.
It is unsurprising that some substrates react with peroxydisulfuric acid faster when in 100 %
sulfuric acid than in oleum. A striking example of this is illustrated by the oxidation of 2,6-
dinitroaniline to 1,2,3-trinitrobenzene in 56 % yield when the sulfuric acid concentration is
96 % whereas in 20 % oleum this substrate is unaffected.^139
The powerful explosive hexanitrobenzene (55) (experimental^155 VOD∼9500 m/s) is pre-
pared from the oxidation of 2,3,4,5,6-pentanitroaniline (31) with peroxydisulfuric acid gener-
atedin situfrom both oleum/hydrogen peroxide^139 ,^153 and oleum/ozone^39 (Table 4.1, Entries
1a and 1b). Hexanitrobenzene is one of the few explosives containing no hydrogen atoms
and belongs to a select group of organic explosives called ‘nitrocarbons’. Decanitrobiphenyl
(65) is another nitrocarbon explosive and is prepared from the oxidation of 4,4′-diamino-
2,2′,3,3′,5,5′,6,6′-octanitrobiphenyl (64) with peroxydisulfuric acid (Table 4.1, Entry 7).^154