Nitration nitrous acid catalysed

In contrast to its effect upon the general mechanism of nitration by the nitronium ion, nitrous acid catalyses the nitration of phenol, aniline, and related compounds. Some of these compounds are oxidised under the conditions of reaction and the consequent formation of more nitrous acids leads to autocatalysis. 

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... 

The nitrosation of phenol and cresols in buffer solutions involves a diffusion-controlled C-nitrosation followed by rate-limiting proton loss. /r-Crcsol is much less reactive than the other substrates.79 Nitrosation in trifluoroacetic acid or in acetic-sulfuric acid mixtures is regioselective (e.g. 4-nitroso-m-xylene is fonned from m-xylene) and possible non-selective nitrous acid-catalysed nitration can be eliminated by purging reaction solutions with nitric oxide.80... 

Scheme 2. Nitrous acid catalysed nitration of phenol, mechanism proposed in literature...

In organic media (Table 2) results are more surprising in that, that according to the proposed nitrous acid catalysed mechanism, one would not have predicted paranitrosophenol to be the main product. Particularly, in toluene or dichloromethane where biphasic nitration of phenol takes place very rapidly, no reaction is observed. 

The carbon balance is not very good when reaction takes place, so interpretation must be cautious. But even if this carbon loss reveals an electron transfer from phenol to NO+, this reaction is slow and does not explains results obtained in nitrous acid catalysed nitration of phenol. 

At the present time, it is not possible to identify the exact role of the dihydroxybenzene in this reaction. Two possibilities can be considered first, whether it acts like a reducing agent on the hydrogen peroxide to give the hydroxyl radical, in a similar way to the Fenton reaction [10] or whether, after its oxidation by hydrogen peroxide into quinone, it causes an electron transfer from the phenol to give the phenoxy radical, in a similar role to the nitrosonium cation in the nitrous acid catalysed nitration of phenols [11]. In either case, this peculiar characteristic of the hydroxylation reaction explains the good behaviour... 

The catalysed nitration of phenol gives chiefly 0- and />-nitrophenol, (< 0-1% of w-nitrophenol is formed), with small quantities of dinitrated compound and condensed products. The ortho para ratio is very dependent on the conditions of reaction and the concentration of nitrous acid. Thus, in aqueous solution containing sulphuric acid (i 75 mol 1 ) and nitric acid (0-5 mol 1 ), the proportion of oriha-substitution decreases from 73 % to 9 % as the concentration of nitrous acid is varied from o-i mol l i. However, when acetic acid is the solvent the proportion of ortAo-substitution changes from 44 % to 74 % on the introduction of dinitrogen tetroxide (4-5 mol 1 ). 

The kinetics of nitration of anisole in solutions of nitric acid in acetic acid were complicated, for both autocatalysis and autoretardation could be observed under suitable conditions. However, it was concluded from these results that two mechanisms of nitration were operating, namely the general mechanism involving the nitronium ion and the reaction catalysed by nitrous acid. It was not possible to isolate these mechanisms completely, although by varying the conditions either could be made dominant. 

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. 

The effect of nitrous acid on the nitration of mesitylene in acetic acid was also investigated. In solutions containing 5-7 mol 1 of nitric acid and < c. 0-014 mol of nitrous acid, the rate was independent of the concentration of the aromatic. As the concentration of nitrous acid was increased, the catalysed reaction intervened, and superimposed a first-order reaction on the zeroth-order one. The catalysed reaction could not be made sufficiently dominant to impose a truly first-order rate. Because the kinetic order was intermediate the importance of the catalysed reaction was gauged by following initial rates, and it was shown that in a solution containing 5-7 mol 1 of nitric acid and 0-5 mol 1 of nitrous acid, the catalysed reaction was initially twice as important as the general nitronium ion mechanism. 

Under the conditions mentioned, i-methylnaphthalene was nitrated appreciably faster than was mesitylene, and the nitration was strongly catalysed by nitrous acid. The mere fact of reaction at a rate greater than the encounter rate demonstrates the incursion of a new mechanism of nitration, and its characteristics identify it as nitration via nitrosation. 

For a given concentration of nitrous acid, the first-order rate coefficients increase as the sixth to seventh power of the nitric acid concentration and this catalysed nitration is presumed to be nitrosation followed by oxidation, reactions (41) and (42), viz. 

As the concentration of nitric acid is increased the kinetic form changes and in 10 M nitric acid the reaction is zeroth-order and anti-catalysed by nitrous acid (as described above) presumably due to nitronium ion nitration. 

Mesitylene was studied using the range 5-7 M nitric acid, and when the nitrous acid concentration is small (< 0.014 M) nitronium ion nitration appears to occur, giving zeroth-order kinetics weakly retarded by nitrous acid. At rather higher nitrous acid concentrations the reaction is catalysed by nitrous acid and the kinetics go over to first-order (at constant nitrous acid concentration). 

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