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 Chapter 21: Ester Enolates 
Ch 21 contents
The Claisen Condensation
Claisen condensation of esters
Reaction type : Nucleophilic Acyl Substitution

Summary

Claisen condensation of ethyl ethanoate
Try to identify the enolate portion and the carbonyl portion in the different representations
QUESTION
STUDY TIPS:
  • Students often find this reaction difficult, probably because they are intimidated by it..... break it down into the component steps.
  • Think of it as a nucleophile adding to the carbonyl of the ester as we saw in chapter 20 (review)
  • Try to recognise the two units that came together and identify the nucleophilic (enolate) portion and the electrophilic (carbonyl) portion.
    • The nucleophilic enolate will still contain the ester unit -CO2R.
    • The electrophilic ester will have become a ketone C=O having lost it's -OR group.
  • Recognise the products, the b-ketoester, as being from a Claisen reaction.
taking a Claisen reaction apart to find the pieces

Related Reactions

MECHANISM OF THE CLAISEN CONDENSATION
Step 1:
First, an acid-base reaction. The alkoxide functions as a base and removes the acidic a-hydrogen giving the reactive ester enolate.
mechanism of the Claisen condensation
Step 2:
The nucleophilic ester enolate attacks the carbonyl C of another ester in a nucleophilic substitution process giving the tetrahedral intermediate.
Step 3:
The intermediate collapses, reforming the C=O, resulting in loss of the leaving group, the alkoxide, leading to the
b-ketoester product.

Note that the reaction is drawn to completion by deprotonation of the active methylene in the product by the ethoxide. The salt typically precipitates and is recovered after acid work-up.


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© Dr. Ian Hunt, Department of Chemistry, University of Calgary