How do you decide the stability of carbon free radical?

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How do you decide the stability of carbon free radical?

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Answer 1 · 2015-08-07T15:59:41

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:

$b e n z y l > a l l y l \geq 3^{\circ} > 2^{\circ} > 1^{\circ} > m e t h y l$ $benzyl > allyl >= 3^@ > 2^@ > 1^@ > methyl$

Allyl and benzyl radicals are stabilized by resonance with the $π$ $pi$ 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 $C - H$ $C-H$ bond increases, the stability of the radical decreases , i.e. $s p^{3} > s p^{2} > s p$ $sp^3 > sp^2 > sp$ ; i.e. acetylenic radicals should be the least stable.

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How do you decide the stability of carbon free radical?

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