Fluorinations, chlorinations and brominations of organic compounds in micro reactors

This paper gives an overview of the application of micro reactors for fluorination and chlorination of organic compounds supplemented by reporting about first investigations by the authors on the topic of bromination reactions in a micro reactor system. After a brief introduction illuminating the status of micro-chemical processing in general and covering the basic advantages of microstructured reactors, the different micro reactors used so far for halogenation reactions will be described with respect to fabrication, range of operation and performance. Thereafter, investigated reaction systems will be described and discussed. All the reactions have in common that halogenation is achieved by using elemental halogens. Beside one gas phase chlorination, the described fluorinations and chlorinations are gas/liquid processes investigated in specialised gas/liquid micro reactors. In contrast, bromination reactions were performed in a micro mixer/tube set-up not specially adopted e.g. to gas/liquid conditions. Phase conditions here are quite complex through evaporated bromine and gaseous hydrogen bromide formed during the reaction. The range of reactions comprises aromatic electrophilic substitutions and free radical substitutions of alkanes and in the side chain of aromatic compounds. The experimental results underpin the benefits of micro reactors for halogenation reactions as improved process control, process safety, improved selectivity and yields, shortening of syntheses and higher space-time yields. Furthermore, another aspect is the potential for an accelerated process development. (C) 2004 Elsevier B.V. All rights reserved.