Purification of Nitromethane by Crystallization There is no purified nitromethane an the market, and to obtain pure nitromethane for research purposes seems to be a problem. It has been said1 that conventional methods of purification were inadequate for the removal of impurities in
commercial grade nitromethane. The impurities are usually higher nitroalkanes plus trace cyanoalkanes and are distinguishable from nitromethane by nmr spectroscopy (Fig. 1).We have tried the reported methods of purifying nitromethane, such as the chemical treatmentsz and the szeotropic distillation^,^,^ but the results were unsatisfactory. Now we find that a treatment of crystallization from ether a t -60-78OC eliminates all the impurities which appear in the nmr spectrum of the commercial product (Fig. 2).
Fbve 1. Nm s m m of commercial MeN02, neat (Varian T-60A at 35%).
Figure 2. Nmr spectrum of purified MeNOs, neat (Varian T-BOA at 3S°C).
This is an excellent example of liquid purification by the crystallization method. The physical constants (m.p. 'C, h.p. 120). OC) of nitramethane (-28.5. . 101). and the oossihle imourities.. e.e.. . .. 1" . nitroethane~.(-90. 115). 2-nitrooraoane (-93. nihopropane I-106,131), propionitrile (-$1.9,97.2l,andacetonitrile (-45.7.60.1) cliarlyikdicate that alow temperature crystnllirar~onshould be more efficient. The solid nitromethane is only slightly soluble indiethylether (m.p. -1 1 6 W below -60°C. and Dry Ice is therefore a perfect cooling agent for this purpose. There is no need to build a vacuum system. The condensed moisture from the atmosphere can be removed easily via an ordinary fractional distillation as nitromethane and water form an azeotrope a t 83% A simple procedure of this method for laboratory operation is outlined as follows: To a stirred 1:l (vlv) mixture of commercial nitromethane and diethyl ether in a Dewar flask, add Dry Ice powder slowly until the temperature is well below -60°C. Stir for five more minutes and transfer the mixture to a deep Bnehner type funnel (with coarse fritted disc) jacketed with Dry Ice. Apply the water pump for several seconds to remove most of the ether. Stop pumping and pack the crystals tight in the funnel with a glass rod. Spread some Dry Ice powder over the surface and suck for 20 sec. Wash twice with ether precooled at -78°C each of half the amount in the original mixture. Pack the crystals and add more Dry Ice as above during each washing. Remove the Dry Ice jacket and allow the solid in the funnel tomelt and filter through into adistilling flask. Distill the liquid through a packed column to the boiling point of nitromethane, discarding the distillate. Clean and dry the condenser thoroughly and continue distillating. Discard a small amount of forerun and collect the rest of the distillate. This would recover 60-80% of the commercial nitromethane in a nure state. The purity of the nitromethane after treatment should be b e k r than 99.9%as the nrnrpeaks of the impurities are no longer drtectahlr compared with the intensities of the ':'C satellites (Fig. 2) which is 0.5590 of the intensity of the nitromethane. This work was supported by the Defence Researeh Board of Canada (Grant No. 9530-102).
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Bardin, J. C., Analusis, 1(2), 140 (1972); C.A., 78,8516f (1973). Thomoson. C. J.. Coleman. H . J.. and Helm. R. V.. J. Arner. Chem. Soc.. 76.3445 (1954). Royal Military College of Canada Kingston. Ontario. Canada K7L 2W3
448 1 Journal of Chemical Education
Frederick William Parrett Mian Shin S u n
