Bond Properties
C=C and C=O double bonds separated by one or more sp3 carbons have the same properties as isolated double bonds. Cumulated and conjugated ones, however, behave differently, as happens in dienes.
Cumulated double bonds (ketenes)
To explain the structure of a ketene (cumulated double bonds), the central carbon must be considered sp hybridized, not sp2.
The greater s character of the sp hybrid orbitals of the central carbon makes the C=X double bonds shorter than those of ordinary alkenes, aldehydes and ketones.
α,β-Unsaturated carbonyl compounds
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The effect of conjugation between C=C and C=O bonds is energetically and electronically similar to that between two C=C bonds, except for the electron-withdrawing effect of the C=O group.
The hydrogenation of 1,3-pentadiene (conjugated diene) turns out to be less exothermic than that of its 1,4-isomer (isolated diene), which means that the conjugated diene is more stable.
The difference is about 7 kcal/mol.
When 2-butenal (conjugated double bonds) is hydrogenated, the reaction releases less energy than that of its isomer 3-butenal (isolated double bonds).
This reveals that the conjugated 2-butenal is about 6 kcal/mol more stable.
The stabilization gained by conjugation of the C=C and C=O bonds is explained in the same way as that between two C=C bonds.
However, the carbonyl group strongly reduces the electron density of the C=C double bond, which can thus be attacked by nucleophiles, just the opposite of what happens with a "normal" C=C double bond.
In α,β-unsaturated carbonyl compounds, both the conjugation and the electronegative oxygen atom stabilize the π molecular orbitals relative to those of a "normal" alkene.
Consequently, an α,β-unsaturated carbonyl compound reacts through its empty LUMO with the HOMO of a nucleophile, which supplies the electrons. In contrast, a "normal" alkene has its LUMO too high in energy and reacts through its HOMO, donating electrons to the LUMO of an electrophile, which accepts them.