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Acetonitrile (methyl cyanide) [75-05-8] M 41.1, b 81.6°, d25
0.77683, n 1.3441, n25
1.34163. Commercial acetonitrile is a byproduct of the reaction of propylene and ammonia to acrylonitrile.
The following procedure that significantly reduces the levels of acrylonitrile, allyl alcohol, acetone and *benzene
was used by Kiesel [Anal Chem 52 2230 1988]. Methanol (30OmL) is added to 3L of acetonitrile fractionated
at high reflux ratio until the boiling temperature rises from 64° to 80°, and the distillate becomes optically clear
down to X = 240nm. Add sodium hydride (Ig) free from paraffin, to the liquid, reflux for lOmin, and then distil
rapidly until about 10OmL of residue remains. Immediately pass the distillate through a column of acidic
alumina, discarding the first 15OmL of percolate. Add 5g of CaH2 and distil the first 5OmL at a high reflux
ratio. Discard this fraction, and collect the following main fraction. The best way of detecting impurities is by
gas chromatography.
Usual contaminants in commercial acetonitrile include F^O, acetamide, NH4OAc and NH3. Anhydrous CaSO4
and CaCl2 are inefficient drying agents. Preliminary treatment of acetonitrile with cold, satd aq KOH is
undesirable because of base-catalysed hydrolysis and the introduction of water. Drying by shaking with silica
gel or Linde 4A molecular sieves removes most of the water in acetonitrile. Subsequent stirring with CaH2
until no further hydrogen is evolved leaves only traces of water and removes acetic acid. The acetonitrile is then
fractionally distd at high reflux, taking precaution to exclude moisture by refluxing over CaH2 [Coetzee Pure
Appl Chem 13 429 1966]. Alternatively, 0.5-1% (w/v) P2O$ is often added to the distilling flask to remove
most of the remaining water. Excess ?2O5 should be avoided because it leads to the formation of an orange
polymer. Traces of ?2O5 can be removed by distilling from anhydrous ^CO3. |
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