Nitrosoalkanes

Nitrosoalkanes

The nitrosoalkanes contain a nitroso-group, ーN=O, directly attached to a carbon atom. They are named as the nitroso-derivatives of the corresponding alkanes, e.g.,

(CH₃)₃CNO                2-methyl-2nitrosopropane

General methods of preparation

1.  By the addition of nitrosyl chloride or bromide to alkenes, whereby alkene nitrosohalides are formed:

CH₂=CH₂   +   NOCl   ⟶    ClCH₂CH₂NO    (2 mol) ⟶   (ClCH₂CH₂NO)₂

2.  By the action of nitrous acid on certain types of compounds, e.g., secondary nitroalkanes.

3. By the oxidation of primary amines containing a tertiary alkyl group with e.g., Caro’s acid (peroxy-(mono) sulphuric acid):

R₃CNH₂   +   2[O]     (H₂SO₄)⟶    R₃CNO   +   H₂O

On the other hand, Emmons (1957) has prepared primary, secondary and tertiary nitroso-compounds by oxidation of amines with neutralized peracetic acid in methylene dichloride (yield: 33-80 per cent). N-Substituted hydroxylamines may also be oxidized (aqueous potassium dichromate-sulphuric acid) to nitoso compounds:

RNHOH    [O] ⟶     RNO

General properties

Nitorso-compounds, which are usually blue or green liquids, tend to associate to give colorless solids (smelling like camphor) which are dimmers. These bimolecular solids regenerate the monomer which fused or when dissolved in solution. Chilton et al. (1955) and Gowenlock et al. (1955) have examined the absorption specturem of the two solid forms of nitrosomethane, and have concluded that they are geometrical isomers.

Nitroso-compounds exist as such when the notroso-group is attached to a tertiary carbon atom. If the nitroso-group is attached to a primary or secondary carbon atom, the nitroso-compound is generally unstable, tending to rearrange to the oxime:

R₂CH-NO   ⟶   R₂C=NOH

Blue and               colorless and

Unstable                     stable

This is known as nitroso-oximino triad system. This system is potentially tautomeric, but no example is known of the isomerisation of the oxime directly to the nitroso-compound. It has also been found that the biomolecular nitroso-chlorides rearrange, if possible, to the oxime when treated with sodium hydroxidy:

(ClCH₂CH₂NO)₂    [OH^-] ⟶    2ClCH₂CH=NOH

The so-called isonitroso-compounds (oximino-compounds) are actually the oximes, e.g., isonitroso-acetone, CH₃COCH=NOH, formed by the action of nitrous acid on acetone, is really the half oxime of methylglyoxal (q.v.).

Nitroso-compounds may be oxidize to the nitro-compounds by nitric acid, and reduced to the primary amines by, e.g., tin and hydrochloric acid:

R₃CNO₂    [HNO₃] ⟵    R₃CNO    [Sn/HCl] ⟶    R₃CNH₂