Nitromethane, sometimes shortened to simply "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 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.
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Preparation
Nitromethane is produced industrially by combining propane and nitric acid in the gas phase at . 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 , 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 also be prepared by other methods that are of instructional value. The reaction of sodium chloroacetate with sodium nitrite in aqueous solution produces this compound, along with sodium chloride and sodium bicarbonate:
Uses
The dominant use of the nitromethane is as a precursor reagent. A major derivative is chloropicrin (), a widely used pesticide. It condenses with formaldehyde (Henry reaction) to eventually give tris(hydroxymethyl)aminomethane ("tris"), a widely used buffer and ingredient in .[
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Solvent and stabilizer
The major application is as a stabilizer in chlorinated solvents. As an organic solvent, nitromethane has an unusual combination of properties: highly polar (εr = 36 at 20 °C and μ = 3.5 Debye) but aprotic and weakly basic. This combination makes it useful for dissolving positively charged, strongly electrophilic species. It is a solvent for acrylate , such as (more commonly known as "super-glues").[
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Fuel
Although a minor application in terms of volume,[ nitromethane also is used as a fuel or fuel additive for sports and hobby. For some applications, it is mixed with methanol in racing cars, boats, and model engines.
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Nitromethane is used as a fuel in motor racing, particularly drag racing, as well as for radio-controlled model power boats, cars, airplane and helicopters. In this context, nitromethane is commonly referred to as "nitro fuel" or simply "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.
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 megajoule/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 propellant that decomposes to release energy without added oxygen. It was first tested as rocket monopropellant in 1930s by from the Italian Rocket Society.
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 (about twice that of methanol), resulting in reasonably low temperatures.
Nitromethane is usually used with Air-fuel ratio 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. The National Hot Rod Association and Academy of Model Aeronautics do not permit its use in competitions.
In model aircraft and model car glow fuel, the primary ingredient is generally methanol with some nitromethane (0% to 65%, but rarely over 30%, and 10–20% lubricants (usually castor oil and/or synthetic oil)). Even moderate amounts of nitromethane tend to increase the power created by the engine (as the limiting factor is often the air intake), making the engine easier to tune (adjust for the proper air/fuel ratio).
Former uses
It formerly was used in the explosives industry as a component in a binary explosive formulation with ammonium nitrate and in shaped charges, and it was used as a chemical stabilizer to prevent decomposition of various halogenated hydrocarbons.
Other
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.
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 acid deprotonates only slowly. Protonation of the conjugate base , which is nearly isosteric with nitrate, occurs initially at oxygen.[
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Organic reactions
In organic synthesis nitromethane is employed as a one carbon synthon.
Purification
Nitromethane is a popular solvent in organic and electroanalytical chemistry. It can be purified by cooling below its freezing point, washing the solid with cold diethyl ether, followed by distillation.
Safety
Nitromethane has a modest acute toxicity. LD50 (oral, rats) is 1210±322 mg/kg.[
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Nitromethane is "reasonably anticipated to be a human carcinogen" according to a U.S. government report.
Explosive properties
Nitromethane was not known to be a high explosive until a railroad tank car loaded with it exploded on .
If mixed with ammonium nitrate, which is used as an oxidizer, it forms an explosive mixture known as ANNM.
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.
Nitromethane reacts with solutions of sodium hydroxide or methoxide in alcohol to produce an insoluble salt of nitromethane. This substance is a sensitive explosive which reverts to nitromethane under acidic conditions and decomposes in water to form another explosive compound, sodium methazonate, which has a reddish-brown color:
Nitromethane's reaction with solid sodium hydroxide is hypergolic.
See also
Cited sources
Further reading
External links
