Nitromethane

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Nitromethane

General
Systematic name nitromethane
Other names nitrocarbol
Molecular formula CH₃NO₂
SMILES C[N+]([O-])=O
Molar mass 61.04 g/mol
Appearance colorless liquid
CAS number [75-52-5]
Properties
Density and phase 1.138 g/cm³, liquid
Solubility in water miscible
Melting point -29 °C (244.15 K)
Boiling point 100-103 °C (373-376 K)
Acidity (pK_a) 10.2
Viscosity ? cP at ? °C
Hazards
MSDS External MSDS
Main hazards Flammable, harmful
NFPA 704
Flash point 35 °C
R/S statement R: R5 R10 R22
S: S41
RTECS number PA9800000
Related compounds
Related nitro compounds nitroethane
Related compounds methyl nitrite
methyl nitrate
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
[Chemical infoboxInfobox disclaimer and references]

Nitromethane is an organic compound with the chemical formula CH₃NO₂. 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.

Contents

1 Reactions
2 Use as an engine fuel
3 Explosive properties
4 See also
5 References
6 External links

Reactions

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 reaction

Use as an engine fuel

Nitromethane 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:

4CH₃NO₂ + 3O₂ → 4CO₂ + 6H₂O + 2N₂

14.6 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:

4CH₃NO₂ → 4CO + 4H₂O + 2H₂ + 2N₂

Nitromethane 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.

Explosive properties

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.

References

K. Owen and T. Coley, "Automotive Fuels Reference Book - 2nd edition", Chapter 13 "Racing Fuels", ISBN 1-56091-589-7 (1995)

External links

For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].

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