radical stability

A free radical has a lonely, unpaired electron and badly wants company, which makes it reactive. But some radicals are more comfortable in their lonely state than others, and that comfort, their relative stability, governs which one forms and how a reaction turns out. Radical stability asks: when there is a choice, which radical does nature prefer to make?

The answer mirrors carbocations almost exactly. The order is tertiary more stable than secondary, more stable than primary, more stable than methyl, and the same two effects explain it. First, hyperconjugation: neighboring C-H bonds donate some of their electron density into the half-filled orbital, spreading the unpaired electron out a little, and more attached carbons means more such bonds. Second, the inductive electron-donating effect of attached alkyl groups. On top of that, resonance is a powerful stabilizer: a radical next to a double bond (allylic) or next to a benzene ring (benzylic) can spread its unpaired electron over several atoms, making it far more stable than even a tertiary radical. There is one key difference from carbocations, though: radicals are uncharged, so they do NOT rearrange by hydride or methyl shifts the way carbocations do.

Radical stability is the hidden hand behind selectivity. In bromination the choosy bromine radical pulls off whichever hydrogen makes the most stable carbon radical, which is why tertiary, allylic, and benzylic positions react preferentially. It is also why NBS halogenates the allylic position so cleanly, and why polyunsaturated fats go rancid at their doubly-allylic carbons, where the resulting radical is especially stable.