Question

How do you decide the stability of carbon free radical?

Answer 1

The question should read "how do you rationalize the stability of a carbon free radical? " Radical stability mirrors the stability of the carbocation.

Explanation:

The longer lived the radical, the more opportunity there is for radical coupling (and skeletal rearrangement), but a few general rules may be applied:

`benzyl > allyl >= 3^@ > 2^@ > 1^@ > methyl`

Allyl and benzyl radicals are stabilized by resonance with the `pi` system; tertiary and secondary radicals are possibly stabilized by hyperconjugation. Interestingly, I was once told by an organic prof that as the percentage s character of the `C-H` bond increases, the stability of the radical decreases , i.e. `sp^3 > sp^2 > sp`; i.e. acetylenic radicals should be the least stable.