Nitration nitrous acid catalysis

Nitrous acid catalysis also takes place in the nitration of such compounds (naphthalene) that are unable to undergo nitrosation on the given conditions or whose nitrosation proceeds slower than nitration. As accepted, the nitrosonium ion is formed from HNOj in acid media. The nitrosonium ion oxidizes an aromatic substrate into a cation-radical and transforms into nitric oxide. The latter reduces nitronium cation to nitrogen dioxide that gives a a-complex with the aromatic cation-radical ... 

The anticatalytic effect of nitrous acid in nitration The effect of nitrous acid was first observed for zeroth-order nitrations in nitromethane ( 3.2). The effect was a true negative catalysis the kinetic order was not affected, and nitrous acid was neither consumed nor produced by the nitration. The same was true for nitration in acetic acid. In the zeroth-order nitrations the rate depended on the reciprocal of the square root of the concentration of nitrous acid =... 

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. 

The catalysis was very strong, for in the absence of nitrous acid nitration was very slow. The rate of the catalysed reaction increased steeply with the concentration of nitric acid, but not as steeply as the zeroth-order rate of nitration, for at high acidities the general nitronium ion mechanism of nitration intervened. 

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). 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3, so-called neutral diazotization) or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. Kozlov and Volodarskii (1969) measured the rates of diazotization of l-amino-2-naphthol-4-sulfonic acid in the presence of one equivalent of 13 different sulfates, chlorides, and nitrates of di- and trivalent metal ions (Cu2+, Sn2+, Zn2+, Mg2+, Fe2 +, Fe3+, Al3+, etc.). The rates are first-order with respect to the added salts. The highest rate is that in the presence of Cu2+. The anions also have a catalytic effect (CuCl2 > Cu(N03)2 > CuS04). The mechanistic basis of this metal ion catalysis is not yet clear. 

Exchange of nitrate oxygen with water has been studied under conditions where it seems improbable that catalysis by nitrous acid contributes appreciably. The exchange is quite slow, and subject to chloride ion catalysis. The rate expression... 

Reactions of nitric and nitrous acid with hydroxylamine will be discussed in a later section. The bulk of published work has been concerned with nitration of organic substrates, such as cinnamic acids and 2-iodo-1,3,5-trialkylbenzenes. Evidence has been produced to show the wider generality of the electron transfer role for nitrous acid in the catalysis... 

A useful general assessment of nitration of a range of substrates in aqueous nitric acid has been presented/ which has centered on the rate profile for nitration in nitric acid, the limiting rate of nitration when reaction occurs on encounter, and the rate of reaction of nitronium ions with the solvent. In this work hydrazine was added when necessary to prevent catalysis by nitrous acid. New Raman data on nitric acid and d-nitric acid have been reported. ... 

The diazotisation of aniline in aqueous nitric acid has also been studied. At low acidities (below 0-2 m), nitrous anhydride is the most important reagent . At higher acidities the kinetics of the reaction are similar to those for equivalent concentrations of perchloric acid and the rate increases rapidly with acidity . This catalysis has been attributed to nitrosation by dinitrogen tetroxide (nitrosyl nitrate) , but other studies of catalysis by neutral nitrate salts indicate the contribution to the overall rate of nitrosation by dinitrogen tetroxide is slight It seems, therefore, that the reagents at high acidity are the nitrous acidium and nitrosonium ions, as for perchloric acid. 

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