That $ceC=C$ bonds are more stable at even more substituted carbon atoms is a common dispute provided throughout Clayden et al., yet is never actually described (at least not in the first 21 chapters). They execute define yet, why the HOMO is boosted in energy for even more substituted double bonds, via the debate of electron donation by implies of hyperconjugation of bordering teams, enhancing the electrophilicity of the $ceC=C$ bond.

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I discover this rather counterintuitive. Normally as soon as something is even more reenergetic, it is much less stable (I understand also that of course by kinetic pathmeans this does not always organize, e.g. through $ceI-$ as nucleophile).

How can both observations be included in a continual explacountry in this case?


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edited Oct 2 "15 at 16:29
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Wildcat
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asked Nov 15 "14 at 16:28
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JoriJori
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Let"s start by offering some advantageous background indevelopment that will certainly help answer your question.

1,3-Butadiene is more steady than 2 separate ethylene molecules due to the extended conjugation possible in the diene. We deserve to illustrate this by illustration straightforward resonance frameworks for the diene that can"t be drawn for ethylene.

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Huckel calculations present the very same thing, 1,3-butadiene is more stable than 2 individual ethylenes. Here is the Huckel MO diagram for ethylene. The 2 electrons in ethylene have actually a net stabilization of 2$eta$ compared to an electron in an isolated p-orbital. 4 electrons in 2 ethylenes would have actually an all at once stabilization energy of 4$eta$

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Here is the Huckel diagram for 1,3-butadiene. The 4 electrons have a net stabilization of <(2 x 0.62) + (2 x 1.62)> = 4.48$eta$.

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Even though the HOMO in 1,3-butadiene is higher in energy then the HOMO for ethylene, overall 1,3-butadiene is even more steady than 2 isolated ethylenes by 0.48$eta$. This is because the MO listed below the HOMO is stabilized a lot even more (0.62 $eta$ compared to ethylene).

Now let"s go back to your question. Hyperconjugation is a reasonable way to define why more highly substituted double bonds are even more stable. Just like in 1,3-butadiene, as soon as we delocalize electrons over a bigger number of atoms, which is what hyperconjugation does, an overall stabilization of the molecule must outcome. And, as the hyperconjugated resonance framework mirrors, electron thickness will certainly shift from the alkyl team to the double bond.

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Even though the HOMO may be increased in power, (at leastern some of) the various other populated MO"s listed below the HOMO will be lowered in energy (just prefer in 1,3-butadiene) and also overall a net stabilization of the molecule will certainly result.