Organic Chemistry of Explosives

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352 Dinitrogen Pentoxide – An Eco-Friendly Nitrating Agent

6 HNO 3 + P 2 O 5 3 N 2 O 5 + 2 H 3 PO 4 (Eq. 9.2)

Figure 9.2

On a laboratory scale the dehydration of nitric acid with phosphorous pentoxide is a con-

venient route to dinitrogen pentoxide (Equation 9.2). Isolation is achieved by sublimation

and collection in a cold trap at− 78 ◦C,^14 but the quality and yield of dinitrogen pentoxide is

poor if the system is not continually flushed with a stream of ozone, a consequence of facile

decomposition to dinitrogen tetroxide.

Phosphorous pentoxide in absolute nitric acid is essentially a solution of dinitrogen pen-

toxide in nitric acid and has been used for some nitrolysis reactions.^15 Nitrogen pentoxide

concentrations up to 30 % can be prepared according to the amount of phosphorous pentoxide

used.

N 2 O 4 + O 3 N 2 O 5 + O 2 (Eq. 9.3)

Figure 9.3

Two routes are feasible for the industrial synthesis of dinitrogen pentoxide and both are

economically dependent on the cost of electricity. The reaction of dinitrogen tetroxide with

ozone (Equation 9.3) was initially reported by Guye^16 and latter by Harris and co-workers^17.

This was later exploited and developed into an industrially useful process by chemists at the

Defence and Evaluation Research Agency (DERA) in the UK.^18 ,^19 It is mainly due to the

continued research effects of these chemists that dinitrogen pentoxide is now recognized as

the future for energetic materials synthesis. In this process a commercial ozonizer generates a

gas stream containing a 5–10 % mixture of ozone in oxygen and this is mixed in a flow reactor

with dinitrogen tetroxide. This is an instantaneous reaction and the dinitrogen pentoxide is

isolated by trapping as a solid in condenser tubes cooled by dry ice–acetone or a similar

coolant. Dinitrogen pentoxide from this process is essentially acid-free and usually stored at

a temperature of− 60 ◦C or below until required for reaction. Chemists at DERA have also

reported ozonolysis in the liquid phase, so enabling the direct production of 10 % (wt./wt.)

solutions of dinitrogen pentoxide in methylene chloride.^18 The reagents prepared in this way

have been used in batch and flow reactors for the pilot plant scale production of energetic

materials. Devendorf and Stacy^20 working at the Naval Surface Warfare Centre (NSWC) have

reported a very similar continuous process which produces 360 g of dinitrogen pentoxide per

hour. The introduction of high capacity ozonizers means that there is no intrinsic limitation to

scale-up.

N 2 O 4 + 2 NO 3 - 2 N 2 O 5 + 2e- (Eq. 9.4)

2e- + 4 HNO 3 N 2 O 4 + 2 NO 3 - + 2 H 2 O (Eq. 9.5)

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4 HNO 3 2 N 2 O 5 + 2 H 2 O (Eq. 9.6)

Figure 9.4