20.11 : Radical Reactivity: Electrophilic Radicals

Radicals adjacent to electron‐withdrawing groups are called electrophilic radicals. These radicals readily react with nucleophilic alkenes. For example, the malonate radical, in which the radical center is flanked by two electron‐withdrawing groups, reacts readily with butyl vinyl ether, which consists of an electron‐donating oxygen substituent. The reaction between electrophilic malonate radical and nucleophilic vinyl ether is favored because the radical has a low‐energy SOMO, which interacts best with the high‐energy HOMO of the alkene.

The non‐carbon‐centered electrophilic radicals also exhibit similar SOMO–HOMO interactions. For instance, the non‐carbon‐centered chlorine radical abstracts a hydrogen atom from the terminal methyl group of propionic acid. This is because chlorine radical has a low‐energy SOMO, and the C–H bond of the terminal methyl group has a high‐energy HOMO. Therefore, interactions between low‐energy SOMO of the chlorine radical and high‐energy HOMO of the terminal C–H bond favor chlorine attack on the terminal carbon of propionic acid.