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Which of the following structures would be least stable?

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I initially approached it like this:

As both are allylic carbocations, and that both are in fact resonating structures of the same molecule, so resonance can't be used to judge them. So I went for hyperconjugation. The first molecule has four alpha hydrogens and the second has only one alpha hydrogen.

Since there is more hyperconjugation in the first, it should be more stable. But I also have an intuition that there might be something more that we should have a look upon but I'm not able to figure it out. Should we always choose hyperconjugation as our second option when we can't make judgements using resonance?

ron
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Prajwal Tiwari
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    To me, the reasoning sounds fair enough. – V.G Apr 12 '21 at 04:54
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    Well, the alkenes have the same degree of substitution, resonance can be ruled out, all that's left is, indeed, hyperconjugation. And you have compared the two correctly. That should be it. – TheRelentlessNucleophile Apr 12 '21 at 05:55
  • In addition to the hyperconjugation (resonance) effect there is also an inductive (sigma) effect that provides stabilization. See this earlier answer for an explanation. – ron Apr 12 '21 at 15:27
  • Will we only be checking for hyperconjugation stability of the cation or should we also check for stability of double bond. I think in general 2-ene are more stable than 1-ene cause of more alpha hydrogen, but here we have a cation too so not sure which one to choose – Ashish Apr 13 '21 at 06:47
  • @Ashish yes! that's what we're doing when we are accounting for hyperconjugation here. More the number of alpha hydrogens, more would be the stability – Prajwal Tiwari Apr 13 '21 at 15:34
  • @PrajwalTiwari but we're doing that for cation at the cost of alpha hydrogens of the double bond. So can we say that cation stabilization has more dominance here ? – Ashish Apr 13 '21 at 16:54

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My teacher says that while comparing the stability of the species concerned, we should remember that stability provided by Resonance>hyper conjugation>inductive effect. Of course there are exceptions(like in the case of chloro benzene where according to resonance the ring should be activated but inductive effect dominates it and the ring gets deactivated) to these but I think we can follow it in majority of these cases and there is no reason to not be confident about it.So likewise in your case resonance is not enough so you look upon the next one i.e hyper conjugation

lakshman
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