Alkane Chlorination Investigated in Micro Reactors

Chlorine molecules are cleaved at high temperatures by photoinduced radical formation. By this means, a gas/liquid reaction can be performed in the side chain of alkyl aromatics quite selectively. The electrophilic ring substitution, instead, is favored using Lewis catalysts in polar solvents at low temperature. 

Via such a gas/liquid reaction, toluene-2,4-diisocyanate reacts with chlorine to give l-chloromethyl-2,4-diisocyanatobenzene [6], As a ring-substituted side product, toluene-5-chloro-2,4-diisocyanate is formed in minor quantities. 

Typically, the reaction mechanism proceeds as follows [6], By photoreaction, two chlorine radicals are formed. These radicals react with the alkyl aromatic to yield a corresponding benzyl radical. This radical, in turn, breaks off the chlorine moiety to yield a new chlorine radical and is substituted by the other chlorine, giving the final product. Too many chlorine radicals lead to recombination or undesired secondary reactions. Furthermore, metallic impurities in micro reactors can act as Lewis catalysts, promoting ring substitution. Friedel-Crafts catalyst such as FeClj may induce the formation of resin-Uke products. 

Conversion/selectivity/yield - by-product analysis - benchmarking 

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