Nitric acid, dimerization reactions with

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 ... 

Fast dimerization of NO2 followed by reaction with water to give nitrous acid and nitric acid (this chemistry is well known from nitric add plants) ... 

Fanning and coworkers have prepared two forms of the complex [Fe(salen)-NO3] (salen = N,iV -ethylenebis(sahcylideneaminato)), namely, the monomeric [Fe(salen)02N0], and the dimeric [Fe(salen)0N02]2 complex by the reaction of the //-oxo-dimer complex [Fe(salen)]20 with nitric acid (Figure 17) (115). 

Clearly, the analogy between the Fast SCR and the Enhanced SCR chemistries has deeper roots than just a combination of stoichiometries. It actually originates from the key mechanistic role played by nitrates adspecies in both reactions. Such nitrates are either formed via NO2 dimerization, disproportionation, and hetero-lytic chemisorption in the Fast SCR chemistry (see Sect. 9.5.3 and Table 9.1), or are formed directly by nitric acid adsorption when feeding aqueous solutions of NH4NO3 or nitric acid in the case of Enhanced SCR. It is well known that ammonium nitrate participates in the dissociation equilibrium (9.16) with nitric acid and ammonia. 

An exception is the case that a critical reaction step takes place in the gas phase only. An interesting example is the nitric acid process, where evolved nitrous oxide is oxidized to nitrogen dioxide in the gas phase. The dioxide (or its dimer) dissolves again and reacts with water. In this process both the gas volume and the gas/liquid interfacial area determine the overall reaction rate. Of course, gas/liquid mass transfer has then to be taken into account (section 4.6). 

The nitric oxide which is formed reacts with oxygen to form nitrogen dioxide. Nitrogen dioxide exists in equilibrium with its dimer, dinitrogen tetroxide. The nitrogen dioxide/dimer mixture is sent to a column, sometimes called an absorption tower. Water is added at the top of the column. The nitrogen dioxide is converted to nitric acid. Byproduct nitric oxide is oxidized to nitrogen dioxide by means of a stream of air passed into the absorption column. The aqueous nitric acid is removed continuously from the base of the column. Overall, the reaction can be written as ... 

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