Nitromethane

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 highly potent racing fuel.

Synthetic chemistry applications

Nitromethane is a popular solvent in organic and electroanalytical chemistry.^[1]

In organic synthesis nitromethane is employed as a one carbon building block. Its weak acidicity 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. Nitromethane can serve as a Michael donor, adding to α,β-unsaturated carbonyl compounds via 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.^[2] In this context, it is commonly referred to as "nitro" or just "fuel".

The oxygen content of nitromethane enables it to burn with much less atmospheric oxygen in comparison to hydrocarbons such as gasoline:

4CH₃NO₂ + 3O₂ → 4CO₂ + 6H₂O + 2N₂

14.6 kg of air are required to burn one kg of gasoline, but only 1.7 kg of air for 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 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. 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, somewhat higher than gasoline, thus making nitromethane suitable for high speed engines. It also has a somewhat higher flame temperature of about 2400 °C. The high heat of vaporisation 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. In a Top Fuel drag racing engine this alone will provide the cooling of the engine, as the cylinder heads are machined from solid pieces of aluminum billet with no water jackets.

Nitromethane is usually used with rich air/fuel mixtures. This is partly because nitromethane can provide power even in the absence of atmospheric oxygen, and also because nitromethane tends to produce severe knock and pre-ignition. Rich mixtures 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 unstable and it poses a severe safety hazard.

In model aircraft and car glow fuel, the primary ingredient is generally methanol with some nitromethane (0% to 65%, but rarely over 30% since nitromethane is expensive compared to methanol) and 10–20% lubricants (usually castor oil or a synthetic oil). Even moderate amounts of nitromethane tends to increase the power created by the engine (as the limiting factor is often the air intake) and make the engine easier to tune (adjust for the proper air/fuel ratio) properly. During combustion, this fuel produces a characteristic blue smoke.

Nitromethane has also been used as a rocket fuel.

Explosive properties

Nitromethane was not known to be a high explosive until the 1950s when a railroad tanker car loaded with it exploded. 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 (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 an 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.

Explosives

There is a group of explosives in which Nitromethane is used. They are in a subgroup called Nitroamines and is also the main fuel in many liquid explosives.

PLX is the most common liquid explosive which uses Nitromethane as the fuel in the explosive.

It can also be added to Ammonium Nitrate which is used as an Oxidizer to form Nitroazine, which is similar to ANFO.

References

^ Coetzee, J. F. and Chang, T. H. (1986). "Recommended Methods for the Purification of Solvents and Tests for Impurities: Nitromethane" (PDF). Pure Appl. Chem. 58: 1541–1545.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^ K. Owen and T. Coley, "Automotive Fuels Reference Book - 2nd edition", Chapter 13 "Racing Fuels", ISBN 1-56091-589-7 (1995)

External links

WebBook page for CH3NO2

Template:ChemicalSources

[1] Coetzee, J. F. and Chang, T. H. (1986). "Recommended Methods for the Purification of Solvents and Tests for Impurities: Nitromethane" (PDF). Pure Appl. Chem. 58: 1541–1545.{{cite journal}}: CS1 maint: multiple names: authors list (link)

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

[1]

[2]