Organic Chemistry of Explosives

246 Synthetic Routes toN-Nitro

5.15.1.4 Reaction of paraformaldehyde with ammonium nitrate in the presence

of acetic anhydride

3 CH 2 O + 3 NH 4 NO 3 + 6 Ac 2 O RDX + 12 AcOH (Eq. 5.25)

Figure 5.102

Paraformaldehyde and ammonium nitrate undergo dehydration in the presence of acetic an-

hydride in a reaction known as the E-method after its discoverer M. Ebele.^201 Ross and

Schiessler^202 discovered the same reaction independently in Canada in 1940. The reaction

proceeds according to the stoichiometry in Equation (5.25). Studies conducted by Wright and

co-workers^192 and later by Winkler and co-workers^203 suggest the reaction occurs in the two

discreet steps shown in Equations (5.26) and (5.27).

6 CH 2 O + 4 NH 4 NO 3 + 6 Ac 2 O

(CH 2 ) 6 N 4 + 4 HNO 3 RDX + 3 CH 2 O + NH 4 NO 3 (Eq. 5.27)

(CH 2 ) 6 N 4 + 4 HNO 3 + 12 AcOH (Eq. 5.26)

Figure 5.103

The E-method is more expensive than other methods of RDX production because of the

relatively large amount of acetic anhydride used. However, the E-method uses no nitric acid

for the formation of RDX, and so must be considered the safest of the methods. The process

can, however, be dangerous if the reaction is not initiated before the reagent addition starts,

due to a potential build up of unreacted material. On safety grounds the paraformaldehyde and

ammonium nitrate are added to previously warmed acetic anhydride, or alternately, a catalytic

amount of boron trifluoride catalyst (∼0.4 %) is added at the start of the reaction, the latter

initiating the reaction at room temperature and lowering the proportion of undesirableN-acetyl

linear nitramines formed during the reaction. The low acidity of the reaction mixtures in the

E-process favours the formation of HMX as a by-product (∼6 % HMX in the RDX product).

The reaction based on Equation (5.25) gives yields of RDX up to 80 %.

5.15.1.5 Reaction of sulfamic acid, formaldehyde and nitric acid

N

N 3 S SO 3 KKO

SO 3 K 238

3 HNO 3 3 NH 2 SO 3 K + 3 CH 2 O 3 KHSO 4 + RDX

Figure 5.104

This unusual process known as the W-method was discovered in Germany by Wolfram^201 and

involves the condensation of the potassium salt of sulfamic acid with formaldehyde to form the

heterocycle (238) followed by treatment with nitric acid. The extreme sensitivity of (238) to

hydrolysis means that nitrolysis has to be conducted under anhydrous conditions using sulfur

trioxide^201 or phosphorous pentoxide^204 dissolved in fuming nitric acid. The yield of RDX

from the W-method is 80–90 %.