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Which is higher in energy? Cyclohexanone enol or cyclohex-2-en-1-ol?

Okay from what I understand a higher pi HOMO is synonymous with more nucleophilic. So I would expect cyclohex-2-en-1-ol to have the higher pi HOMO because in the case of cyclohexanol enol there would be inductive withdrawal by the oxygen connected to one of the sp2 carbons involved in the pi bond.

Or would it be the other way around? Resonance donation by oxygen into the pi bond of the enol makes it more nucleophilic? How can we tell how the HOMO/LUMO energies split?

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from what I understand a higher pi HOMO is synonymous with more nucleophilic.

Yes, that's generally true. If you have a substituent on a double bond that increases the electron density in the double bond, then the double bond (usually one carbon more than the other) becomes more nucleophilic and the energy of the HOMO and LUMO are raised. Conversely, electron withdrawing groups attached to a double bond withdraw electron density, making the double bond less nucleophilic and lowering the energies of the HOMO and LUMO.

Comparing cyclohexanone enol (cyclohex-1-en-1-ol) to cyclohex-2-en-1-ol, you can draw a resonance structure for the former that increases electron density in the double bond (especially the $\beta$ carbon). This electron donating resonance effect overwhelms the electron withdrawing inductive effect of the hydroxyl group. Therefore, the double bond in the cyclohex-1-en-1-ol will be more nucleophilic and its HOMO and LUMO will be higher in energy than those for cyclohex-2-en-1-ol.

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  • $\begingroup$ "Yes, that's generally true." - any exceptions? $\endgroup$ – Dissenter Mar 4 '15 at 19:47
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    $\begingroup$ I'm sure there are many (we're talking about chemistry after all). When a carbonyl is attached to a double bond the double bond becomes less nucleophilic, but sometimes the HOMO will move up - not down - in energy. Conjugation, rings, strain, etc. can sometimes cause HOMO-LUMO movement that doesn't correspond with the generalities presented above. $\endgroup$ – ron Mar 4 '15 at 20:11
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    $\begingroup$ interesting I might open a new question about this $\endgroup$ – Dissenter Mar 4 '15 at 20:13

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