The nitrolysis of hexamine

The phenol (237) can be synthesized via a Mannich condensation between phenol, formaldehyde and dimethylamine, followed by nitration-nitrolysis of the product (236) with concentrated nitric acid. The phenolic group of (237) is acidic, and so enables the formation of metal salts which are very impact sensitive explosives. 

The importance of cyclic nitramines as military explosives has meant that an enormous amount of research has been conducted in this area. Only some of the rich array of products and by-products obtainable from hexamine nitrolysis are discussed in this section. For mechanistic smdies and detailed analysis of these reactions the primary research papers should be consulted.  

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J. Solomon, Pi Study of the Nitrolysis of Hexamine to Increase HMX Yields, Illinois Institute of Technology, Chicago, 1973. 

The replacement of amine and amide hydrogen with a nitro group via direct nitration is an important route to A-nitro functionality. However, the cleavage of other bonds is also important. In the case of C-N bond cleavage the process is known as nitrolysis and is an invaluable route to many energetic materials (Section 5.6). The nitrolysis of hexamine and the syntheses of the important explosives HMX and RDX are discussed in Section 5.15. This area of chemistry could easily demand a separate chapter of its own and is the most complex and diverse in the field of nitramine chemistry. 

The nitrolysis of A,A-disubstituted amides is one of the key tools for the synthesis of nitramine containing energetic materials. The present synthesis of the high performance explosive HMX is via the nitrolysis of hexamine (Section 5.15). This is an inefficient reaction requiring large amounts of expensive acetic anhydride. An alternative route to HMX (4) is via the nitrolysis of either l,3,5,7-tetraacetyl-l,3,5,7-tetraazacyclooctane (79) (79%) or 1,5-dinitro-3,7-diacetyl-l,3,5,7-tetraazacyclooctane (80) (98 %) with dinitrogen pentoxide in absolute nitric acid. These reactions are discussed in more detail in Section 5.15. 

The most important nitrolysis reaction to date is the formation of RDX (3) and HMX (4) from the caged methylenediamine known as hexamine (104). These important military explosives were first mass manufactured by this route towards the end of the Second World War and they are still prepared by this route today. The process uses a mixture of acetic anhydride, ammonium nitrate and nitric acid. The nitrolysis of hexamine is one of the most widely studied reactions in the history of explosives. Many other cyclic and linear polynitramines have been isolated from these reactions and this rich chemistry is discussed in more detail in Section 5.15. 

The nitrolysis of hexamine with 40 equivalents of a 25 % solution of dinitrogen pentoxide in absolute nitric acid in carbon tetrachloride at —20 °C is reported to give a 57 % yield of RDX. The product is free from HMX as determined by NMR (>95 % pure)." ... 

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). ... 

Other nitramine products from the nitrolysis of hexamine... 

Treatment of DPT (239) with dinitrogen pentoxide in pure nitric acid leads to the isolation of the nitrate ester (249), an unstable explosive which is highly sensitive to impact and readily undergoes hydrolysis. A low nitration temperature favours the formation of (249) and its presence during the nitrolysis of hexamine is clearly undesirable. The nitrolysis of DPT (239) with one equivalent of pure nitric acid in an excess of acetic anhydride yields 1-acetomethyl-3,5,7-trinitro-l,3,5,7-tetraazacyclooctane (251), a useful starting material for the synthesis of other explosives. ... 

The nitrolysis of hexamine ean be used to obtain the linear nitramines (247), (259) and (260) depending on the conditions and reagents used. Thus, the nitrolysis of hexamine with a mixture... 

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 nitrolysis of hexamine is a direct route to the military high explosives 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and l,3,5,7-tetranitro-13,5,7-tetraazacyclooctane (HMX). " The direct nitrolysis of hexamine with dinitrogen pentoxide in absolute ifitric acid provides RDX in 57 % yield. RDX prepared by this process is exceptionally pure, but other reagents, like ammonium nitrate-nitric acid-acetic anhydride, give much higher yields, partly because they use ammonium nitrate to supplement for ammonium nitrogen deficiency in the reaction. 

The synthesis of HMX from the nitrolysis of hexamine with conventional reagents is far more problematic. However, HMX (25) is synthesized in high yield from the nitrolysis of l,3,5,7-tetraacetyl-l,3,5,7-tetraazacyclooctane (TAT) (23) and l,5-diacetyl-3,7-dinitro-1,3,5,7-tetraazacyclooctane (DADN) (24) with dinitrogen pentoxide in nitric acid. DADN (24) is readily synthesized from the acetolysis of hexamine followed by mild nitration with mixed acid. The synthesis of HMX (25) via the nitrolysis of DADN (24) is now a pilot plant... 

The nitrolysis of hexamine can be controlled by changes in conditions to follow either of two paths. Strongly acidic conditions lead to triazine and linear triamine products. With nitric acid and acetic anhydride the dinitrotetraazabicyclo[3.3.1]nonane (328) is produced, and opening of the methylene bridge then gives the tetrazocane (329) and finally, in the presence of ammonium nitrate, HMX or the tetramine (330) (51JA2773). 

If the nitrolysis of hexamine dinitrate (IV) is conducted at a very low temperature, e.g. -40°C, then, as reported by Hirst et ah [41] it is not cyclonite which is formed, but a dinitrate of a dinitro derivative (la) (3,5-dinitro-3,5-diazapiperidinium nitrate) ... 

Vroom and Winkler believed substance (la) to be an intermediate compound in the preparation of cyclonite by the nitrolysis of hexamine. This view was shown to be incorrect by Wright, Berman and Meen [46] who proved that the substance... 

According to Karpukhin and Chetyrkin [49] the nitrolysis of hexamine may proceed with the formation of trihydroxymethylamine nitric ester (XVI) ... 

Kirsch and Winkler [52] studied the influence of acetic acid on the nitrolysis of hexamine to cyclonite. 

On the other hand, Wright et al. [47] found that the nitrolysis of hexamine with nitric acid in the presence of acetic anhydride but in the absence of ammonium nitrate involves a decrease of the yield of cyclic products. The amount of the chain compound (XXIII) formed is then increased. 

Tracer Studies on the Nitrolysis of Hexamine to RDX and HMX. The formation of RDX and/or HMX molecules from the nitration or nitrolysis of Hexamethylenetetramine (Hexamine) is a complex process and has been postulated to tate place via two separate paths. One involves the selective cleavage of the Hexamine molecule to the appropriate cyclic nitramine (RDX, HMX or both) depending on the specific... 

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