Nitromethane decomposition

Nitromethane is the simplest and perhaps the most studied prototypical energetic material. It has been subjected to most of the applicable experimental techniques and its chemical and physical properties have been investigated for the solid, liquid, and gas phases. Significant progress in theoretically modeling of nitromethane in the various phases is being made. It appears that the initial steps in the chemical decomposition mechanisms are the same in the liquid and gas phases [83], 

The initial decomposition chemistry involves unimolecular reactions. This was the conclusion of the first gas-phase kinetics study [84] and has been repeatedly confirmed by subsequent bulb and shock-tube experiments [85, 86]. That first study used shock heating to induce thermal decomposition [84], The data were interpreted in terms of simple C-N bond fission to give CH2 and N02. A more extensive and definitive shock-tube study was reported by Zhang and Bauer in 1997 [85]. Zhang and Bauer presented a detailed kinetics model based on 99 chemical reactions that reproduced their own data and that of other shock-tube experiments [84, 86]. An interesting conclusion is that about 40% of the nitromethane is lost in secondary reactions. 

Although the mechanism is far from completely determined (which is not surprising given its complexity), there are sufficient details to invite attempts to simulate the chemistry of this prototypical energetic material. Its size permits accurate ab initio calculations to determine the potential energy surface of the reactions in the proposed mechanism many of the secondary reactions are of interest in other combustion systems and have been studied by various experimental and theoretical approaches. Even a cursory discussion of the theoretical studies of secondary reactions involved in the decomposition of nitromethane is beyond the scope of this chapter we limit the discussion to the initial decomposition steps. 

The most explicit and definitive study of the decomposition of isolated nitromethane molecules is that by Wodtke, Hintsa, and Lee [87, 88]. They used infrared multiphoton excitation to dissociate nitromethane in a 

They determined the branching ratio of reaction I to reaction II to be 0.6 0.2. By modeling the reactions with RRKM theory they estimated the barrier to the nitro-nitrite rearrangement to be 55.5 1.5 kcal/mol, which is about 4 kcal/mol lower than that for C-N bond rupture. Previously, there had been ab initio predictions of the energy barrier to nitro-nitrite isomerization [89-91], but the values were significantly higher (by 20-30 kcal/mol) than that estimated by Wodtke et al. based on their data, as were the values published subsequently [92, 93]. 

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Tris(dimethyl sulfoxide)indium(III) chloride is a white crystalline nonhygro-scopic compound, soluble in alcohols, ethyl acetate, and nitromethane. Decomposition occurs at 130°. The infrared spectrum and the results of thermal stability studies have been reported.6 The presence of dmso can be verified from the infrared spectrum,6 which shows C—H vibrations, and =0 at 945, 960, and 995 cm. ... 

Mader then reprogrammed his computations, for an Eulerian code and considered the interactions of 4 cylindrical voids rather than a single void (Ref 16). He showed that shock interactions with four holes lead to much greater faster computed nitromethane decomposition than the shock interaction with a single hole for the same initial conditions... 

This proposed reaction scheme for nitromethane decomposition at elevated pressures suggests a strong intermolecular interaction of nitromethane in the condensed phase. If this is indeed the case, then it is important to investigate the effect of pressure on the vibrational bands in nitromethane and to analyze the observed shifts in... 

Nitromethane is another interesting case. Engelke et al. [39] have provided a variety of evidence that the early critical intermediate in nitromethane decomposition is the aci ion (CH2NO2 ). In pure nitromethane the driving force at high pressure is the electrostriction effect of the formation of the CH3N02H -CH2N02 ion pair. 

Thompson points out that there is no evidence that adducts give other than acetates on thermolysis. The exocyclic methylene intermediate (iv) postulated by Robinson could arise by proton abstraction from a Wheland intermediate analogous to (vll) above, rather than from the adduct (in). Similarly its decomposition does not necessarily require the intermediacy of the adduct (v). The fact that i -methyl-4-nitromethylnaphthalene is the product even when the nitrating medium is nitric acid and nitromethane would then require no separate explanation. 

The concentration of nitromethane, CH3NO2, can be determined from the kinetics of its decomposition in basic solution. In the presence of excess base the reaction is pseudo-first-order in nitromethane. For a standard solution of 0.0100 M nitromethane, the concentration of nitromethane after 2.00 s was found to be 4.24 X 10 M. When a sample containing an unknown amount of nitromethane was analyzed, the concentration remaining after 2.00 s was found to be 5.35 X 10 M. What is the initial concentration of nitromethane in the sample ... 

Methylene chloride is one of the more stable of the chlorinated hydrocarbon solvents. Its initial thermal degradation temperature is 120°C in dry air (1). This temperature decreases as the moisture content increases. The reaction produces mainly HCl with trace amounts of phosgene. Decomposition under these conditions can be inhibited by the addition of small quantities (0.0001—1.0%) of phenoHc compounds, eg, phenol, hydroquinone, -cresol, resorcinol, thymol, and 1-naphthol (2). Stabilization may also be effected by the addition of small amounts of amines (3) or a mixture of nitromethane and 1,4-dioxane. The latter diminishes attack on aluminum and inhibits kon-catalyzed reactions of methylene chloride (4). The addition of small amounts of epoxides can also inhibit aluminum reactions catalyzed by iron (5). On prolonged contact with water, methylene chloride hydrolyzes very slowly, forming HCl as the primary product. On prolonged heating with water in a sealed vessel at 140—170°C, methylene chloride yields formaldehyde and hydrochloric acid as shown by the following equation (6). 

The transformation of l-methylthio-l-(methylsulfonyl)alkanes (254) to methyl esters can be efficiently carried out by oxidation or by a-chlorination followed by methanolysis (equation 152)145. The lithium or the sodium salt of (phenylsulfonyl)nitromethane (256) is a very useful reagent for the preparation of higher homologues of nitromethanes by alkylation since the salts are air insensitive, non-hygroscopic, and easily handled without decomposition. The oxidation of the resulting secondary a-nitro sulfone (257) gives... 

Storage stability Extremely unstable in presence of trace metals or other impurities. Traces of iron chloride may cause explosive decomposition. Pure material is stable for only 1 or 2 months. It may be stabilized by nitromethane, chloropicrin, glycine, ethyl acetate, or ether - but only in glass vessels below 20°C. Apparently, it is most stable in aromatic solvents. 

The heat of decomposition (238.4 kJ/mol, 3.92 kJ/g) has been calculated to give an adiabatic product temperature of 2150°C accompanied by a 24-fold pressure increase in a closed vessel [9], Dining research into the Friedel-Crafts acylation reaction of aromatic compounds (components unspecified) in nitrobenzene as solvent, it was decided to use nitromethane in place of nitrobenzene because of the lower toxicity of the former. However, because of the lower boiling point of nitromethane (101°C, against 210°C for nitrobenzene), the reactions were run in an autoclave so that the same maximum reaction temperature of 155°C could be used, but at a maximum pressure of 10 bar. The reaction mixture was heated to 150°C and maintained there for 10 minutes, when a rapidly accelerating increase in temperature was noticed, and at 160°C the lid of the autoclave was blown off as decomposition accelerated to explosion [10], Impurities present in the commercial solvent are listed, and a recommended purification procedure is described [11]. The thermal decomposition of nitromethane under supercritical conditions has been studied [12], The effects of very high pressure and of temperature on the physical properties, chemical reactivity and thermal decomposition of nitromethane have been studied, and a mechanism for the bimolecular decomposition (to ammonium formate and water) identified [13], Solid nitromethane apparently has different susceptibility to detonation according to the orientation of the crystal, a theoretical model is advanced [14], Nitromethane actually finds employment as an explosive [15],... 

Decomposition of nitromethane on the (111) face of nickel has been studied. 

An inhomogeneous mixture of the dry salt with a little water exploded violently after 30 min [1], This was probably owing to exothermic decarboxylation generating the ad-salt of nitromethane, which is explosively unstable. The decomposition of sodium nitroacetate proceeds exothermically above 80° C [2],... 

Urticants are relatively unstable and tend to decompose spontaneously unless stored at low temperatures. Below —4°F, they can be kept for extended periods. Solvents including 1, 2-dimethoxybenzene, ether, dioxane, nitromethane, and glycine act as stabilizers and may be added to help prevent decomposition of agents during storage. Agents can be stored in glass or enamel-lined containers. Urticants rapidly attack rubber and metals, especially iron. 

Nitromethane is the most easily accessible aliphatic nitro-com-pound Kolbe s method of preparation is much less satisfactory when applied to higher members of the series. The course of the reaction is clear, and the reasons for the decomposition which takes place are similar... 

In formamide, acetone, and nitromethane the bromide ion is the most mobile of the halides. The difference is slight in nitromethane, but pronounced in the other two solvents. Because mobilities reflect a variety of factors it is possible that opposing effects could result in an ion of intermediate size being more mobile than others in the series. Another possible factor could be the presence of impurities in formamide and acetone, formamide because of decomposition on standing even a short time, and acetone because of the difficulty in removing last traces of water. The presence of impurities could have a significant but unpredictable effect on mobilities. 

Contact with metal oxides increases the sensitivity of nitromethane, nitroethane and 1-nitropropane to heat (and of nitromethane to detonation). Twenty-four oxides were examined in a simple quantitative test, and a mechanism was proposed. Cobalt, nickel, chromium, lead and silver oxides were the most effective in lowering ignition temperatures [1]. At 39 bar initial pressure, the catalytic decomposition by chromium or iron oxides becomes explosive at above 245° C [2],... 

Blower, C. J., Smith, T. D. The gas-phase decomposition of nitromethane over metal ion-exchanged sodium Y zeolite and sodium X zeolite. Zeolites, 1993, Volume 13, Issue 5, 394-398. 

Dinitrogen pentoxide is readily soluble in absolute nitric acid and chlorinated solvents. The polarity of the solvent has a significant effect on the rate of decomposition in solution. The rate is fastest in nonpolar solvents like chloroform and slower in polar solvents like nitromethane. ° The decomposition rate for solutions of dinitrogen pentoxide in nitric acid is very slow and these solutions are moderately stable at subambient temperatures. ... 

Recently Lee et al (Ref 3) re-examined the behavior of PETN under 10 to 50 kbars of external pressure. They also find a reduction in decomposition rate with increasing applied pressure. HMX behaves similarly to PETN. TNT whose explosion products contain a high proportion of solid carbon, as expected from LeChatelier s Principle, shows little pressure effect on its thermal decomposition. Nitro-methane, however, appears to decompose more rapidly under an external pressure of 50 kbars than 10 kbars. This effect is not completely understood but Lee et al suggest that high pressure may favor the formation of the thermally less stable aci form of Nitromethane ... 

The solution is heated slowly until the first appearance of bubbles of carbon dioxide, which occurs when the temperature has reached about 8o°. The flame is then removed and the reaction allowed to proceed by itself (Note 2). If no rise in temperature occurs, heat is very cautiously applied until the temperature rises to 85°, when the flame is again removed. At this temperature the exothermic decomposition of the sodium nitroacetate becomes so rapid that the temperature rises almost to ioo° without further application of external heat. If heat is applied after the temperature of the liquid reaches 85°, violent frothing will occur, with serious loss of nitromethane. If the... 

This preparation represents a convenient, albeit inefficient, procedure, as the 3h reflux time corresponds to the point of diminishing returns for acetyl formation versus decomposition. Other procedures using THF or acetonitrile in place of nitromethane have been reported, but the reaction times are substantially longer.12... 

The benzobisazole family of rigid-rod polymers is soluble in acidic solvents such as PPA, methanesulfonic acid, chlorosulfonic acid, 100% sulfuric acid and Lewis acid salts such as antimony trichloride and bismuth trichloride. More recently, PBZT has been reported [22] to form liquid crystalline solutions in nitromethane containing aluminum trichloride or gallium trichloride. Since the glass transition temperature of these materials is above their decomposition temperature, they must be processed from solution. 

Nitromethane (Vol. I, p. 579) may be used as a monergol propellant. However its negative oxygen balance may be reduced by the addition of liquid oxidants, e.g. of tetranitromethane to form a bipropellant. In practice, however, nitromethane decomposes too slowly, and it is difficult therefore to obtain hypergolic mixtures from it. To facilitate and accelerate the decomposition of mixtures with nitromethane it is necessary to add a catalyst, such as a salt of chromic acid. 

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