Nitric acidium ion

In equimolar mixtures of nitric acid and water a monohydrate is formed whose Raman spectrum has been observed. There is no evidence for the existence of appreciable concentrations of the nitric acidium ion in aqueous nitric acid. 

The rates of nitration of mesitylene-a-sulphonate anion (iii) and iso-durene-a -sulphonate anion (iv) in mixtures of aqueous nitric and perchloric acid followed a zeroth-order rate law. Although the rate of exchange of oxygen could not be measured because of the presence of perchloric acid, these results again show that, under conditions most amenable to its existence and involvement, the nitric acidium ion is ineffective in nitration. 

If it be assumed that the ionising characteristics of nitric acid are similar to those of the organic indicators used to define the scales of acidity, then a correspondence between the acidity-dependence of nitration and would suggest the involvement of the nitronium ion, whereas a correspondence with Hq would support the h)rpothesis that the nitric acidium ion were active. The analogies with and Hg are expressed in the first and last pairs of the followii equations respectively. The symbol AQ represents anthraquinone, the indicator originally used in this way for comparison with the acidity dependence of the rate of nitration of nitrobenzene ... 

The nitric acidium ion undergoes slow heterolysis to yield water and the nitronium ion ... 

In the presence of sulphuric acid this route to the nitric acidium ion is almost entirely dominant, the additional route of protonation via the... 

Neither of the above schemes for forming the nitric acidium ion involves water. However, the addition of moderate quantities of water depresses the zeroth-order rate by up to a factor of four, without disturbing the kinetic form. This last fact shows that an inappreciable fraction of the nitronium ions is reacting with water, and therefore to explain the results it is necessary to postulate the existence of a means, involving water, for the consumption of nitric acidium ions ... 

The depletion of the concentration of nitric acidium ion by appreciable quantities of water is expressed by the equilibrium... 

NO3-] oc [N20J and so [NOai oc Now nitrate ions reduce the rate of formation of nitronium ion by de-protonating nitric acidium ions, and this effect must also depend upon [HN02]"toich> as was observed. 

In first-order nitration the anticatalysis is of the same form because the deprotonation of nitric acidium ion diminishes the stationary concentration of nitronium ion and therefore diminishes the rate of nitration. 

The anticatalytic action is ascribed to the deprotonation of nitric acidium ions by nitrite ions, which, being more basic than nitrate ions, will be more effective anticatalysts. The effect of nitrite ions should depend upon [HNOaJaioich it does. 

The kinetic effect of increased pressure is also in agreement with the proposed mechanism. A pressure of 2000 atm increased the first-order rates of nitration of toluene in acetic acid at 20 °C and in nitromethane at 0 °C by a factor of about 2, and increased the rates of the zeroth-order nitrations of p-dichlorobenzene in nitromethane at 0 °C and of chlorobenzene and benzene in acetic acid at 0 °C by a factor of about 559. The products of the equilibrium (21a) have a smaller volume than the reactants and hence an increase in pressure speeds up the rate by increasing the formation of H2NO. Likewise, the heterolysis of the nitric acidium ion in equilibrium (22) and the reaction of the nitronium ion with the aromatic are processes both of which have a volume decrease, consequently the first-order reactions are also speeded up and to a greater extent than the zeroth-order reactions. 

About (i) and (2) there can be no dispute, but (3) must be rejected. The implication that the nitronium ion, effectively freed from a close association with another entity, is not the nitrating agent in those reactions of benzene and its homologues, under conditions in which substantial intermolecular selectivity is observed, conflicts with previous evidence ( 3.2). Thus, in nitration in organic solvents and in aqueous nitric acids, the observation of kinetically zeroth-order nitration, and the effect of added nitrate on this rate, is compelling evidence for the operation of the nitronium ion. The nitric acidium ion is not the electrophile under these conditions, and it is difficult to envisage how a species in which the water is loosened but not yet completely eliminated could be formed in a slow step independent of the aromatic and be capable of a separate existence. It is implicit that this species should be appreciably different from the nitronium ion in its electrophilic properties. There is no support to be found for the participation of the aromatic in the formation of the electrophile. 

However, the nitronium ion in aqueous systems must surely be hydrated to some extent and the difference in kinetics to be expected between hydrated nitronium and nitric acidium ion will be related to the degree of bonding between water and nitronium ion and the moment in the reaction process at which the water molecule is released. 

There have been a number of papers on nitration processes. Species formally corresponding to the nitric-acidium ion H2NO3 have been reported in mass-spectrometric studies (H20)NO J is considered to be more stable than (HO)2NO . In recent years the possibility of a radical pathway in aromatic nitration has attracted a lot of attention. It had been suggested that the radical pathway might... 

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