Which of the following carbocations is more stable?

Question

Which of the following carbocations is more stable?

I know two major factors in determining carbocation stability: resonance and the number of carbon atoms neighboring the positive carbon. Does the double bond in the substituent of the first molecule participate in resonance?

The one with the aliphatic C=C bond may be more stable since there may neighbouring group participation, allowing dispersal of positive charge — Tan Yong Boon, Oct 17 '19 at 06:23
@TanYongBoon There is an $\mathrm{sp^3}$ carbon in-between which breaks resonance so the shorter chain allows for more hyperconjugation. — Jan, Oct 17 '19 at 06:39
@Jan I am not talking about resonance. When I mean "neighbouring group participation", I am referring to the $\pi$ electron density being donated directly to the positively charged carbon atom, forming a cyclopropyl or cyclobutyl structure — Tan Yong Boon, Oct 17 '19 at 06:54
@TanYongBoon Oh, I think I see what you mean; analogous to a metal-ethene complex? If that’s the case, I’m not sure whether the single $\ce{CH2}$ group provides enough flexibility but I would have to build a model. — Jan, Oct 17 '19 at 06:56
@TanYongBoon that mechanism would require a "six membered ring". In term or charge stabilisation/relative stability the smaller carbocation should be favoured. — Alchimista, Oct 17 '19 at 09:45
@TanYongBoon actually now I feel stupid. The cation on the top is just the linear form of a cyclopropylmethyl cation … *head meets table* — Jan, Oct 17 '19 at 14:57
@Jan Yes, that is one possible stable structure that can be derived from the "neighbouring group participation" I suggested — Tan Yong Boon, Oct 17 '19 at 14:59
Are we talking about: What is the reason for the exceptional stability of the cyclopropylmethyl carbocation? — Martin - マーチン, Oct 17 '19 at 15:49

score 1 · Answer 1 · answered Oct 18 '19 at 03:55

The second one will be more stable. We usually first check for resonance as it is more effective. But here it has to be explained on basis of hyper conjugation because the resonance due to the ring is same in both. But the second one will have more hyper conjugating structures as greater number of alpha hydrogens are present. More about hyper conjugation concept: https://en.wikipedia.org/wiki/Hyperconjugation

Edit : No the double bond wont participate in resonance.

score 0 · Answer 2 · answered Oct 18 '19 at 03:46

The Second One Will be More Stable as it has one of the resonating structure with octet configuration that is when the the double bond of the ring comes out and participates in the formation of double bond with the carbocation which is out of ring in no case the double bond from the first molecule will be involved in resonance ....

Which of the following carbocations is more stable?

2 Answers2