20.9:
Radical Reactivity: Steric Effects
Recall that the presence of electron-donating, electron-withdrawing, and conjugating groups adjacent to a radical center results in electronically-stabilized radicals.
Some of these electronically-stabilized radicals are known as persistent radicals. These radicals are remarkably stable; some can even be isolated and purified.
In addition to the electronic factors, steric factors also contribute to the stability of these persistent radicals.
For example, consider the triphenylmethyl radical. The exceptionally high stability of this radical is due to the presence of three phenyl rings.
The three surrounding rings are twisted out of the plane by 30° in a propeller-like conformation.
As a result, the central carbon bearing the radical character is sterically shielded by the twisted phenyl rings, making the radical highly stable and unavailable for reactions.
Overall, steric effects make a radical more stable and less reactive.
20.9:
Radical Reactivity: Steric Effects
The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
Along with electronic factors, steric factors also account for the stability of these persistent radicals. For instance, the exceptionally high stability of triphenylmethyl radical is due to the presence of three surrounding phenyl rings that are twisted out of the plane by 30° in a propeller conformation. These twisted phenyl rings sterically shield the central carbon, which bears most of the radical character. As a result, the radical becomes highly stable and unavailable for molecules to react. Therefore, steric hindrance imparts stability to the radicals making them less reactive.