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Primary and secondary nitro paraffins (nitroalkanes) are acidic in nature and form sodium salts. When mineral acid is added to these sodium salts, the nitro paraffins are regenerated. The action is different, however, when the sodium salts are added to an excess of the acid. In the latter case aldehydes or ketones are formed with the evolution of nitrous oxide. The action may be summarized by the following equation but the mechanism of the reaction is not known:
2 RCH-NO2Na + 2 H2SO4 → 2 RHCO + N2O + 2 NaHSO4 + H2O (aldehydes)
2 R2C-NO2Na + 2 H2SO4 → 2 R2CO + N2O + 2 NaHSO4 + H2O (ketones)
Nef2 prepared acetaldehyde from nitroethane by adding a 10% solution of the sodium salt to 20% sulfuric acid. The yield was 70%. Nef also prepared acetone by this method. Konowaloff1 prepared ketones from the sodium salts of several nitro paraffins but his reactions were probably accompanied by simultaneous reduction, as in several cases he dropped the salt into a mixture of zinc and acetic acid, and in another, into an acid solution of Stannous chloride.
The general procedure is: one-sixth of a mole of nitro paraffin is dissolved in 150 ml of a solution containing 8 g of sodium hydroxide, and then added dropwise to an ice cold mixture of 25 ml of concentrated sulfuric acid in 160 ml. of water. Efficient agitation is used during the addition. Gas is evolved as the solution is added. The mixture is distilled, and the aldehyde or ketone determined quantitatively by its reaction with hydroxylamine hydrochloride.
The procedure was modified in one series of experiments by using 7 g of calcium hydroxide instead of the sodium hydroxide.
By these two procedures the aldehydes and ketones available from nitroethane, 2-nitropropane, 1-nitropropane, 1-nitrobutane, 2-nitrobutane, and 1-nitro-2-methylpropane were prepared. With the exception of isobutyraldehyde, all of these aldehydes and ketones were obtained in 80-85% yield. It might be noted that in several attempts to regenerate the nitro paraffin, from the sodium salt, the recovery of nitro paraffin was limited to about 85%.
The calcium salts gave as good yields as the sodium salts, but more time had to be given for the reaction between the calcium hydroxide and nitro paraffins in order to assure an appreciable concentration of the enolic or salt form. The calcium salts of the nitrobutanes are not very soluble in water.
The decomposition of the salts of the nitro alkanes is practically instantaneous. The nitrous oxide is evolved as soon as the salt solution comes in contact with the acid.