Radical Reactivity: Electrophilic Radicals

An electrophilic radical has an electron-withdrawing group attached to the radical center. This electron-withdrawing substituent lowers the energy of the radical's SOMO (singly occupied molecular orbital), making it electron-deficient and highly reactive toward nucleophilic species like electron-rich alkenes and other electron-donating groups.

Electrophilic radicals react readily with nucleophilic alkenes because of favorable SOMO-HOMO interactions. The low-energy SOMO of the electron-deficient radical interacts effectively with the high-energy HOMO of the electron-rich alkene, driving the reaction forward and stabilizing the transition state between these two reactive species.

The malonate radical, formed from diethyl chloromalonate, reacts quickly with vinyl ether. The malonate radical is electrophilic because it is flanked by two electron-withdrawing groups, while vinyl ether is nucleophilic due to its electron-donating oxygen substituent, making this a favorable and rapid combination for reaction.

Non-carbon-centered electrophilic radicals, such as chlorine radicals, exhibit similar SOMO-HOMO interactions as carbon-centered radicals. A chlorine radical's low-energy SOMO interacts well with the high-energy HOMO of C–H bonds, allowing it to abstract hydrogen atoms from organic molecules like propionic acid effectively.

A chlorine radical preferentially abstracts hydrogen from the terminal methyl group of propionic acid because the low-energy SOMO of the electrophilic chlorine radical interacts favorably with the high-energy HOMO of the terminal C–H bond, making this position the most reactive site for hydrogen abstraction.

Orbital energy is central to electrophilic radical reactivity. Electron-withdrawing groups lower the SOMO energy of the radical, while electron-donating groups raise the HOMO energy of alkenes. This energy difference maximizes orbital overlap and reaction favorability between electrophilic radicals and nucleophilic alkenes.

Electron-withdrawing groups attached to a radical center lower the SOMO energy, creating an electrophilic radical that readily reacts with nucleophilic alkenes and other electron-rich species. This electronic effect makes the radical more electron-deficient and significantly increases its reactivity toward electron-rich reaction partners.