Nitronium salts kinetics

It has already been noted that, as well as alkylbenzenes, a wide range of other aromatic compounds has been nitrated with nitronium salts. In particular the case of nitrobenzene has been examined kinetically. Results are collected in table 4.4. The reaction was kinetically of the first order in the concentration of the aromatic and of the nitronium salt. There is agreement between the results for those cases in which the solvent induces the ionization of nitric acid to nitronium ion, and the corresponding results for solutions of preformed nitronium salts in the same solvent. 

Kinetic studies on the nitration of nitrobenzene by nitronium borofhioride in the polar solvents sulphuric acid, methane-sulphuric acid, and acetonitrile show the reaction to be first-order in both nitronium salt and aromatic110. With the first two solvents, the rate coefficients are similar for nitration by nitric acid and by the nitronium salts, indicating a common nitrating entity. With acetonitrile the rate coefficients are very much lower, consistent with a much lower concentration of free nitronium ions in this medium and thus with the nitronium salts existing as ion pairs in organic solvents (see Table 25). 

Most of the support for this mechanism comes from evidence about the nature of the attacking particle in each of these reactions evidence, that is, that substitution is electrophilic. This evidence, in turn, comes largely from kinetics, augmented by various other observations the nitrating power of preformed nitronium salts (Sec. 11.8), for example, or carbonium ion-like rearrangements in some Friedel-Crafts alkylations (Problem 11.3 above). The electrophilic nature of these reactions is supported in a very broad way by the fact that other reactions which show the same reactivity and orientation features also fit into the same mechanistic pattern. 

Determination of the mechanism of nitration of azoles is complicated by the fact that they can be nitrated both in the free form and in the protonated form. The kinetics of the nitration of azoles in media with various acidities is therefore compared with the kinetics of the nitration of related model compounds known to exist in the form of ions (e.g., the quaternary salt of the investigated azole). If nitration takes place through the protonated form, the rate initially increases with increase in the acidity on account of the increase in the concentration of the nitronium ion. After reaching a maximum the rate then remains almost unchanged or decreases slightly. 

Dihydrodiazepinium salts are readily nitrated at the 6-position.28 43-44 Kinetic studies show that the dihydrodiazepinium monocation is indeed the substrate species involved in the reaction, the reagent being nitronium ion.46... 

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