Nitronium salts ortho/para ratio

Low meta substitution allows favorable regiocontrol in the subsequent preparation of dinitrotoluenes. In general, the nitric add-trifluoro-methanesulfonic acid system shows less meta substitution than other nitrating systems at comparable temperatures (Table IV). The major factor, however, effecting low meia nitration is the use of extremely low temperatures. Solubility of the formed nitronium salt at low temperature in halomethane solutions is limited and unusual ortho/para ratios ma> be also a consequence of the heterogeneous nature of the reaction mixtures. 

Kameo et ai. reported [50] the use of polystyrenesulfonic acid as a catalyst in the nitration of aromatics with HNO3. Nitration of toluene with 90% HNO3 over dried sulfonated polystyrene resin (Rohm and Haas amberlite IR-120) was also reported by Wright et al. at 65-70°C to give an ortho para isomer ratio of only 0.68, much lower than usual ortho-para ratios in acid-catalyzed nitrations [11]. It is considered that the nitroniura ion is strongly ion paired to the resinsulfonic acid. The ion-pair salt thus formed is much bulkier than the free nitronium ion or such nitronium salts as NO2 BF4. 

In order to study this problem, Olah and Lin carried competitive studies of nitration of benzene and toluene with nitry chloride, catalyzed by Lewis acid halides. When carbon tetrachloride or excess aromatics were used as solvent, the data summarized in Table XII were obtained. The data show that the ortho para ratios are smaller than in nitrations with nitio-nium salts. The observed changes point to the fact that the nitrating agents arc the conesponding donor acceptor complexes and not the nitronium ion itself. The lower orthojpara ratios than those obtained in case of NOj, particularly point to bulkier nitrating agents. 

At ambient temperatures the nitration of toluene produces the ortho and para Isomers dominantly with between 2 and 4% meta product. A similar product ratio is obtained when the reaction is carried out heterogeneously with mixed nitric and sulfuric acid. However, as expected, greater selectivity is achieved at low temperatures as shown by the reactions with nltronlum salts. The fact that the reaction yields virtually the same product distribution under all conditions supports the view that the same reagent, the nitronium ion, is the effective agent under all conditions. 

A useful feature of this reagent is that high ratios of ortho to para products are found for some substituted aromatics. Finally, it possible to use nitronium-ion salts for nitration. Nitronium fluoroborate has been extensively studied. The trifluoromethanesulfonate salt is also readily prepared and is an active nitrating... 

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