Nitromethane is produced commercially through vapor-phase nitration of propane, a process that yields a mixture of nitroalkanes that must then be separated and purified. It can also be synthesized on a smaller scale through several well-documented chemical reactions involving sodium nitrite. The compound is widely used as a high-performance racing fuel, an industrial solvent, and a feedstock for pharmaceutical and agricultural chemical manufacturing.
Industrial Production: Vapor-Phase Nitration
Nearly all commercial nitromethane comes from the nitration of propane gas with nitric acid. In this process, propane is vaporized and heated to between 370°C and 450°C, then fed into a reactor at 125 to 175 psig. Inside the reactor, 50 to 60 percent nitric acid is sprayed directly into the hot propane gas. The reaction requires no catalyst. At these extreme temperatures, the nitric acid breaks apart propane molecules and attaches nitrogen-containing groups to the resulting fragments.
This process doesn’t produce pure nitromethane. It yields a mixture of four nitroalkanes: nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane. The mixture then goes through fractional distillation to separate these compounds by their different boiling points. Even after distillation, commercial-grade nitromethane typically reaches only 94 to 97% purity, with the remaining 3 to 6% consisting of those higher nitroalkane byproducts.
Purifying Commercial-Grade Nitromethane
Getting from 96% to 99%+ purity requires an additional step. One patented method adds water and a simple hydrocarbon like hexane to the crude nitromethane, then distills the mixture through a tall column. The nitromethane, water, and hexane come over as a three-layer distillate, while the heavier nitroalkane impurities stay behind in the boiler as residue. The bottom layer of the distillate contains the nitromethane with only small amounts of water and hexane, which can be removed by heating just below nitromethane’s boiling point of 101.5°C. This technique pushes purity above 99%, with one example achieving 99.8% nitromethane with only a 0.2% trace of nitroethane remaining.
Laboratory-Scale Synthesis Methods
Two main chemical routes produce nitromethane outside of industrial propane nitration. Both rely on sodium nitrite as the nitrogen source but use different starting materials.
The Kolbe Synthesis
The older method, published in Organic Syntheses, reacts sodium chloroacetate with sodium nitrite in a 1:1 molar ratio. Chloroacetic acid is first dissolved in water and made slightly alkaline with sodium hydroxide, converting it to its sodium salt. Sodium nitrite is then added. The solution is heated slowly until carbon dioxide bubbles begin to appear, which happens around 80°C. At that point, external heat is removed because the reaction becomes self-sustaining. An intermediate compound forms and then rapidly decomposes once the temperature climbs to about 85°C, releasing enough heat on its own to push the temperature nearly to 100°C. Nitromethane begins distilling out of the mixture at around 90°C and is collected as it comes over.
The Dimethyl Sulfate Route
A more recent approach reacts dimethyl sulfate with sodium nitrite. Two molecules of sodium nitrite react with one molecule of dimethyl sulfate to produce two molecules of nitromethane and sodium sulfate as a byproduct. The basic version of this reaction achieves 50 to 70% yield with about 90% purity. An improved version, described in a Chinese patent, adds a solvent to better control the reaction temperature. This suppresses the breakdown of dimethyl sulfate and reduces the formation of an unwanted byproduct called methyl nitrite (an isomer where the nitrogen-oxygen group attaches through the oxygen atom rather than the nitrogen). With these refinements, yields reach 81 to 85% with purity above 99.5%.
Key Physical Properties
Nitromethane is a clear, oily liquid that is noticeably denser than water at 1.137 grams per cubic centimeter. It boils at 101.1°C, which is almost identical to water’s boiling point, making separation from aqueous mixtures slightly tricky. Its flash point is 35°C (95°F), meaning it can produce ignitable vapors at temperatures not far above room temperature on a warm day. This relatively low flash point is part of what makes it effective as a racing fuel, but it also demands careful handling and storage away from heat sources.
Safety Considerations
Nitromethane is moderately toxic if swallowed. Oral toxicity testing shows lethal doses in the range of 940 to 950 mg per kilogram of body weight in rats and mice. Dogs are considerably more sensitive, with toxic effects appearing at much lower doses. The compound can be absorbed through the skin and its vapors irritate the respiratory tract, so proper ventilation and protective gloves are essential during handling. It is also sensitive to detonation under confinement or when mixed with certain other chemicals, particularly strong bases and amines.
Regulatory Status in the United States
The Department of Homeland Security lists nitromethane as a Chemical of Interest under the Chemical Facility Anti-Terrorism Standards, alongside compounds like ammonium nitrate and potassium chlorate. Any facility that possesses 400 pounds or more of nitromethane must complete and submit a security screening to DHS within 60 calendar days. This threshold applies specifically to the risk of theft or diversion for explosive purposes. For individual consumers, nitromethane remains legal to purchase in smaller quantities. It is commonly sold as racing fuel (often blended with methanol) and as a component of certain hobby fuels for radio-controlled model engines.