Pictures of the Day 2-18-08

 
The Enolate Anion: Stabilized by Resonance
Shown above is the electrostatic potential energy surface for an enolate ion. Note how the negative charge, (red color) is distributed over both the alpha carbon atom and the oxygen atom of the enolate. You should be able to draw resonance forms that describe this charge distribution. This delocalization of charge makes this anion more stable than an anion with a localized charge. Thus, carbonyl groups that have an alpha proton are MUCH more acidic than other types of C-H bonds such as alkanes or alkenes. In addition, enolates are very reactive nucleophiles, being able to react at either the alpha carbon atom or the oxygen atom. That is, they are not only highly reactive, they are also "schizophrenic" nucleophiles. Because carbon-oxygen double bonds are stronger than carbon-carbon double bonds, as well as other factors like solvation etc., reactions with electrophiles, especially carbon electrophiles, occur primarily at the carbon atom of the enolate.
Keto-Enol Equilibrium Changes the Personality of the Carbonyl Compound
Even though keto-enol equilibrium strongly favors the keto form, the enol form is important in some reactions like alpha halogenation. The key idea here is that the enol reacts like an alkene with reagents like Br2 or Cl2 to give alpha halogenation. Acid catalyzes the keto-enol equilibrium, so adding acid to these reactions increases the rate of enol formation, and therefore the rate of the reaction.