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My teacher told me that the hyperconjugation of 9 alpha hydrogen domintes over resonance (for a carbocation )and so the stability of a tertiary carbocation is more than benzyl carbocations due to exceptional hyperconjugation .

I was convinced to the fact and started applying to the questions but then i got stuck and was confused when i saw the stability order of the following groups of free radical . According to me the order told by my teacher for carbocation should still be valid for free radical .but in the following picture benzyl free radical is ahead of tertiary free radical in terms of stability.enter image description here

chemophilic
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    Not entirely sure but from my experience, resonance provides a far better delocalisation of electrons (or absence of them). So I would agree with the image you've presented. – Αντώνιος Κελεσίδης Nov 07 '18 at 13:49
  • this issue has been addressed on ChemSE: https://chemistry.stackexchange.com/questions/26570/how-is-a-free-radical-on-allylic-position-more-stable-than-at-benzylic-position/87653#87653 – user55119 Nov 07 '18 at 20:13

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Yes correct....in case of t-butyl cation it is more stable than benzylic cation due to 9 alpha hydrogen hyperconjugation, 3 methyl gropu inductive effect...it is proved by hydride ion affinity that t-butyl cation have ∆H -231 kcal/mol while benzylic cation have ∆H -234 kcal/mol...if talk about any other tertiary cation excpt t-butyl cation then benzylic cation have more stability....and it is the same case in free redical

KAILASH
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