Nitro-hydrocarbons aliphatic, preparation

Nitro compounds. Aliphatic nitro compounds are acidic. They are freed from alcohols or alkyl halides by standing for a day with concentrated sulphuric acid, then washed with water, dried with magnesium sulphate followed by calcium sulphate and distilled. The principal impurities are isomeric or homologous nitro compounds. In cases where the nitro compound was originally prepared by vapour phase nitration of the aliphatic hydrocarbon, fractional distillation should separate the nitro compound from the corresponding hydrocarbon. Fractional crystallisation is more effective than fractional distillation if the melting point of the compound is not too low. 

Olah showed that nitrations can be split into the three categories of electrophilic, nucleophilic and free radical nitration. Free radical nitrations are extensively used for the industrial synthesis of low molecular weight nitroalkanes from aliphatic hydrocarbons. Nucleophilic nitration is the basis for a number of important methods for the synthesis of nitro and polynitro alkanes. Generally speaking only electrophilic nitration is of preparative importance for the... 

Aliphatic nitro compounds cannot as a rule be prepared in the same way as the aromatic nitro compounds. The more rapid oxidation of aliphatic hydrocarbons by nitric acid is the main interfering factor, so that conditions must be chosen which minimize oxidation and promote nitration. The oxidation reactions are of such complexity in these cases that no attempt will be made to formulate them. Only a summary of the conditions favoring nitration will be given. The use of a solvent such as ether for carrying out the reaction is often successful. Also dilute nitric acid has been used, and alkyl (generally ethyl) nitrate. In the Friedel-Crafts reaction with ethyl nitrate, aluminium chloride is used as catalyst. In aliphatic nitrations with ethyl nitrate, alkalis such as metal alkoxides (NaOC Hs) are found to be best. The use of alkalis brings out the similarity of this reaction to aldol condensations which are also favored by alkalis. An example of aliphatic nitration, in comparison with an aromatic one may be given ... 

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