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C-nitration 353
The second route, involving the electrolysis of dinitrogen tetroxide solutions in 100 %
nitric acid (Equations 9.4, 9.5 and 9.6), was pioneered at the Lawrence Livermore National
Laboratory (LLNL)^21 and enables the preparation of nitric acid solutions containing up to
30 % by weight strength^22 of dinitrogen pentoxide in specially designed cells. This technology
was further developed into a commercial process by chemists at DERA^23 and is now used
for energetic materials synthesis on an industrial scale. Rodgers and Swinton^24 working for
ICI Explosives Division in collaboration with DERA have reported a procedure where the
dinitrogen pentoxide generated in this system can be phase separated and isolated as the pure
crystalline solid for use in nitrations in nonpolar solvents. Bloom, Fleischmann and Mellor^25
reported a modification of the electrolytic process and used tetrabutylammonium nitrate and
tetrafluoroborate as electrolytes for the electrochemical oxidation of solutions of dinitrogen
tetroxide in methylene chloride. Chapman and Smith^26 reported the use of sodium fluoride as a
base to remove nitric acid from electrochemically generated solutions of dinitrogen pentoxide
in nitric acid, so enabling the preparation of acid-free solutions of dinitrogen pentoxide in inert
solvents.
Equipment needed for the above procedures is not always available in the standard labo-
ratory. A useful and widely used method for preparing solutions of dinitrogen pentoxide in
nitric acid involves the distillation of mixtures of oleum and potassium nitrate in absolute nitric
acid.^27 Another method uses a solution of sulfur trioxide and ammonium nitrate in nitric acid.^27
Although the original report^27 states that solutions of 28–42 % dinitrogen pentoxide in nitric
acid can be prepared via this method, a later report^22 suggests that concentrations higher than
30 % are not attainable.
Dinitrogen pentoxide has also been synthesized from the reaction of nitric oxide (NO) and
dinitrogen trioxide (N 2 O 3 ) with ozone.^28 The reaction of nitric acid containing a trace of picric
acid also yields dinitrogen pentoxide.^29
9.5 C-nitration
Dinitrogen pentoxide has been used as a solution in organic solvents and in absolute nitric
acid for aromatic nitrations. The former are mild and nonoxidizing nitrating agents and well
suited for the nitration of acid-sensitive or activated substrates prone to oxidation i.e. phenols.
Solutions of dinitrogen pentoxide in absolute nitric acid are similar to powerful nitrating agents
like anhydrous mixed acid and valuable for the synthesis of polynitroarylenes. In this way the
systems are complimentary in nature and quite unique amongst conventional nitrating agents.
The nitration of aromatic substrates with dinitrogen pentoxide in various organic solvents
has been part of an extensive study by Ingold and co-workers.^30 The kinetics and mechanism
of these reactions are complex and beyond the scope of this discussion but some important
observations have been made. In polar solvents, nitrations with dinitrogen pentoxide are be-
lieved to involve the nitronium ion^31 and the activity is directly dependent on the concentration
of nitronium ion in that medium. In nonpolar solvents the nitration probably proceeds via
unionized dinitrogen pentoxide. The addition of salts like tetraethylammonium nitrate has a
large accelerating effect on reaction rates in organic solvents, a point explained by the greater
ionization of dinitrogen pentoxide in this medium which leads to an increase in nitronium
ion concentration. This effect is seen with dinitrogen pentoxide in nitric acid where the dini-
trogen pentoxide becomes fully ionized and a powerful nitrating agent.^7 ,^10 ,^32 For an in depth
mechanistic discussion the readers are directed to an excellent review by Fischer.^3
