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 is also used as a racing fuel.

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Preparation

Nitromethane is produced industrially by treating propane with nitric acid 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.^[1]

Although inexpensively available, nitromethane 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:

ClCH₂COONa + NaNO₂ + H₂O → CH₃NO₂ + NaCl + NaHCO₃

Nitromethane is distilled from the reaction and then dried over a mild desiccant.^[2]

Uses

The principle use of nitromethane is as a stabilizer for chlorinated solvents, which are used in dry cleaning, semiconductor processing, and degreasing. It is also used as a solvent for acrylate monomers.^[1]

Derivatives

In organic synthesis nitromethane is employed as a one carbon building block. Its 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. Some important derivatives include the pesticides Chloropicrin, Cl₃CNO₂ and tris(hydroxymethyl)nitromethane, (HOCH₂)₃CNO₂. Reduction of the latter gives tris(hydroxymethyl)aminomethane, (CH₂OH)₃CNH₂, better known as “tris,” a widely used buffer.

In more specialized organic synthesis, nitromethane serve as a Michael donor, adding to α,β-unsaturated carbonyl compounds via 1,4-addition in the Michael reaction.

As an engine fuel

In a minor application, nitromethane is used as a fuel in racing, particularly drag racing, as well as for rockets and RC Models. In car racing, nitromethane is commonly referred to as "nitro," "top fuel," 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. Nitromethane, however, has a lower energy density: Gasoline provides about 42-44 MJ/kg whereas nitromethane provides only 11.3 MJ/kg.

Nitromethane can also be used as a monopropellant, i.e. a fuel that burns without added oxygen. The following equation describes this process:

4 CH₃NO₂ → 4 CO + 4 H₂O + 2 H₂ + 2 N₂

Nitromethane has a laminar combustion velocity of approx. 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 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.

Nitromethane is usually used with rich air/fuel mixtures because it provide power even in the absence of atmospheric oxygen Otherwise, rich mixtures cause ignition problems and a lower combustion speed.

When rich air/fuel mixtures are used, hydrogen and carbon monoxide are two of the combustion products. These gases often ignite, sometimes spectacularly, when they and any unburned fuel contacts atmospheric oxygen at the end of the exhaust pipes.

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.

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), making the engines easier to tune (adjust for the proper air/fuel ratio). During combustion, this fuel produces a characteristic blue smoke.

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 with a VoD of approximately 6200 m/s, 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.

PLX is the most common liquid explosive which uses nitromethane as the fuel and ethylene diamine as a sensitizer. Various other amine groups can be used, such as triethylene tetramine and ethanolamine.

It can also be added to ammonium nitrate which is used as an oxidizer to form an explosive composition commonly referred to as ANNM. This mixture is more powerful than ANFO. For proper oxygen balance, a mixture of 67% Ammonium Nitrate and 33% Nitromethane would be used. However, since this particular mixture will often produce a very "runny" product that is often difficult to manage, a mixture of 75% AN and 25% NM is often used instead.

Purification

Nitromethane is a popular solvent in organic and electroanalytical chemistry.^[3]

See also

• Adiabatic flame temperature for a thermodynamic calculation of the flame temperature of nitromethane.

References