Ester Enolates Essay

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Overview of Ester Enolate reactions | Chapter 21: Ester Enolates | The most important reactions of ester enolates are the Claisen and Dieckmann condensations. However, ester groups are common as part of active methylene enolates. Remember that enolates are sources of nucleophilic C that react with electrophiles. | Claisen condensation | | Dieckmann condensation | The Claisen Condensation Reaction type : Nucleophilic Acyl Substitution Summary * The Claisen condensation is the ester analogue of the Aldol condensation. * Reagents : most commonly the base would be the alkoxide, R'O- * The reaction involves an ester enolate reacting with another molecule of the ester. * Remember enolates are good nucleophiles and the ester carbonyl C are electrophilic. * The products of these reactions are ketoesters which are important, useful synthetic intermediates. * Note that the product is the original ester with an acyl group added i.e. an acylation reaction has occurred. * A simple example of a Claisen condensation is the reaction of ethyl ethanoate which is shown below in two representations: | | | Try to identify the enolate portion and the carbonyl portion in the different representations | | Highlight enolate component Nu Highlight carbonyl component E | Question Methanoate esters can not undergo Claisen self condensations. Why ? 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.

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