Chapter 21: Ester Enolates

Summary

• The enolates of active methylenes are easily prepared using a base (such as ethoxide, EtO^-)
• These enolates are good nucleophiles and react with alkyl halides via S_N2 type reactions.
• This allows alkyl groups to be introduced in the a-positions.
• Since the reaction is an S_N2 reaction, methyl and primary halides are most suitable for alkylation reactions.
• In principle both of the a-H can be replaced with alkyl groups:

• This can be utilised to form a cyclic system by using a dihalide:

Related Reactions

• Other substitution reactions of alkyl halides
• Alkylation of other Enolates

MECHANISM OF ALKYLATION
Step 1: First, an acid-base reaction. The ethoxide functions as a base and removes an acidic a-hydrogen from between the two carbonyl groups giving the reactive enolate.
Step 2: The nucleophilic enolate attacks the alkyl halide at the electrophilic carbon carrying the halogen via an SN2 type process giving the alkylation product and a bromide ion.

© Dr. Ian Hunt, Department of Chemistry