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 Chapter 20: Carboxylic Acid Derivatives. NucleophilicAcyl Substitution 
Ch 20 contents

Reduction of Esters
(review of Chapter 15)

reduction of carboxylic acids using LiAlH4
Reactions usually in Et2O or THF followed by H3O+work-ups

Reaction type:  Nucleophilic Acyl Substitution then NucleophilicAddition

Summary

MECHANISM OF THE REACTION OF LiAlH4 WITH AN ESTER

Step 1:
The nucleophilic H from the hydride reagent adds to the electrophilic C in the polar carbonyl group of the ester. Electrons from the C=O move to the electronegative O creating the tetrahedral intermediate a metal alkoxide complex.
reduction of an ester using hydride
Step 2:
The tetrahedral intermediate collapses and displaces the alcohol portion of the ester as a leaving group, in the form of the alkoxide, RO-. This produces an aldehyde as an intermediate.
Step 3: 
Now we are reducing an aldehyde (which we have already seen)
The nucleophilic H from the hydride reagent adds to the electrophilic C in the polar carbonyl group of the aldehyde. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex. 
Step 4:
This is the work-up step, a simple acid/base reaction. Protonation of the alkoxide oxygen creates the primary alcohol product from the intermediate complex. 

 


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