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350 Dinitrogen Pentoxide – An Eco-Friendly Nitrating Agent
elaborate reactors and blast-proof buildings. Product separation is a frequent problem with the
mixed acid nitration of polyols. The mixed acid residue and the aqueous washings often
contain considerable amounts of dissolved nitrate ester and so, presenting both a safety and
waste disposal problem. Similar problems are encountered during the nitrolysis of hexamine
via the Bachmann process, with the waste liquors containing many unstable species including
acetyl nitrate. Such liquors need additional plant to remove or destroy organic residues before
recycling or discharging as effluents.
European and American countries are slowly moving towards the use of a new nitration
methodology based on dinitrogen pentoxide (N 2 O 5 ).^3 –^5 Agrawal has recently described the
potential of dinitrogen pentoxide methodology for high energy materials synthesis.^2 It is now
widely believed that this new nitration technology is the future for energetic materials syn-
thesis. This chapter discusses the recent advances made in energetic materials synthesis with
dinitrogen pentoxide as the nitrating agent.
9.2 Nitrations with dinitrogen pentoxide
Dinitrogen pentoxide was first prepared^6 over 150 years ago but received little attention as
a nitrating agent. This is probably due to technical difficulties in preparation and its low
thermal stability, requiring temperatures of− 60 ◦C for its long term storage.^4 Its real potential
for the synthesis of polynitroarylenes, nitramines and nitrate esters, has only recently been
recognized.^3 ,^5 This is in large due to sustained research at the Defence Evaluation and Research
Agency (DERA) in the UK. Dinitrogen pentoxide is now seen as a real alternative to nitration
with conventional nitrating agents. Environmental restrictions on hazardous waste effluents
are also pushing this technology forward, and in many cases, dinitrogen pentoxide constitutes
an ‘environmentally friendly’ alternative.
There are several advantages of using dinitrogen pentoxide over mixed acid; (1) reactions are
considerably faster, less exothermic and easier to control, and cleaner, often due to an absence
of oxidation by-products, (2) yields are higher, (3) product isolation is often easier, (4) there is
no acid waste which needs treatment and disposal, and lastly, (5) some high energy materials
such as nitrated hydroxy-terminated polybutadiene (NHTPB), poly[NIMMO], poly[GLYN]
and ammonium dinitramide (ADN), which cannot be synthesized with the use of conventional
nitrating agents, are readily synthesized by this versatile nitrating agent. In fact, dinitrogen
pentoxide can achieve all the reactions of conventional nitrating agents but usually more
efficiently.
Nitrations with dinitrogen pentoxide are conducted with two types of reagent:
(1) Solutions of dinitrogen pentoxide in anhydrous nitric acid. This is a highly acidic and
powerful nitrating agent. It can be used to synthesize all types of explosives i.e. polyni-
troarylenes, nitramines and nitrate esters. It is particularly valuable for the nitration of
highly deactivated nitroaromatics, and the synthesis of energetic nitramines by nitrolysis
where mixed acid fails in many cases. However, like nitrations with many conventional
nitrating agents, it is unselective.
(2) Solutions of dinitrogen pentoxide in chlorinated solvents. These are less active nitrating
agents but much milder and more selective in their nitrations. These reactions are more
controlled and usually very clean, nonoxidizing and, consequently, high yielding. With
