Nitrous acid Subject

Chloroanisole and p-nitrophenol, the nitrations of which are susceptible to positive catalysis by nitrous acid, but from which the products are not prone to the oxidation which leads to autocatalysis, were the subjects of a more detailed investigation. With high concentrations of nitric acid and low concentrations of nitrous acid in acetic acid, jp-chloroanisole underwent nitration according to a zeroth-order rate law. The rate was repressed by the addition of a small concentration of nitrous acid according to the usual law rate = AQ(n-a[HN02]atoioh) -The nitration of p-nitrophenol under comparable conditions did not accord to a simple kinetic law, but nitrous acid was shown to anticatalyse the reaction. 

Nitration at the encounter rate and nitrosation As has been seen ( 3.3), the rate of nitration by solutions of nitric acid in nitromethane or sulpholan reaches a limit for activated compounds which is about 300 times the rate for benzene imder the same conditions. Under the conditions of first-order nitration (7-5 % aqueous sulpholan) mesitylene reacts at this limiting rate, and its nitration is not subject to catalysis by nitrous acid thus, mesitylene is nitrated by nitronium ions at the encounter rate, and under these conditions is not subject to nitration via nitrosation. The significance of nitration at the encounter rate for mechanistic studies has been discussed ( 2.5). 

Ferrous Sulfdte Titration. For deterrnination of nitric acid in mixed acid or for nitrates that are free from interferences, ferrous sulfate titration, the nitrometer method, and Devarda s method give excellent results. The deterrnination of nitric acid and nitrates in mixed acid is based on the oxidation of ferrous sulfate [7720-78-7] by nitric acid and may be subject to interference by other materials that reduce nitric acid or oxidize ferrous sulfate. Small amounts of sodium chloride, potassium bromide, or potassium iodide may be tolerated without serious interference, as can nitrous acid up to 50% of the total amount of nitric acid present. Strong oxidizing agents, eg, chlorates, iodates, and bromates, interfere by oxidizing the standardized ferrous sulfate. 

The reactivity of the amino groups at the pteridine nucleus depends very much upon their position. All amino groups form part of amidine or guanidine systems and therefore do not behave like benzenoid amino functions which can usually be diazotized. The 4-, 6-and 7-amino groups are in general subject to hydrolysis by acid and alkali, whereas the 2-amino group is more stable under these conditions but is often more susceptible to removal by nitrous acid. 

Oxidation of the trioxane ( paraldehyde ) to glyoxal by action of nitric acid is subject to an induction period, and the reaction may become violent if addition of the trioxane is too fast. Presence of nitrous acid eliminates the induction period. 

A series of /> ra-substituted ort o-nitroanilines 1274 is converted in this way to benzotriazolyl derivatives 1277, which are of interest as potassium channel activators. In the first step, nitroanilines 1274 are treated with salicylyl chloride to provide salicylamides 1275 in 70-95% yield. The nitro group is catalytically reduced, and the obtained intermediates 1276 are subjected to a reaction with nitrous acid, generated in situ from NaN02, to afford 5-substituted T(2-hydroxybenzoyl)-17/-benzotriazoles 1277 in 52-96% yield (Scheme 213) <2001FA827>. 

David and Lubineau191 reported the synthesis of pseudocytidine [5-/3-D-ribofuranosylcytosine (270)] and its a anomer by a procedure analogous to that used in preparing pseudouridine.155-157 Thus, 2,4 3,5-di-O-benzylidene-a/de/iydo-D-ribose (223) was condensed with the dilithio derivative of 2-0,4-N-(trimethylsilyl)cytosine, and the resulting, epimeric, acyclic derivatives were subjected to acid-catalyzed cyclization. The anomeric configuration of the free C-nucleosides was ascertained by spectroscopic methods and by their transformation into a- and /3-pseudouridine in the presence of nitrous acid. The anomeric 5-(/3-D-ribofuranosyl)isocytosines have also been prepared by Fox and coworkers.1913... 

Here was a clue to their search. They repeated the experiments of Cavendish and isolated a small volume of gas from the nitrogen of the air. They subjected it to every test for an unknown, and identified a new element. Small wonder that this colorless, odorless, insoluble gas would not form nitrous acid, as Cavendish had remarked. This idle gas, argon, was found to be incapable of combining with even the most active element. It was present in the atmosphere to the extent of one part in 107 by volume. Henry Cavendish had recorded one part in 120—remarkable accuracy in the light of a century of experimental advance. 

Nitroso derivatives of a more complex and more stable character than the foregoing were discovered by Roussin in 1858.2 This investigator observed that a black voluminous precipitate is obtained when a mixture of ammonium sulphide and alkali nitrite is added to an aqueous solution of ferrous sulphate. On boiling, the precipitate passes into solution. The liquid is filtered, and, upon cooling, black crystals separate out, the composition of which has been the subject of considerable discussion. The reaction does not proceed in perfectly neutral solution, a green liquor only being produced, consisting of sulphides of iron and sodium, entirely free from any nitroso derivative. The presence of a small quantity of acid, however, results in the formation of the nitroso derivative, probably because it liberates nitrous acid, which acts direct upon the ferrous salt. 

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