The synthesis of RDX

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. 

In this method, first established by Herz and later studied by Hale, 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. 

The 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) °° and Equation (5.22). ° Based on the assumption that one mole of hexamine produces one mole of RDX the Hale nitrolysis reaction commonly yields 75-80 % of RDX. 

2 Nitrolysis of hexamine dinitrate with nitric acid - ammonium nitrate - acetic anhydride 

Both Koffler in Germany (1943) and Bachmann in the US (1941) discovered this method independently. In Germany the reaction was known as the KA-process. The process 

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In addition to the Ross-Schiessler process, utilising p-CH20 and AN, the synthesis of RDX/ HMX mixts has also been reported starting with other smail molecules (Ref 7), namely, methyl-amine nitrate methylene diamine dinitrate and nitramine (NH2N02) in combination with CH20. In these reactions, the intermediate formation of Hexamine or a cyclic analog, is not necessarily established... 

The synthesis of RDX via the Triazine Process (Ref 13) involves the condensation of formaldehyde with acetonitrile followed by nitration ... 

Proceeding from a precursor polyaza-caged structure, which may be different from the desired product, but includes the final structure within the cage. Although not a caged compound the synthesis of RDX from the nitrolysis of hexamine would fit this category. 

The only known adamantane containing more than three endocyclic ring nitrogens is hexamethylenetetramine (hexamine, 1,3,5,7-tetraaza-adamantane, 4). It is readily prepared by reaction of formaldehyde and ammonia and was first described by Butlerov in 1859 (Ref. 82). Its preparation and properties have been reviewed (Ref. 83). The mechanism of formation from formaldehyde and ammonia has been studied (Refs. 84,85). Hexamine is employed as a reactant in the synthesis of RDX (3) and HMX (1). 

The chemistry of the prepn of RDX is highly complex and remains not fully understood. What follows is a synthesis of the views of British and Canadian investigators (Refs 37a 41a) as summarized by Urbanski (Ref 82, pp88-89). The following reactions presumably occur when hexamine is treated with nitric add to produce RDX ... 

In conclusion, there appears to be some supporting evidence, other than these tracer studies, that the synthesis of HMX and RDX molecules can be accomplished thru a build-up from single methylene-containing spedes or other small molecules, and that this route can also take place via a total degradation and resynthesis from molecules such as Hexamine. However, die development of an economical process for the large scale production of these expl nitramines, in particular HMX, via a method precluding the use of Hexamine, is vet to be accomplished... 

Chapman studied the nitrolysis of symmetrical methylenediamines. The nitrolysis of N, N, N, M-tetramethylmethylenediamine with nitric acid-acetic anhydride-ammonium nitrate mixtures gives both dimethylnitramine and RDX the latter probably arises from the nitroT ysis of hexamine formed from the reaction of ammonium nitrate and formaldehyde released from the hydrolysis of the methylenediamine. The same reaction with some morpholine-based methylenediamines (105) allows the synthesis of l,3,5-trinitro-l,3,5-triazacycloalkanes (106). 

While the present industrial synthesis of RDX via the Bachmann process has many faults, it is high yielding and would be difficult to match by an alternative synthesis. Even so, Gilbert and co-workers reported on a study investigating the nitrolysis of a series of 1,3,5-trisubstituted-... 

Variations in the conditions used for the nitrolysis of hexamine have a profound effect on the nature and distribution of isolated products, including the ratio of RDX to HMX. It has been shown that lower reaction acidity and a reduction in the amount of ammonium nitrate used in the Bachmann process increases the amount of HMX formed at the expense of Bachmann and co-workers ° were able to tailor the conditions of hexamine nitrolysis to obtain an 82 % yield of a mixture containing 73 % HMX and 23 % RDX. Continued efforts to provide a method for the industrial synthesis of HMX led Castorina and co-workers to describe a procedure which produces a 90 % yield of a product containing 85 % HMX and 15 % RDX. This procedure conducts nitrolysis at a constant reaction temperature of 44 °C and treats hexamine, in the presence of a trace amount of paraformaldehyde, with a mixture of acetic acid, acetic anhydride, ammonium nitrate and nitric acid. Bratia and co-workers ° used a three stage aging process and a boron trifluoride catalyst to obtain a similar result. A procedure reported by Picard " uses formaldehyde as a catalyst and produces a 95 % yield of a product containing 90 % HMX and 10 % RDX. 

Gilbert and co-workers showed that the nitrolysis of 1,3,5-triacyl-1,3,5-triazacyclohexanes offered little benefit over the conventional synthesis of RDX via the nitrolysis of hexamine. This is not the case for HMX where its synthesis via the Bachmann process is far from perfect. This process and its modifications are expensive, requiring large amounts of acetic anhydride. The rate of production is slow and the maximum attainable yield is 75 %. In fact, HMX is five times as expensive as RDX to produce by this process and this prevents the widespread use of this high performance explosive. Many efforts have focused on finding more economical routes to HMX. 

The nitrolysis of hexamine at low temperature has led to the synthesis of a number of cyclic nitramines. The reaction of hexamine dinitrate (241) with 88 % nitric acid at -40 °C, followed by quenching the reaction mixture onto crushed ice, leads to the precipitation of 3,5-dinitro-3,5-diazapiperidinium nitrate (242) (PCX) in good yield PCX is an explosive equal in power to RDX but is slightly more sensitive to impact. The reaction of PCX (242) with sodium acetate in acetic anhydride yields l-acetyl-3,5-dinitro-l,3,5-triazacyclohexane (82) (TAX), which on further treatment with dilute alkali in ethanol yields the bicycle (243). ... 

The above observations allow the selective formation of RDX, HMX or the two linear nitramines (247) and (248) by choosing the right reaction conditions. For the synthesis of the linear nitramine (247), with its three amino nitrogens, we would need high reaction acidity, but in the absence of ammonium nitrate. These conditions are achieved by adding a solution of hexamine in acetic acid to a solution of nitric acid in acetic anhydride and this leads to the isolation of (247) in 51 % yield. Bachmann and co-workers also noted that (247) was formed if the hexamine nitrolysis reaction was conducted at 0 °C even in the presence of ammonium nitrate. This result is because ammonium nitrate is essentially insoluble in the nitrolysis mixture at this temperature and, hence, the reaction is essentially between the hexamine and nitric acid-acetic anhydride. If we desire to form linear nitramine (248) the absence of ammonium nitrate should be coupled with low acidity. These conditions are satisfied by the simultaneous addition of a solution of hexamine in acetic acid and a solution of nitric acid in acetic anhydride, into a reactor vessel containing acetic acid. 

Tetranitrohexahydropyrimidine (DNNC) (94) has been synthesized from the nitrolysis of the A(iV -di-fert-butylpyrimidine (93). Levins and co-workers reported the synthesis of DNNC (94) from the nitrolysis of the analogous A. V -di-wo-propylpyrimidine (92). DNNC is a high performance explosive with a detonation velocity of 8730 m/s, impact sensitivity lower than RDX and a very favourable oxygen balance. DNNC has been suggested " for use as an oxidizer in propellant compositions. This is also considered as an excellent oxidant for pyrotechnic compositions. ... 

PETN was first prepared in 1894 by nitration of pentaerythritol. Commercial production of PETN could not be achieved until formaldehyde and acetaldehyde required in the synthesis of pentaerythritol became readily available about a decade before World War II. During World War II, RDX was utilized more than PETN because PETN was more sensitive to impact and its chemical stability was poor. Explosive compositions containing 50% PETN and 50% TNT were developed and called Pentrolit or Pentolite . This composition was used for filling hand and anti-tank grenades, and detonators. 

Bachmann s products were known as Type B RDX and contained a constant impurity level of 8-12%. The explosive properties of this impurity were later utilized and the explosive HMX, also known as Octogen, was developed. The Bachmann process was adopted in Canada during World War II, and later in the USA by the Tennes-see-Eastman Company. This manufacturing process was more economical and also led to the discovery of several new explosives. A manufacturing route for the synthesis of pure RDX (no impurities) was developed by Brockman, and this became known as Type A RDX. 

G. A. Olah in Chapter 7 reviews some of the most useful methods in preparing nitro compounds (i.e., electrophilic nitrations with superacid systems, nitronium salts, and related Friedel-Crafts type complexes). Polynitro compounds were traditionally and still are the most widely used explosives [e.g., nitroglycerol, trinitrotoluene (TNT), and Af-nitramines (RDX and HMX)]. Methods of preparing nitro compounds thus remain a key part of the synthesis of energetic materials. 

Trifluoroacetaldehyde figures in recent work on the synthesis of analogues of nitramine-type explosives, having been used to prepare 2,2,2-trifluoro-1,1-dinitraminoethane (20) by application of the route used to procure Medina itself [CHj(NHNOj)2] (Scheme 19), and to obtain the 2,4,6-tris(trifluoromethyl) derivative (22) of RDX (Scheme 20). Nitration of 2,4,6-tris(trifluoromethyl)hexahydro-sym-triazine (21) yields the heterocyclic nitramine (22) directly. The acetamido-analogue [CF3-CH(NH COMe)2] of the bisformamide (19a) can be prepared by heating anhydrous trifluoroacetaldehyde with acetamide, and the trifluoroacetamido-compound... 

The original practical method of RDX synthesis, as developed by Hale of PicArsn in 1925 (Ref 1), involves treatment of Hexamine with a large excess of abs nitric acid and results in 40% yield based on 2 moles of RDX from 1 mole of Hexamine. Ross and Schiessler (Ref 15), in 1940 at McGill University, succeeded in synthesizing the same compd from paraformaldehyde (p-CH20) and AN in Ac20 in 35% yield based on p-CH20. (Subsequently, after WWII, this was found to be identical to the secret process of Eble, developed in Ger). 

Bell and Dunstan (Ref 16) have reported that the addition of me thy lene dinit ramine to Hexamine nitrolysis mixts, aged at 0° for 5 and 120 minutes, gave RDX in yields of 120 and 106%, respectively, compared with 52 and 83% in the absence of the dinitramine. However, rather than supporting a synthesis from small molecules, the authors contend that these results substantiate the existence of a postulated bis(nitroxymethyl) aminomethyl precursor of RDX, namely ... 

Thyagarajan and Majumdar (Ref 18) have studied the condensations of urethanes with formaldehyde under various exptl conditions and accomplished the selective synthesis of either six-membered 1,3,5-triazines or eight-membered 1,3,5,7-tetrazocines. These are nonnitrated analogs of RDX and HMX respectively. Their results are summarized in Fig 4... 

SNPE, France, produced reduced sensitivity RDX (RS-RDX) by the Woolwich synthesis by employing a proprietary recrystallization process. This RS-RDX displayed reduced sensitivity to shock initiation. Subsequently, some other manufacturers also claimed to produce some form of RDX that exhibits reduced sensitivity to shock compared with the conventional RDX produced by the Bachmann process. EURENCO has also developed a process to mass-manufacture a variety of low sensitive Hexogen (RDX), called I-RDX. 

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