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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?

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    $\begingroup$ To me, the reasoning sounds fair enough. $\endgroup$ Apr 12 at 4:54
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    $\begingroup$ 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. $\endgroup$ Apr 12 at 5:55
  • $\begingroup$ In addition to the hyperconjugation (resonance) effect there is also an inductive (sigma) effect that provides stabilization. See this earlier answer for an explanation. $\endgroup$
    – ron
    Apr 12 at 15:27
  • $\begingroup$ 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 $\endgroup$
    – Ashish
    Apr 13 at 6:47
  • $\begingroup$ @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 $\endgroup$ Apr 13 at 15:34
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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

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  • $\begingroup$ suggestions regarding anything is appreciated $\endgroup$
    – lakshman
    Apr 13 at 5:47

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