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Chapter 21: Ester Enolates



Summary | Enolates | Reactions of Ester Enolates | Reactions of Other Enolates | Self Assessment | Quiz |


Ester Enolates

Chapter 21: Ester Enolates

In Chapter 18 we introduced the enolates of aldehydes and ketones (review) and looked at their reactions as C nucleophiles (review).  These enolates were formed by treating the aldehyde or ketone with a suitable base :

formation and resonance stabilisation of a ketone enolate

Now we will investigate another group of carbonyl containing compounds, the esters, which behave in a very similar fashion.....

When preparing ester enolates, the base is normally chosen to match the alcohol portion of the ester...
So if you have:
a methyl ester, RCOOCH3 you use methoxide, -OCH3, or,
an ethyl ester, RCOOCH2CH3 you use ethoxide, -OCH2CH3

This avoids problems caused by transesterification (conversion of one ester into another)......

transesterification
or hydrolysis......
hydrolysis

Note that in both cases the problem arises because the base reacts as a "nucleophile"


  Common Bases for preparing enolates
 
Active Methylenes
acidity of an active methylene system
Some common active methylene compounds are shown below as CHIME images.... make sure you can see the acidic active methylene hydrogen atoms.....
Highlight acidic H
Highlight acidic H
Highlight acidic H

As we will see later is this Chapter, these compounds are useful synthetic intermediates.
Their enolates can be formed readily and can be alkylated, then further manipulated.

Acidity of a-Hydrogens

pKa data for various systems   active methylene compound acidity

Why are the protons adjacent to carbonyl groups acidic?
As we have advocated before, look at the stabilization of the conjugate base.
Notice the proximity of the adjacent p system, and hence the possibility for RESONANCE stabilization by delocalization of the negative charge to the more electronegative oxygen atom.

stabilisation of the conjugate base by resonance

The more effective the resonance stabilization of the negative charge, the more stable the conjugate base is and therefore the more acidic the parent system.

Let's compare pKa of the common systems: aldehyde pKa = 17, ketone pKa = 19 and an ester pKa = 25, and try to justify the trend.

comparing aldehydes and ketones
resonance in an ester enolate
The difference in the 3 systems is in the nature of the group attached to the carbonyl.  The aldehyde has a hydrogen, the ketone an alkyl- group and the ester an alkoxy- group.

H atoms are regarded as having no electronic effect : they don't withdraw or donate electrons.

Alkyl groups are weakly electron donating, they tend to destabilize anions (you should recall that they stabilize carbocations).
This is because they will be "pushing" electrons towards a negative system which is unfavorable electrostatically.
Hence, the anion of a ketone where there are extra alkyl groups is less stable than that of an aldehyde, and so, a ketone is less acidic.

In the ester, there is also a resonance donation from the alkoxy group towards the carbonyl that competes with the stabilization of the enolate charge. This makes the ester enolate less stable than those of aldehydes and ketones so esters are even less acidic.
 
 


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