Primary Amines

Primary Amines

These are very sensitive to oxidation and generally darken on exposure to air, through autoxidaion at the surface, to give mixtures of complex products. This is particularly true of aromatic amines; a number of products appear to arise by oxidation of nitrso and nitro compounds followed by condensation e.g., ArNH2 àArNO: ArNH2 +ArNO àArN=NAr. Repeated condensations give aniline blacks

Synthetically useful methods have therefore to be highly selective. The most successful reagents are hydrogen peroxide and peracids.

[1]Hydrogen peroxide converts primary aliphatic amines into aldoximes, e.g, presumably by nucleophilic displacement by the amines on the peroxide

Oxidation of aromatic amines to the corresponding level [nitroso compound] is generally brought about with perdilsulpuric acid, HSO3-O-OSO3H or Caro’s acid. For example, the former oxidant converts o-nitroaniline into o-nitroso-nitrobenzene in 75% yield

:

Aromatic nitroso compounds can also be prepared from the corresponding hydroxylamine by oxidation with dichromate at low temperature [to prevent further oxidation].For example, phenylhydroxylamine, sodium dichromate, and 50% sulphuric acid at 0 0C give nitrosobenzene at about 50% yield[3]

Since aryl-hydroxylamine are readily obtained by the reduction of nitro compound in neutral solution [PhNO2+Zn+NH4Cl/H2Oà PhNHOH},this method is convenient for introducing nitroso group via nitration.

[2]Triflouroperacetic acid, a more powerful oxidant than hydrogen peroxide, converts primary amines directly into nitro compounds. The yields with aromatic amines are generally high: e.g., o-nitroaniline is oxidized in refluxing methylene chloride to o-dinitrobenzene in92% yield.

With aliphatic amines, however, the yields are low. Other oxidants have been used with moderate success, for example, n-hexyl amine is oxidized in 33% yield to nitro compound by peracetic acid.

Aromatic nitro compounds can also be obtained by of nitroso compounds with dichromate or dilute nitric acid,

but it is normally more convenient to oxidize the amine.

[b]Secondary Amines

Oxidation with hydrogen peroxide gives hydroxylamine [cf. Primary amines]

[c]Tertiary Amines hydrogen peroxide converts tertiary amines into their N-oxide hydrates, by nucleophilic displacement analogous to the reaction of primary amine. The N-Oxide is obtained by warming the hydrate in vacuo.

Aromatic amines behave similarly e.g. pyridine give pyridine N-oxide.