Nitromethane
Nitromethane is an
organic compound with the chemical formula CH
3NO
2. It is the simplest organic
nitro compound. It is a slightly viscous, highly 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. It also finds use as a racing fuel.
The weak acidicity of nitromethane allows it to undergo reactions analogous to those of carbonyl compounds:
*Nitromethane can add to
aldehydes in 1,2-addition in the
nitroaldol reaction *Nitromethane can act as a Michael donor, adding to α,β-unsaturated carbonyl compounds by 1,4-addition in the
Michael reactionNitromethane is used as a fuel in racing, particularly
drag racing, to provide more power. In this context, it is commonly referred to as "nitro" or "fuel".
The
oxygen in the molecular structure of nitromethane enables it to burn with much less atmospheric oxygen in comparison to hydrocarbons such as gasoline. Part of the oxygen needed for combustion is carried by nitromethane itself. At stoichiometric the combustion will be as follows::4CH
3NO
2 + 3O
2 â†' 4CO
2 + 6H
2O + 2N
214.6 kg of air are needed to burn one kg of gasoline, but only 1.7 kg of air are needed to burn one kg of nitromethane. Since an engine's cylinder can only contain a limited amount of air on each stroke, 8.7 times more nitromethane than gasoline can be burned in one stroke. However, nitromethane has a lower energy density. Gasoline provides about 42-44 MJ/kg, nitromethane provides only 11.3 MJ/kg. This would indicate that it with nitromethane is possible to generate about 2.3 times the power of gasoline. This is however not the complete story, nitromethane can also be used as a monopropellant. Without additional oxygen nitromethane will combust according to::4CH
3NO
2 â†' 4CO + 4H
2O + 2H
2 + 2N
2Nitromethane has a laminar combustion velocity of approx. 0.5 m/s, that is somewhat higher than gasoline and makes the fuel suitable to engines running at high speed. Somewhat higher is also the flame temperature at about 2400°C. The high heat of vaporisation of 0.56 MJ/kg together with the high fuel flow does however provide a high cooling of the incoming charge (about twice that of methanol), resulting in reasonably low temperatures. In a Top Fuel dragracing engine this alone will provide the cooling of the engine.
Nitromethane is usually used with rich air/fuel mixtures. This is partly because nitromethane can provide power even in the absence of atmospheric oxygen, as described above, but it's also because nitromethane tends to produce severe knock and pre-ignition. Rich mixtures do however cause ignition problems and a lower combustion speed.
When rich air/fuel mixtures are used, hydrogen and carbon monoxide will be two of the combustion products, when these and any unburned fuel comes into contact with the oxygen in the atmosphere at the end of the exhaust pipes they often ignite. The result is spectacular flames from the exhaust system.
A small amount of hydrazine blended in nitromethane can increase the power output even further. With nitromethane, hydrazine forms an explosive salt that can combust by using only the oxygen in the nitromethane. This mixture is however unstable, so it poses a severe safety hazard.
In addition, model
aircraft and
car fuel contains from 0% to 65% nitromethane. It has also been used as a model rocket fuel. It is normally mixed with methanol.
Nitromethane was not known to be an
explosive until the
1950s, when a whole railroad tanker car of it exploded, leaving a huge crater. Much testing later it was realized that nitromethane was a more energetic high explosive than
TNT. However TNT has a higher velocity of detonation and
brisance (shattering power against hard targets). Both of these explosives are oxygen poor and some benefits are gained from mixing with an
oxidizer, such as
ammonium nitrate. One graphic example of this was the use of nitromethane and ammonium nitrate on the
Alfred P. Murrah Federal Building at
Oklahoma City. Pure nitromethane is a very insensitive explosive, 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 be
sensitized by adding a base to raise the
pH.
Adiabatic flame temperature for a thermodynamic calculation of the flame temperature of nitromethane.
K. Owen and T. Coley, "Automotive Fuels Reference Book - 2nd edition", Chapter 13 "Racing Fuels", ISBN 1-56091-589-7 (1995)
