Reactions at Solid-Liquid Interfaces. The Mechanism and Kientics of the Fluorination of 2,4-Dinitrochlorobenzene Using Solid Potassium Fluoride in Dimethylformamide

摘要

The halogen exchange reaction of 2,4-dichloronitrobenzene with potassium fluoride in dimethylformamide containing tetrabutylammonium salts has been studied employing an electrochemical detectin methodology based upon the use of square wave voltammetry to follow the loss of reactant and the formation of the product and intermediates. The results obtaiend show that the kientics of los of parent material behave on one hand as a dissolution-rate-controlled procedss and on the other as a homogeneous chemical process. Initially homogeneous reaction dominates the observed kinetics as the presaturated solution is stripped of fluoride ion; at longer time, the observed kinetics are controled by the rate of KF dissolution. Modeling the system using a fully implicit finite difference method with Richtmyer modifiecation (FIRM algorithm) yielded a mean value for the homogeneous rate constant for the formation of 2,4-dinitrofluorobenzene by reaction of 2,4-dichloronitrobenzene with fluoride ion in DMF at 85deg C of 640+-250 mol~(-1) cm~3 s~(-1) and a mean value for the saturation concentration of fluoride ion of (6.5+-0.5)X10~(-6) mol cm~(-3). Ultrasound was found not to significantly enhance the rate of the reaction in the intensity range studied. Furthermore, the utility of microelectrodes for obtaining simple quantifiabe voltammetric responses from compounds of which the macroelectrode resonses are complicated by chemical followup steps is demosntrated. Ultrasonically induced mixing has been shown to facilitate reproducible microelectrode responses in intrinsically heterogeneous systems.