Modern molecular modeling and instrumentation allow us to return to radical halogenation reactions with a new perspective. This research revisits these reactions in order to further understand underlying processes that may have been overlooked. The current belief that halogen radicals prefer hydrogen abstraction at electron rich positions away from electronegative substituents does not accurately explain the product ratios we have analyzed using capillary gas chromatography. One cannot only consider the inductive electron withdrawal of electronegative substituents when interpreting radical halogenation product ratios. We argue that alpha position halogen orbital interaction with radicals has a significant impact on their stability, greatly affecting hydrogen abstraction. The formation of bridged halogen radical intermediates due to beta position radical stabilization has been proposed by many; new experimental data and spin density calculations using Spartan Molecular Modeling support these claims. Product distributions of radical bromination reactions performed on chloroalkanes show a significant increase in bromine substitution at the carbon alpha to chlorine atoms. This suggests that chlorine stabilizes radicals formed via hydrogen abstraction by bromine radicals at this position. This stabilization is specific to carbon radicals formed by bromine radical hydrogen abstraction and is not observed in the chlorination of bromoalkanes, chlorination of chloroalkanes or bromination of bromoalkanes. Previous research is consistent with our observations but fails to explain the mechanisms by which they are occurring.

Effect of chloro-stabilization on free radical halogenation

253rd National Meeting (2017)

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