20.9 : 自由基反应性：空间效应

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.