20.11
Radical Reactivity: Electrophilic Radicals
Radicals adjacent to electron‐withdrawing groups are called electrophilic radicals. These radicals readily react with nucleophilic alkenes. For exampl…
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.
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Q1: What makes a radical electrophilic?
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.
Q2: Why do electrophilic radicals react readily with nucleophilic alkenes?
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.
Q3: What is an example of an electrophilic radical reacting with a nucleophilic alkene?
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.
Q4: How do non-carbon-centered electrophilic radicals behave?
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.
Q5: Why does a chlorine radical attack the terminal carbon of propionic acid?
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.
Q6: What role does orbital energy play in electrophilic radical reactivity?
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.
Q7: How do electron-withdrawing groups affect radical reactivity?
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.