20.11: 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.