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Saytzeff rule says that more alkylated alkene is more stable and the reason is hyper conjugation. Going by this, 2,3 Dimethyl but-2-ene should be most stable alkene due to 12-alpha hydrogen. But on drawing hyper conjugative structures of the molecule and a less alkylated alkene, say 2-Methyl propene; there was formation of a tertiary carbanion in 2,3 Dimethyl but-2-ene! and not in 2-Methyl propene. Instead in this molecule there is a primary carbanion which is much more stable than tertiary. I know that there will be 12 hyper conjugative structures in 2,3 Dimethyl but-2-ene rather than 6 structures but isn’t a tertiary carbanion poorly stable and make overall molecule less stable than 2-Methyl propene?

Can anyone please prove me wrong?

Thanks in advance

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There are two types of description of hyperconjugation: (1) Valence bond approach & (2) Molecular orbital approach. Your question is pertaining to valence bond approach as you are interested in the stability of the canonical forms.

In Valence bond approach, hyperconjugation is said to happen due to conjugation of C—H bond electron pair with adjacent double bond. We call this phenomenon 'hyper'-conjugation because there is no real negative charge that is getting delocalized. Rather, the partial negative charge which is accumulated on the carbon due to electronegativity difference with hydrogen is getting delocalized.

In a real carbanion, the carbon has ~100% share of the negative charge. However, in this case, the carbon has only partial share of the negative charge. To be precise, about 7%, because 7% is the ionic character of a C—H bond (Calculated using Hannay-Smith's Relationship for Calculating Percentage Ionic Character [1]).

In case of conjugation of real carbanion, the ~100% negative charge is getting delocalized, and you can give importance to the stability of canonical forms to detrmine whether that conjugation is relevant. However, when ~7% negative charge is getting delocalized, it is more important to give importance to the number of such conjugations, than the relevance of each conjugation based on stability of canonical forms, because anyway, ~7% negative charge won't cause much destabilization even in a tertiary carbanion!

In Molecular orbital approach, hyperconjugation is said to happen due to overlap of bonding σ oribitals of C—H bond with adjacent antibonding π* orbital (in case of double bond). Owing to energy & orientation mismatch, the overlap is minimal and thus stabilization arising from hyperconjugation becomes significant only when the number of hyperconjugations is greater. So, this explains why 2,3-dimethyl-but-2-ene assumes much greater stabilization from hyperconjugation compared to 2-methyl-propene.

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  • $\begingroup$ " carbocation, the ~100% negative charge" - negative? You're mixing up too many things IMO. $\endgroup$ – Mithoron Feb 27 '20 at 16:45
  • $\begingroup$ Corrected! That is carbanion. What else have I mixed? $\endgroup$ – Sujay Ghosh Feb 27 '20 at 16:55
  • $\begingroup$ Also removed reference of hyperconjugation-aided-carbocation-stability for adding clarity to the answer. $\endgroup$ – Sujay Ghosh Feb 27 '20 at 16:58
  • $\begingroup$ Thank you Sujay Ghosh for such a logical explanation! $\endgroup$ – Kartik Aggarwal Feb 28 '20 at 1:19

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