Kinetics of heterogeneous nitration in emulsions

Abstract

The two-phase nitration of aromatic hydrocarbons has been studied in a model system consisting of a solution of toluene in hexane dispersed as an emulsion in aqueous sulphuric acid, in the concentration range 63–76 wt%. Reaction half-lives were found to range from 400 to 0.4 s over this acid concentration range. The progress of the reaction was monitored by spectrophotometric determination of the nitrotoluenes produced; a stopped-flow apparatus was used for the faster runs. The experimental results are compared with those calculated from a theoretical model. The aqueous acid phase around each organic droplet is subdivided into a number of concentric spherical shells. The change in the amount of toluene in each shell owing to diffusion in and out and to reaction is calculated for a large number of short time intervals, during each one of which the concentration is assumed constant. Good agreement between theoretical and experimental results is obtained except for the fastest runs. For these, diffusion is faster than predicted for stagnant conditions, showing that stirring is occurring within 1 µm of the liquid/liquid interface.