Kinetic Studies of Nitration with Nitronium Fluoroborate

Abstract

Using nitronium fluoroborate as a nitrating agent, rates of nitration of nitrobenzene to m-dinitrobenzene were measured in methanesulfonic acid, in sulfuric acid and in acetonitrile. For comparison, the nitration rates were also determined with nitric acid in each of these solvents and with anhydrous nitric acid alone. The reaction was followed by polarographic analysis of quenched samples. Conditions for purification of the fluoroborate, analysis and preparation of kinetically reproducible methanesulfonic acid are described. The reaction is first order in nitrobenzene and first order in the nitrating agent (except for HNO_3 in CH_3SO_3H). Second order rate constants are about equal for the two nitrating agents in sulfuric acid and, when the HNO_3 data are extrapolated to zero HNO_3, in methanesulfonic acid. The rate constants in the two solvents are also comparable in magnitude. When a second order rate constant in anhydrous nitric acid is computed from the estimated NO_2^+ concentration, its value at - 13° is about the same as those observed in the above acidic solvents at 25°, and its approximate value at room temperature is appreciably greater. In acetonitrile, the rate constant with NO_2BF_4 is much less, in qualitative agreement with the negligible content of free NO_2^+ in this medium as deduced from infrared spectra. The nitration rate with HNO_3 in acetonitrile is negligible. The study extends somewhat the range of solvents suitable for the investigation of medium effects on the attack of aromatics by NO_2^++.