Nitrolic acids formation

Pseudonitrole or nitrolic acid formation can be a side-reaction during the acidification of nitronate salts, particularly if the acid addition is slow. This process has been studied, optimized, and patented as a route to these compounds. " ... 

Organic nitroxides (136a, 5), nitroparafins, and nitrolic acids (107) react with aqueous solutions of pentacyanocobaltate(II) salts to give alkyl pentacyanocobalt(III) nitroxide anion radicals, spin traps involving the formation of an N—Co bond (5). 

Nitrolic acids (II) are relatively stable when R is alkyl but can lose nitrous acid with fiiroxane (IV) formation (Wieland and Semper [14]). Aci-nitroparaffins (III) readily yield aldehydes when R is alkyl. At lower temperatures the product V (R=C6H5) predominates. 

The reaction leading to the formation of nitrolic acids was described for the first time by V. Meyer [49], The compounds are formed by treating aci-forms of primary paraffins with NaN02, followed by acidification of the whole mass. According to Veibel [61], the mechanism of the reaction may be presented by the following multistage scheme ... 

Again V. Meyer was the first to describe the reaction. As in the formation of nitrolic acids, here also it is the aci-form of the secondary nitro compound that takes part in the reaction. It reacts with sodium nitrite to give a compound which, after acidification, forms pseudonitrole. 

Direct dehydrogenation of oximes is also possible, particularly for sterically hindered nitrile oxides. The most successful oxidizing agents are alkaline hypobromite and NBS in the presence of base, the latter being valuable when basic groups are present. Lead tetraacetate and alkaline hypochlorite have also been employed, but the yields are lower. The formation of furoxans from the reaction of alkoximes with nitrogen oxides is believed to proceed via the corresponding nitrolic acids. 

Further oxidation (N204) of the dehydro dimers [(ArCH NO)2] of aromatic aldoximes produces furoxans.196 Here the nitrolic acids may not be intermediates, as there are alternative routes to the nitrile oxides.197 Some authors have assumed that the formation of furoxans in this reaction... 

---