How do you decide the stability of carbon free radical?

1 Answer
Aug 7, 2015

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


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.