Nitronium salts reaction order

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). 

A solution of a stable nitronium salt (generally the hexa-fluorophosphate N02" PFg but also the hexafluoroantimonate N02 SbFg or tetrafluoroborate N02 BFjj ) in solvents such as methylene chloride-tetramethylene sulfone, a nd the like,is reacted with the alkane (cycloalkane), with usual precautions taken to avoid moisture and other impurities. Reactions were carried out at roan temperature (25°) and in the dark in order to avoid or minimize the possibility of radical side reactions and/or proto-lytic cleavage reactions (tertiary nitroalkanes particularly undergo ready protolytic cleavage reactions). No attempt was made to optimize conditions or increase yields by raising react-... 

We stated in our original paper "The method of ccmpetitive reaction rate determination can be applied only if the observed relative rates are dependent on the aromatic substrate. As our observed relative rates showed only low selectivity concerning the nature of the aromatic substrate it could have been attributed to a very fast reaction taking place when the solution of nitronium salt reagent is dropped into the solution of the mixed aromatic substrate. Thus the NO2 ion would have no possibility of reaching uniform distribution in the solution before reaction occurs with the aromatics and observed relative rates may be influenced rather by statistical factors than real competition". In order to clarify this possibility and also to determine the accuracy of the method used, we carried out experiments to establish whether real competitive reaction actually occured under our experimental conditions and whether the methods employed provided results which could establish small differences between the alkylbenzenes investigated ( ). 

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