Enols, Enolates and Tautomerism

Chapter 18: Enols and Enolates

Keywords to grasp : enols, enolates and tautomerism
Trying to put it as simply as possible, enols are compounds that have alcohol groups, -OH, substituted directly onto alkenes, C=C, hence "alkene-ols" or enols.
Enols can be viewed a alkenes with a strong electron donating substituent. Since alkenes themselves (review) typically react as nucleophiles, the -OH group makes them even more reactive than a simple alkene.
the structural features of an enol are a C=C with an -OH attached.
Enolates are the conjugate bases or anions of enols (like alkoxides are the anions of alcohols) and can be prepared using a base. enolates are the conjugate bases of enols

CHIME images of a ketone and the corresponding enol and enolate are shown below for comparison:


Both enols and enolates are potential nucleophiles, but enolates are better nucleophiles than enols.... Why ?enolates are negatively charged which increases the nucleophilicity.
electrostatic potential for acetone
Acetone electrostatic potential
When comparing the two electrostatic potentials note the increased electron density on both the O (it's more red) and the C (less blue) in the enolate compared to the ketone. 

(The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density)

electrostatic potential for the enolate of acetone
Acetone enolate electrostatic potential


In general enols are unstable compounds and they are in an equilibrium with a more favorable carbonyl group. This process is known as tautomerism and is catalyzed by both acids and bases.  Try to draw the mechanism for these processes before you look at the answers
In the example shown, 2-propanone ( = acetone) and 2-propenol can be described as tautomers. This is really a specific type of constitutional isomerism. 

If you look at the bond strengths of the C=C and O-H compared to C=O and C-H, you should be able to verify the greater stability of the carbonyl group.

carbonyl to enol equilibrium
the aromatic enol, phenol a hydrogen bonded enol
However, some enols are reasonably stable (usually there is an extra stabilizing effect): for example phenols and 1,3-dicarbonyl compound

Enolates are in principle, capable of reacting as nucleophiles at either the C atom or at the O atom (see below). 
This is shown by the drawing out the main two resonance contributors, see right. 
potential reactions of an enolate
enolate resonance
Which of these two would you rank as being the more important contributor ? the O anion : electrons on the more electronegative atom

In fact, most of the reactions we will cover are reactions of the carbanion contributor !
This shows the merit of considering "minor" resonance contributors since they can be the ones controlling reactivity.

Which is the best nucleophile, a negatively charged C or a negatively charged O ?the C anion : C is less electronegative and is more willing to share the electrons

Mechanism of Tautomerisation

Step 1:
First, an acid-base reaction. The Lewis basic O atom of the carbonyl is protonated by the acid catalyst giving an oxonium ion.
mechanism of acid catalysed tautomerisation
Step 2:
Another acid-base reaction.  Removal of an a-hydrogen by a water molecule functioning as a base allows formation of the C=C and neutralizes the positive charge on the O giving the enol and regenerates the catalyst.
Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic a-hydrogen giving the reactive enolate.
mechanism of base catalysed tautomerisation
Step 2:
The negative charge is resonance stabilized to the more electronegative O atom.
Step 3:
An acid-base reaction. The alkoxide deprotonates a water molecule reforming the catalyst hydroxide and the enol