Nitroalkane

Nitroalkane

The nitroalkanes are colorless (when pure), pleasant-smelling liquids which are sparingly soluble in water. Most of them can be distilled at atmospheric pressure.

Nomenclature

The nitroalkanes are named as nitro-derivative of the corresponding alkane, the positions of the nitro-groups being indicated by numbers, e.g.

CH₃NO₂                                             nitromethane

         NO₂    NO₂

         |           |

CH₃CHCH₂CHCH₂CH₃                     2,4-dinitrohexane

The nitroalkanes, the structure of which is RNO₂, are isomeric with the alkyl nitrites, RONO. The evidence that may be adduced for these respective formulas is to be found in the study of the reactions of these two groups of compounds. It is, however, worthwhile at this stage to mention the reaction which most clearly indicates their respective structures, viz., reduction. When alkyl nitrites are reduced, an alcohol and ammonia or hydroxylamines are formed. This shows that the alkyl group in nitrites is attached to an oxygen atom:

R-ONO    (e; H⁺) ⟶    ROH   +   NH₃   +   H₂O

On the other hand, when nitroalkanes are reduced, a primary amine is formed. This shows that the alkyl group is attached to the nitrogen atom, since the structure of a primary amine is known to be R-NH₂ from its method of preparation:

RNO₂     (e; H⁺) ⟶     RNH₂   +   2H2O

The structure of the nitro-compounds is best represented as a resonance hybrid:

For most purposes, however, the non-commercial formula RNO₂, is satisfactory.

General method of preparation

1.  By heating an alkyl halide with silver nitrite in aqueous ethanolic solution:

RX   +   AgONO    ⟶     RNO₂   +   RONO   +   AgX

This method is only useful for the preparation of primary nitroalkanes. With s-halides the yield is about 15 per cent, and with t-halides 0-5 per cent; the yield of nitrite increases from primary to tertiary halides.

2.  Nitro-compounds are prepared industrially by direct nitration, which is carried out in two ways.

In liquid-phase nitration, the hydrocarbon is heated with concentrated nitric acid under pressure at 140^oC. Nitration under these conditions is always slow, and a large amount of polynitro-compounds is produced.

In vapour-phase nitration, the hydrocarbon is heated with nitric acid at 150-475^oC; each hydrocarbon has its optimum temperature.

Vapor-phase nitration is more satisfactory than liquid-phase nitration.

3.  Kornblum et al. (1956) have prepared t-nitro-compounds by the oxidation of t-carbinamines with potassium permanganate

R₃CNH₂   (KMnO₄) ⟶    R₃CNO₂        (70-80%)

Reactions

1.  nitroalkanes are reduced catalytically, by lithium aluminium hydride, or by metal in acid solution to primary amines.

RNO₂   ⟶    RNH₂   +   2H₂O

When the reaction with metal is carried out in neutral solution, e.g., with zinc dust and ammonium chloride solution, nitro-compounds are converted into an N-alkylhydroxylamine.

RNO₂    (Zn/NH₄Cl) ⟶    RNHOH   +   H₂O

2.  Primary nitro-compounds are hydrolysed by boiling hydrochloric acid or by 85 per cent sulphuric acid to a carboxylic acid and hydroxylamine:

RCH₂NO₂    +   H₂O    (HCl)⟶   RCO₂H   +   NH₂OH

This reaction is used for manufacturing hydroxylamine.

Secondary nitro-compounds are hydrolysed by boiling hydrochloric acid to ketones and nitrous oxide:

2R₂CHNO₂    (HCl) ⟶   2R₂CO   +   N₂O   +   H₂O

Tertiary nitro-compounds are generally unaffected by hydrochloric acid.

3.  Primary and secondary nitro-compounds are readily halogenated in alkaline solution in the a-position only:

R2C=NO₂^-Na⁺   +   Br₂     (NaOH)⟶   R₂CBrNO₂   +   NaBr

Primary nitro-compounds can form the mono- and dibromo-derivatives, whereas secondary form only the monobromo-derivative: nitromethane is exceptional in that it can form the tribromo-derivative.