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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 :
Now we will investigate another group of carbonyl containing compounds, the esters, which behave in a very similar fashion.....
This avoids problems caused by transesterification (conversion of one ester into another)......
Note that in both cases the problem arises because the base reacts as a "nucleophile"
As we will see later is this Chapter, these compounds are useful synthetic
Their enolates can be formed readily and can be alkylated, then further manipulated.
Acidity of a-Hydrogens
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.
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.
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.