15.2 : 烯醇离子的反应性

The α hydrogens of carbonyls are acidic in nature, as their conjugate bases are resonance-stabilized. So, enolate ions are often more useful in reactions than enols are, because of their solution stability and better nucleophilicity.

Like the allylic anion, the enolate ion behaves as a three-atom four-electron conjugated system. Replacing carbon with oxygen lowers the energy of the two occupied π molecular orbitals and distorts them.

In the lowest occupied π orbital, electrons are delocalized across the three constituent atoms, with greater contribution from the electronegative oxygen. This additional overlap and stabilization makes carbonyl α hydrogens more acidic than allylic hydrogens of alkenes.

The HOMO has a significant α-carbon character, with a node at the carbon–oxygen bond. While reactions dominated by interaction with the HOMO tend to occur on the α carbon, leading to α-substitution, those driven by electrostatic interactions tend to occur on oxygen, forming enol derivatives.

Since enolates have two distinct reactive nucleophilic centers, they are ambident nucleophiles.