Purification of Nitromethane

Nitromethane (CAS NO. 75-52-5) is generally manufactured by gas-phase nitration of methane. The usual impurities include aldehydes, nitroethane, water and small amounts of alcohols. Most of these can be removed by drying with CaCl2 or by distn to remove the watednitromethane azeotrope, followed by drying with CaSO4. Phosphorus pentoxide is not suitable as a drying agent. The purified material should be stored by dark bottles, away from strong light, in a cool place. Purifications of Nitromethane using extraction are commonly used. For example, Van Looy and Hammett mixed about 150mL of conc H2SO4 with 1L of nitromethane and allowed it to stand for 1 or 2 days. The solvent was washed with water, aqueous Na2CO3, and again with water, then dried for several days with MgSO4, filtered again with CaSO4. It was fractionally distd before use. Smith, Fainberg and Winstein washed successively with aqueous NaHCO3, aqueous NaHSO3, water, 5% H2SO4, water and dilute NaHCO3. The solvent was dried with CaSO4, then percolated through a column of Linde type 4A molecular sieves, followed by distn from some of this material (in powdered form). Buffagni and Dunn refluxed for 24h with activated charcoal while bubbling a stream of nitrogen through the liquid. The suspension was filtered, dried (Na2SO4) and distd, then passed through an alumina column and redistd. It has also been refluxed over CaH2, distd and kept under argon over 4A molecular sieves.
Nitromethane can also be purified by zone melting or by distn under vacuum at 0 °C, subjecting the middle fraction to several freeze-pump-thaw cycles. An impure sample containing higher nitroalkanes and traces of cyanoalkanes was purified (on the basis of its NMR spectrum) by crystn from diethyl ether at -60 °C (cooling in Dry-ice).
Fractional crystn was more effective than fractional distn from Drierite in purifying nitromethane for conductivity measurements. Specific conductivities around 5x10-9 ohm-1cm-1 were obtained