244 Synthetic Routes toN-Nitro
of composite explosives like Composition B, Torpex, Cyclotols, DBX, HBX, Hex-24, PTX-1
etc. and plastic explosives like Composition C-4, Semtex-H, PVA-4 etc. is extensive.
RDX is usually prepared from the nitrolysis of hexamine and this is the most widely studied
reaction for any explosive. As previously discussed, the nitrolysis of the CH 2 –N bonds of hex-
amine can produce products other than RDX. The higher homolog, HMX is a common impurity
in crude RDX prepared via this route. However, the presence of HMX is not undesirable if the
RDX is to be used as an explosive. Other impurities are less desirable, such as complex linear
nitramine-nitrates, which lower the melting point of RDX in addition to increasing impact
sensitivity and lowering thermal stability. However, these nitramine-nitrate impurities are less
stable than RDX and can be hydrolyzed on prolonged treatment with boiling water, a process
known as ‘degassing’, which releases toxic volatiles such as nitrogen oxides and formaldehyde.
Studies have shown that these linear nitramine-nitrate by-products can be re-subjected to the
conditions of nitrolysis^192 and form additional RDX; this process is practised on an industrial
scale.
RDX has been synthesized by the different methods discussed below. Only methods 5.15.1.2
and 5.15.1.3 have received industrial importance. Method 5.15.1.7 is a convenient laboratory
route to analytically pure RDX.
5.15.1.1 Treatment of hexamine with nitric acid
In this method, first established by Herz^199 and later studied by Hale,^200 hexamine is introduced
into fuming nitric acid which has been freed from nitrous acid. The reaction is conducted at
20–30◦C and on completion the reaction mixture is drowned in cold water and the RDX
precipitates. The process is, however, very inefficient with some of the methylene and nitrogen
groups of the hexamine not used in the formation of RDX. The process of nitrolysis is complex
with formaldehyde and some other fragments formed during the reaction undergoing oxidation
in the presence of nitric acid. These side-reactions mean that up to eight times the theoretical
amount of nitric acid is needed for optimum yields to be attained.
(CH 2 ) 6 N 4 + 4 HNO 3(CH 2 ) 6 N 4 + 6 HNO 3RDX + 3 CH 2 O + NH 4 NO 3 (Eq. 5.21)RDX + 6 H 2 O + 3 CO 2 + 2 N 2 (Eq. 5.22)Figure 5.99The stoichiometry of the Hale nitrolysis reaction is very dependent on reaction conditions.
Even so, this reaction has been postulated to conform to the stoichiometry in Equation (5.21)^200
and Equation (5.22).^201 Based on the assumption that one mole of hexamine produces one mole
of RDX the Hale nitrolysis reaction commonly yields 75–80 % of RDX.
5.15.1.2 Nitrolysis of hexamine dinitrate with nitric acid – ammonium
nitrate – acetic anhydride
Both K ̈offler^201 in Germany (1943) and Bachmann^195 ,^197 in the US (1941) discovered this
method independently. In Germany the reaction was known as the KA-process. The process