Hydrogen halides, electron scavenging

As to the next step, namely, the reaction of aryl radicals with nucleophiles, we should take into account the fact that air molecular orbital, which initially accommodates the incoming electron, is available in the aryl halide. The electron is subsequently transferred in-tramolecularly from the it to the o molecular orbital of the carbon-halogen bond. Aryl radicals effectively scavenge H atoms. Therefore, an abstraction of a hydrogen atom from the solvent may occur. However, in the case of nucleophiles that can act as effective traps of aryl radicals, the addition of a nucleophile to the phenyl radical takes place. At this point, let us focus on the step of addition of the nucleophile (Y ) to the intermediate radical (Ph). When a new a bond begins to form between the sp3 carbon-centered radical (H5C6) and... 

The byproduct HX can frequently influence the course of reaction, by both re-adding to elimination products and serving as an efficient hydrogen atom donor, and is best removed from the irradiation mixture as it is formed. Triethylamine can be used as a scavenger in nonpolar solvents. In polar solvents, however, it leads to increased formation of the reduction product RH because of competing reaction via the amine-halide exciplex 1 (Scheme 1). The poorer electron donor HO" has been found to be a suitable HX scavenger in alcohoHc solvents. ... 

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