Nitromethane
Nitromethane, sometimes shortened to just Nitro, is an organic compound with the chemical formula. It is the simplest organic nitro compound. It is a polar liquid commonly used as a solvent in a variety of industrial applications such as in extractions, as a reaction medium, and as a cleaning solvent. As an intermediate in organic synthesis, it is used widely in the manufacture of pharmaceuticals, pesticides, explosives, fibers, and coatings. Nitromethane is used as a fuel additive in various motorsports and hobbies, e.g. Top Fuel drag racing and miniature internal combustion engines in radio control, control line and free flight model aircraft.
Preparation
Nitromethane is produced industrially by combining propane and nitric acid in the gas phase at 350–450 °C. This exothermic reaction produces the four industrially significant nitroalkanes: nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane. The reaction involves free radicals, including the alkoxyl radicals of the type CH3CH2CH2O, which arise via homolysis of the corresponding nitrite ester. These alkoxy radicals are susceptible to C—C fragmentation reactions, which explains the formation of a mixture of products.Laboratory methods
It can be prepared in other methods that are of instructional value. The reaction of sodium chloroacetate with sodium nitrite in aqueous solution produces this compound:Uses
The principal use of nitromethane is as a stabilizer for chlorinated solvents, which are used in dry cleaning, semiconductor processing, and degreasing. It is also used most effectively as a solvent or dissolving agent for acrylate monomers, such as cyanoacrylates. It is also used as a fuel in some forms of racing.It can be used as an explosive, when gelled with several percent of gelling agent. This type of mixture is called PLX. Other mixtures include ANNM and ANNMAl – explosive mixtures of ammonium nitrate, nitromethane and aluminium powder.
As an organic solvent, it is considered to be highly polar but is aprotic and possesses very low Lewis basicity. Thus, it is a rare example of a polar solvent that is also weakly coordinating. This makes it useful for dissolving positively charged, strongly electrophilic species. However, its relatively high acidity and explosive properties limit its applications.
Reactions
Acid-base properties
Nitromethane is a relatively acidic carbon acid. It has a pKa of 17.2 in DMSO solution. This value indicates an aqueous pKa of about 11. The reason of that being so acidic is due to the resonance structure below:It is slow to deprotonate. Protonation of the conjugate base O2NCH2−, which is nearly isosteric with nitrate, occurs initially at oxygen.
Organic reactions
In organic synthesis nitromethane is employed as a one carbon building block. Its acidity allows it to undergo deprotonation, enabling condensation reactions analogous to those of carbonyl compounds. Thus, under base catalysis, nitromethane adds to aldehydes in 1,2-addition in the nitroaldol reaction. Some important derivatives include the pesticides chloropicrin, beta-nitrostyrene, and trisnitromethane,. Reduction of the latter gives trisaminomethane, 3CNH2, better known as tris, a widely used buffer. In more specialized organic synthesis, nitromethane serves as a Michael donor, adding to α,β-unsaturated carbonyl compounds via 1,4-addition in the Michael reaction.As an engine fuel
Nitromethane is used as a fuel in motor racing, particularly drag racing, as well as for radio-controlled models. In this context, nitromethane is commonly referred to as "nitro", and is the principal ingredient for fuel used in the "Top Fuel" category of drag racing.The oxygen content of nitromethane enables it to burn with much less atmospheric oxygen than conventional fuels. During nitromethane combustion nitric oxide is also one of the major product along with CO and HO. Recent study suggest the correct stoichiometric equation for nitromethane is described by the following equation:
The amount of air required to burn of gasoline is, but only of air is required for 1 kg of nitromethane. Since an engine's cylinder can only contain a limited amount of air on each stroke, 8.6 times as much nitromethane as gasoline can be burned in one stroke. Nitromethane, however, has a lower specific energy: gasoline provides about 42–44 MJ/kg, whereas nitromethane provides only 11.3 MJ/kg. This analysis indicates that nitromethane generates about 2.3 times the power of gasoline when combined with a given amount of oxygen.
Nitromethane can also be used as a monopropellant, i.e., a fuel that burns without added oxygen. The following equation describes this process:
Nitromethane has a laminar combustion velocity of approximately 0.5 m/s, somewhat higher than gasoline, thus making it suitable for high-speed engines. It also has a somewhat higher flame temperature of about. The high heat of vaporization of 0.56 MJ/kg together with the high fuel flow provides significant cooling of the incoming charge, resulting in reasonably low temperatures
Nitromethane is usually used with rich air–fuel mixtures because it provides power even in the absence of atmospheric oxygen. When rich air–fuel mixtures are used, hydrogen and carbon monoxide are two of the combustion products. These gases often ignite, sometimes spectacularly, as the normally very rich mixtures of the still burning fuel exits the exhaust ports. Very rich mixtures are necessary to reduce the temperature of combustion chamber hot parts in order to control pre-ignition and subsequent detonation. Operational details depend on the particular mixture and engine characteristics.
A small amount of hydrazine blended in nitromethane can increase the power output even further. With nitromethane, hydrazine forms an explosive salt that is again a monopropellant. This unstable mixture poses a severe safety hazard and the Academy of Model Aeronautics does not permit its use in competitions.
In model aircraft and car glow fuel, the primary ingredient is generally methanol with some nitromethane. Even moderate amounts of nitromethane tend to increase the power created by the engine, making the engine easier to tune.
Explosive properties
Nitromethane was not known to be a high explosive until a railroad tanker car loaded with it exploded on. After much testing, it was realized that nitromethane was a more energetic high explosive than TNT, although TNT has a higher velocity of detonation and brisance. Both of these explosives are oxygen-poor, and some benefits are gained from mixing with an oxidizer, such as ammonium nitrate. Pure nitromethane is an insensitive explosive with a VoD of approximately, but even so inhibitors may be used to reduce the hazards. The tank car explosion was speculated to be due to adiabatic compression, a hazard common to all liquid explosives. This is when small entrained air bubbles compress and superheat with rapid rises in pressure. It was thought that an operator rapidly snapped shut a valve creating a "hammer-lock" pressure surge.Nitromethane can also be mixed with ammonium nitrate, which is used as an oxidizer, to form an explosive mixture known as ANNM. One graphic example of this was the use of nitromethane and ammonium nitrate in the Oklahoma City bombing.
Nitromethane is used as a model explosive, along with TNT. It has several advantages as a model explosive over TNT, namely its uniform density and lack of solid post-detonation species that complicate the determination of equation of state and further calculations.