Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl 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 Nucleophilic Addition

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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