20.11:
Radical Reactivity: Electrophilic Radicals
Electrophilic radicals have an electron-withdrawing group attached to the radical center. These radicals readily react with nucleophilic alkenes.
For instance, the malonate radical, formed from diethyl choloromalonate, reacts quickly with vinyl ether, which possesses an electron-donating oxygen substituent.
This reaction is favored because the electron-deficient, electrophilic radical has a low-energy SOMO, which readily interacts with the high-energy HOMO of the electron-rich, nucleophilic alkene.
Similar SOMO-HOMO interactions are also observed for non-carbon-centered radicals.
For instance, when a chlorine radical reacts with propionic acid, it abstracts a hydrogen from the terminal carbon.
This happens because the low-energy SOMO of the electrophilic chlorine radical interacts well with the high-energy HOMO of the C–H bond of the terminal methyl group.
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.