Rearrangement reactions aromatic nitramines

The aromatic nitramine rearrangement has been known already for more than 100 years. This reaction can proceed as an acid-catalyzed, thermal and photochemical reaction (see reviews in References 23,250-252). /V-Nitroanilinc 169, being treated with strong acids, rearranges to give ort/ro-nitroanilinc 170 as main product (50-60%), para-nitroaniline 171 (5-10%) and some side products (equation 66)251. 

Properties and reactions of nitramines Secondary nitramines are neutral, primary nitramines form salts with bases, but an excess of alkali often causes decomposition to the carbonyl compound, nitrogen, and water. Secondary nitramines and aqueous alkali afford nitrous acid, aldehyde, and primary amine. Acids decompose primary aliphatic nitramines with formation of nitrous oxide in a reaction that has not yet been clarified thus these compounds cannot be hydrolysed by acid to amines in the same way as nitrosamines, although, like the latter, they can be reduced to hydrazines. Primary and secondary aromatic nitramines readily rearrange to C-nitroarylamines in acid solution. Most nitramines decompose explosively when heated, but the lower aliphatic secondary nitramines can be distilled in a vacuum. 

Depending on the amine substrate and the nitrating conditions used, it is not uncommon for an intermediate nitramine to undergo direct rearrangement to the ring-nitrated product without prior isolation, in which case, the formation of the nitramine as an intermediate can only be postulated. Due to the high o-selectivity often observed with this type of reaction the o/p-ratio can be very different to that where the aromatic ring is directly nitrated. 

The reaction of many aromatic amines with nitric acid in concentrated sulphuric acid is well known to give C-nitro products but whether this is a direct C-substitution or an N-substitution followed by rearrangement is usually unknown. The currently accepted view (1) appears to be that nitramine intermediates are seldom involved this conclusion is based on the marked difference in the product compositions obtained in the nitration of aniline and the rearrangement of phenylnitramine when both reactions are carried out in 85% sulphuric acid (2). 

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