Nitrations with solutions of nitronium salts in organic

4 NITRATIONS WITH SOLUTIONS OF NITRONIUM SALTS IN ORGANIC SOLVENTS 

1 Preparation and properties of nitronium salts The first preparation of a nitronium salt by Hantzsch, who isolated the perchlorate mixed with hydroxonium perchlorate, and some of the subsequent history of these salts has already been recounted ( 2.3.1). 

Nitronium tetrafluoroborate was first prepared by adding a mixture of anhydrous hydrofluoric acid and boron trifluoride to a solution of dinitrogen pentoxide in nitromethane. Nitric acid can be used in place of dinitrogen pentoxide, and by replacing boron trifluoride by other Lewis-acid fluorides Olah and his co-workers prepared an extensive series of stable nitronium salts.  

Nitronium salts are colourless, crystalline and very hygroscopic nitronium perchlorate and sulphate are unstable and liable to spontaneous decomposition, whereas nitronium tetrafluoroborate and other complex fluoro-salts are relatively stable. 

Cryoscopic investigations suggest that in sulpholan nitronium tetrafluoroborate exists predominantly as ion pairs. - The specific conductivity of these solutions increases linearly with the concentration of the salt (up to 0-4 mol 1 ), and is attributed to the existence of ion-triplets rather than free ions.  

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