Nitrous acid, in nitration

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 =... 

Nitrous acid anticatalyses nitration in aqueous nitric acid more strongly than in pure nitric acid ( 4.3.2). 

Solutions of dinitrogen tetroxide (the mixed anhydride of nitric and nitrous acids) in sulphuric acid are nitrating agents ( 4.3.2), and there is no doubt that the effective reagent is the nitronium ion. Its formation has been demonstrated by Raman spectroscopy and by cryoscopy ... 

We are not concerned here with the mechanism of nitrosation, but with the anticatalytic effect of nitrous acid upon nitration, and with the way in which this is superseded with very reactive compounds by an indirect mechanism for nitration. The term nitrous acid indicates all the species in a solution which, after dilution with water, can be estimated as nitrous acid. 

The weak effect of nitrous acid upon nitration in nitric acid is a consequence of the already considerable concentration of nitrate ions supplied in this case by the medium. 

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. 

Despite the fact that solutions of acetyl nitrate prepared from purified nitric acid contained no detectable nitrous acid, the sensitivity of the rates of nitration of very reactive compounds to nitrous acid demonstrated in this work is so great that concentrations of nitrous acid below the detectable level could produce considerable catalytic effects. However, because the concentration of nitrous acid in these solutions is unknown the possibility cannot absolutely be excluded that the special mechanism is nitration by a relatively unreactive electrophile. Whatever the nature of the supervenient reaction, it is clear that there is at least a dichotomy in the mechanism of nitration for very reactive compounds, and that, unless the contributions of the separate mechanisms can be distinguished, quantitative comparisons of reactivity are meaningless. 

Analogies for such a mechanism in diazotization are found in the nitrous acid-catalyzed nitration of A,A-dimethylaniline, mesitylene, 4-nitrophenol, and some related compounds, which were investigated by 15N NMR spectroscopy in Ridd s group (Ridd and Sandall, 1981 Ridd et al., 1992 Clemens et al., 1984a, 1984b, 1985 Johnston et al., 1991 review Ridd, 1991). Ridd and coworkers were able to demonstrate clearly that not only the nitration proper, but also the preceding C-nitrosation, is accompanied by a marked 15N nuclear polarization. This was at-... 

The effect of nitrous acid on nitration in nitromethane and acetic acid is also attributed to the effect of nitrate ions even though the ionisation of the dinitrogen tetroxide is much less in these solvents. As noted above (p. 31), the anticatalytic effect of nitrous acid is not governed by k x = a+6[HN03] at nitrous acid concentrations above 0.1 M. 

The kinetics of aromatic nitrosation at ring carbon have received little attention. The first attempt to determine the nature of the electrophile was made by Ingold et a/.117, who measured the rates of the nitrous acid-catalysed nitration of 4-chloroanisole by nitric acid in acetic acid which proceeds via initial nitrosation of the aromatic ring. Assuming that the electrophiles are the nitrosonium ion and... 

A further point of preparative significance still requires explanation, however. Highly reactive aromatic compounds, such as phenol, are found to undergo ready nitration even in dilute nitric acid, and at a far more rapid rate than can be explained on the basis of the concentration of N02 that is present in the mixture. This has been shown to be due to the presence of nitrous acid in the system which nitrosates the reactive nucleus via the nitrosonium ion, NO (or other species capable of effecting nitrosation, cf. p. 120) ... 

The reaction (NO + N02 NO+ + NO2 ) is recommended by Ridd (1998) to put in parentheses to imply that it illustrates the stoichiometry of the process, not the mechanism. The mechanism is more complex because the rate of these nitrous acid-catalyzed nitrations greatly exceed the rate of formation of nitronium ions in the solution. 

A kinetic study of nitrous acid-catalyzed nitration of naphthalene with an excess of nitric acid in aqueous mixture of sulfuric and acetic acids (Leis et al. 1988) shows a transition from first-order to second-order kinetics with respect to naphthalene. (At this acidity, the rate of reaction through the nitronium ion is too slow to be significant the amount of nitrous acid is sufficient to make one-electron oxidation of naphthalene as the main reaction path.) The reaction that initially had the first-order in respect to naphthalene becomes the second-order reaction. The electron transfer from naphthalene to NO+ has an equilibrium (reversible) character. In excess of the substrate, the equilibrium shifts to the right. A cause of the shift is the stabilization of cation-radical by uncharged naphthalene. The stabilized cation-radical dimer (NaphH)2 is just involved in nitration ... 

The nitration of phenols can result in anomalous and large differences in product isomer ratios, showing a high dependence on both nitrating agent and reaction medium. Here the situation is complicated by the intervention of an alternative nitration mechanism - that of nitrous acid catalyzed nitration, which proceeds via in situ nitrosation-oxidation (see Section 4.4). 

An alternative method for the synthesis of amrinone from 3-cyano-5-(4-piridyl)-2(l//)-pyridinone (17.2.2) is based on it s acidic hydrolysis to the corresponding acid, 3-carboxy-5-(4-piridyl)-2(l//)-pyridinone (17.2.5), nitration of which with nitrous acid in the presence of sulfuric acid forms 3-nitro-5-(4-piridyl)-2(l//)-pyridinone (17.2.6). Reducing the nitro group of this product with hydrogen gives the desired amrinone (17.2.4) [19, 20]. 

Or with nitrous acid in the presence of nitric acid, manganese(ll) nitrate is formed ... 

The other products of the equivalents of K S, HS are nitrate of potassa—KO, NOs—and a little ammonia. No trace of nitrate could be discovered in the solution, which was evaporated to dryness with excess of acetic acid, to expel the nitrons acid. An attempt was made to turn the reaction of KS, HS upon gun-cotton to account in effecting the determination of the nitrogen by this means but the results were unsatisfactory, in consequence of the co-existence of ammonia and nitrous acid in the liquid, The solution could not bo heated to expel the former without risk of loss in the weight of the roducad cotton, neither could the ammonia be retained by addition of an acid, without escape of NO,-from the action of sulphide of hydrogen and nitrous acid on each other. 

The nitrous acid in the reaction is oxidised to nitric acid, and this produces nitration. 

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