nimgoldman
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Removing MeNO2/EtNO2 contamination in MeOH/EtOH
I have several liters of methanol and ethanol, likely contaminated with nitromethane/nitroethane.
I usually recycle alcohols by distillation and refluxing with KOH (to remove esters and carbonyl compounds), but not sure whether this will remove the
nitro compound as well...
I think about removing the nitro compound by reduction, but not sure how this will proceed in alcohol. Some purification procedures speak about using
amalgamated aluminium, but I am not sure whether this will completely remove the nitro compounds (by conversion to amine, which can be removed with
acid).
Another options I think about are azeotropic distillation and purifying my alcohols via esters (lots of work).
I tried purifying my alcohols with a small amount of H2SO4 and KMnO4, hoping that nitro compounds will decompose by oxidation (in a "combustion" sense
to CO2, water, nitric acid and nitrogen oxides).
Knowing the amount of nitro compounds in my alcohols, I would simply distill off the azeotrope and burn it. Unfortunately, I found no way to assay the
amount of nitro in alcohol, apart from density measurement (assumes anhydrous alcohol and can be misleading).
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karlos³
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I hope you don't distill them to dryness.
Because the nitroalkanes will form potassium salts under that treatment, and those can be explosive.
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nimgoldman
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No I definitely never distill to dryness.
Actually I first neutralize any residual base (after the reflux) with H2SO4, then filter out the sulfate precipitate. Then distill.
Maybe I will take first 100-200 mL of the distillate, dry it, determine approx. concentration of the nitro compound from density.
If the concentration is below azeotropic, it means there is virtually no more nitro in the boiling flask since the azeotrope have already came over.
As for the destruction, perhaps using excess sodium borohydride and Cu(II) or Ni(II) salt as catalyst will work (reduction to amine), then adding acid
to neutralize unreacted borohydride and salt the amine. The alcohol, free of nitro compound, can then be recovered by simple distillation.
There is however no need to reduce all the way to amine. Even more convenient, perhaps, will be reduction to oxime with a metal salt, such as SnCl2 or Fe/FeCl3, followed by acid hydrolysis to 1. carbonyl compound, 2. hydroxylamine, 3.
carboxylic acid. All these compounds will be removed in the next step by refluxing with strong base (carbonyl compounds will condense, carboxylic acid
and hydroxylamine will form salt after acid neutralization).
I just found that aliphatic nitro compounds alone hydrolyze in a presence of strong acid. such as sulfuric.
This is convenient, since the alcoholic nitro compound azetrope can be diluted with some water, then H2SO4 added, the mixture refluxed and the
resulting carboxylic acid neutralized to salt (with e.g. Na2CO3). Finally the alcohol will be distilled off the salt.
Another destruction method seems to be the reaction of nitro compounds with aldehydes and ketones (nitro-aldol condensation), but this requires an
excess of suitable carbonyl compound (e.g. not acetone in methanol as there will be another azeotrope) and a maybe also a catalyst to proceed. The
product is still a nitro compound so this is rather destruction of the azeotropic mixture to something non-azeotropic, rather than destruction of the
nitro group.
I will probably stick with refluxing alcohol w. aq. H2SO4.
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Bedlasky
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Maybe reaction between nitro compound and nascent hydrogen (from acid and zinc) will work? But I am not a organic chemist, so I am not sure about
that.
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