Nitramines mechanism

Polymer-based rocket propellants are generally referred to as composite propellants, and often identified by the elastomer used, eg, urethane propellants or carboxy- (CTPB) or hydroxy- (HTPB) terrninated polybutadiene propellants. The cross-linked polymers act as a viscoelastic matrix to provide mechanical strength, and as a fuel to react with the oxidizers present. Ammonium perchlorate and ammonium nitrate are the most common oxidizers used nitramines such as HMX or RDX may be added to react with the fuels and increase the impulse produced. Many other substances may be added including metallic fuels, plasticizers, stabilizers, catalysts, ballistic modifiers, and bonding agents. Typical components are Hsted in Table 1. 

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

The term nitrolysis is usually applied to a nitrating mechanism in which both the rupture of the C-N bond and the formation of a nitramine occur simultaneously with the formation of an alcohol which subsequently undergoes esterification (1) ... 

Delpeyroux and coworkers16 have developed a set of molecular mechanics parameters for nitramines (R—N—NO2) for the EMO program and used it in conjunction with MM2-85 parameters to calculate the structures of 1,4-dinitro-glycoluryl (8), 1,3-dinitro-4,6-diacetylglycoluryl (9) and 2,5,7,9-tetranitro-tetraazabicyclo(4.3.0)nonanone (10). The complete parameter list for the nitramine functionality is provided in Reference 16 but the parameterization procedure is not discussed. 

Confirmation of the radical mechanism was provided by Ridd and Sandall35, who detected radical cations by nitrogen-15 NMR of the reaction mixture for the nitramine rearrangement of 2.6-dibromo-jV-nitroaniline and of A -methyl/V-nitroaniline labelled with nitrogen-15 in the nitro group. The results did not allow the authors to indicate whether the products were formed within the solvent cage or from separated radicals. 

If nitration under acidic conditions could only be used for the nitration of the weakest of amine bases its use for the synthesis of secondary nitramines would be severely limited. An important discovery by Wright and co-workers " found that the nitrations of the more basic amines are strongly catalyzed by chloride ion. This is explained by the fact that chloride ion, in the form of anhydrous zinc chloride, the hydrochloride salt of the amine, or dissolved gaseous hydrogen chloride, is a source of electropositive chlorine under the oxidizing conditions of nitration and this can react with the free amine to form an intermediate chloramine. The corresponding chloramines are readily nitrated with the loss of electropositive chlorine and the formation of the secondary nitramine in a catalytic cycle (Equations 5.2, 5.3 and 5.4). The mechanism of this reaction is proposed to involve chlorine acetate as the source of electropositive chlorine but other species may play a role. The success of the reaction appears to be due to the chloramines being weaker bases than the parent amines. 

Similar to nitramine composite propellants and TAGN composite propellants, AN composite propellants produce halogen-free combustion products and thus represent smokeless propellants. However, their ballistic properties are inferior to those of other composite propellants the burning rate is too low and the pressure exponent is too high to permit fabrication of rocket propellant grains. In addition, the mechanical properties of AN composite propellants vary with temperature due to the phase transitions of AN particles. 

Kubota, N Combustion Mechanisms of Nitramine Composite Propellants, 18th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA (1981), pp. 187-194. 

The concentration of alkali required depends on the properties of the radicals R and R1. The more electrophilic the radicals and the more dcidic the nitramine, the easier the course of reaction. Secondary nitramines are decomposed by an aqueous solution of sodium hydroxide. The reaction conditions, including the concentrations of NaOH solutions differ according to the substance. Van Erp and Franchimont [21] found that the reaction proceeded by the following mechanism ... 

Different research groups have different approaches to elucidating decomposition pathways. Thus, a wide variety of activation parameters have been published (Table 1). This is a result of examining the decomposition on different timescales or temperatures. Often if the rate constants are calculated for a common temperature, they will be similar in magnitude [97], It is rare that the first paper published about a compound tells the entire story. For example, the papers on nitramine decomposition, cited herein, span thirty-five years. Each reveals a different piece of the story or supports previous postulates. Consider the mechanisms illustrated by Figs. 8-10. These are the results of several researchers approaching the problem from different perspectives yet, they make similar conclusions. As new ways of probing reaction chemistry become available, they should be implemented. At the same time the reaction chemistry should be examined by the more conventional techniques, and an effort should be made to correlate the results. 

D. Naud, R. Brower, Pressure Effects on The Thermal Decomposition of Nitramines, Nitrosamines, and Nitrate Esters, J. Org. Chem., 57 (1992) 3303-3308. J. Wang, K.R. Brower, D.L. Naud, Evidence of an Elimination Mechanism in Thermal Decomposition of Hexahydro-l,3,5-Trinitro-l,3,5-Triazine and Related Compounds Under High Pressure, J. Org. Chem., 62 (1997) 9055-9060. 

J.C. Oxley, M.A. Hiskey, D. Naud, R. Szekeres, "Thermal Decomposition of Nitramines Dimethylnitramine, Diisopropyl-nitramine, and N-Nitropiperidine," J. Phys. Chem., 96 (1992) 2505-2509. J.C. Oxley, A. Kooh, R. Szeckeres, W. Zheng, "Mechanisms of Nitramines Thermolysis," J. Phys. Chem., 98 (1994) 7004-7008. 

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