Chapter 18: Enols, Enolates and Tautomerism

Chapter 18: Enols and Enolates

Enols, Enolates and Tautomerism

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:


ketone	enol	enolate

Question:
Both enols and enolates are potential nucleophiles, but enolates are better nucleophiles than enols.... Why ?

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 favourable carbonyl group. This process is known as tautomerism and is catalysed 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.


However, some enols are reasonably stable (usually there is an extra stabilising effect): for example phenols (aromaticity) and 1,3-dicarbonyl compound (hydrogen bonding and conjugation of C=O and C=C)

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

Question:
Which of these two would you rank as being the more important contributor ?

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.

Question:
Which is the best nucleophile, a negatively charged C or a negatively charged O ?

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.
Enolates are the conjugate bases or anions of enols (like alkoxides are the anions of alcohols) and can be prepared using a base.