Protonated nitric acid

Strong acids catalyse nitration by protonating nitric acid, as shown below H2NO3+ + HSO4-,... 

Accordingly, the role of water might be explained in the following way in the absence of water, protonation of toluene can induce arylation, whereas, in the presence of water, the acidity of the clay is just sufficient to protonate nitric acid and to favour the formation of an ipsosubstituted Wheland intermediate. The most reasonable reaction sequence compatible with our observations is depicted in scheme 1 and eq. 4. 

That the nitronium ion not only exists but also can be the reactive species has been shown. For example, the rate of nitration of toluene (and of other aromatics) in solutions of nitric acid in nitromethane were independent of the concentration of toluene.143 Thus the slow step must be the formation of the reactive species prior to attack on the toluene ring. This rules out HN03 as the nitrating agent. That protonated nitric acid, formed as shown in Equation 7.62, is not the reactive species follows from the fact that the rate does not become first-order in... 

The protonated nitric acid dissociates to form a nitronium ion (+N02). 

The accepted reaction mechanism for the electrophilic aromatic nitration was postulated by Ingold in 1969[3] and involves several steps (Scheme 5.1). Firstly, the nitric acid is protonated by a stronger acid (sulfuric). The protonated nitric acid gives water and the nitronium ion (N02+) which is the electrophilic active species for nitration of aromatics. Nitric acid heterolysis is considered to be accelerated by the polarity of the solvent, and solvation of nitronium ion in different media affects its reactivity and the selectivity of the reaction. Combination of nitronium ion and an aromatic molecule form an intermediate named the Wheland complex or er-complex. The loss of a proton from the er-complex gives the aromatic nitrocompound (Scheme 5.1). 

The mechanism of aromatic nitration is shown in Figure 1. It may be seen that the sulfuric acid serves as the source of hydrogen ion (proton) which protonates nitric acid to form nitronium ion and water. The NO, ion is the active electrophile that causes the nitration reactions. 

Note that two molecules of sulphuric acid are consumed the first to produce the protonated nitric acid that decomposes to produce the nitronium ion and the second to protonate the water molecule, which is a side product formed from the decomposition of the protonated nitric acid. 

Evidence to support this "counterion modified" model comes from the investigation of hydrated Ln(NC>3)Cl2 prepared in the usual way in water (LnCl3, AgN03). The lanthanide(III) chlorides themselves show little or no catalytic activity for nitrations. A possible rationale for this is obtained by noting that HC1 is a poor activator of nitric acid in nitration chemistry since it is not sufficiently acidic to protonate nitric acid. However, the IR spectra of the Ln(NC>3)Cl2 salts are essentially identical to those of Ln(N03)(0Tf)2 indicating that the chloride ions are outer sphere in these complexes. Additionally the nitrate bands show the same trend as demonstrated for the triflate series (e.g., for La(N03)Cl2 the characteristic nitrate stretch is observed at 1459 cm1 and for Yb(NC>3)Cl2 the band appears at 1497 cm-1)- This indicates that the lanthanide chlorides are capable of activating nitric acid (via metal-nitrate interactions) but critically, the counterion (i.e. chloride) is incapable of fulfilling its role (in whatever capacity that may be) and hence no nitration occurs. 

The case of nitration with at 70 % sulfonitric mixture seems particular. In this zone of acidity, the main specy is neither the nitronium ion neither nitric acid but protonated nitric acid H2NCb+ (ref. 13). In this case one can invoque a cyclic transition state to explain the ortho selectivity that is observed. 

First the nitric acid is protonated by the stronger sulfuric acid. Protonated nitric acid forms water and the N02" ion, the nitronium ion is the active agent in acid-catalyzed aromatic nitration. Its solvation in different media affects its reactivity and the selectivity of the reaction. With reactive aromatics an additional kinetic step must be added to the above mechanism between steps (2) and (3) ... 

To generate the necessary electrophile, sulfuric acid protonates nitric acid. Loss of water from protonated nitric acid forms a nitronium ion, the electrophile required for nitration. Remember that any base (=B) present in the reaction mixture (H2O, HS04, solvent) can remove the proton in the second step of the aromatic substitution reaction. 

Any acid or base is constantly in a Lowry-BrjzJnsted acid-base equilibrium. Nitric acid, therefore, is constantly in acid-base equilibrium where one molecule of nitric acid serves as acid, and another serves as base (step (1), see below). Once a protonated nitric acid molecule is formed, it loses water to give the nitronium ion. (Step (2)). ... 

To generate the necessary electrophile, sulfuric acid protonates nitric acid. Protonated nitric acid then loses water to form a nitronium ion, the electrophile required for nitration. 

In Summary Nitration of benzene requires the generation of the nitronium ion, N02, which functions as the active electrophile. The nitronium ion is formed by the loss of water from protonated nitric acid. Sulfonation is achieved with fuming sulfuric acid, in which sulfur trioxide, SO3, is the electrophile. Sulfonation is reversed by hot aqueous acid. Benzenesulfonic acids are used in the preparation of detergents, dyes, compounds containing leaving groups, and sulfa drugs. 

Because H2SO4 (HOSO3H) is a stronger acid, a mixture of it and HNO3 will contain a higher concentration of protonated nitric acid tiian will nitric acid alone. 

---