Chapter 15: Alcohols, Diols and Thiols

Reductions of Carboxylic Acids and Esters

Reactions usually in Et₂O or THF followed by H₃O⁺ work-ups

Summary

• Carboxylic acids and esters are less reactive to Nu than aldehydes or ketones
• As a result they can only be reduced by LiAlH₄ but NOT by the less reactive  NaBH₄
• Each reaction requires that 2 hydrides (H^-)  be added to the carbonyl of acids or esters

• Reduction of aldehydes and ketones
• Reaction of RLi and RMgX with aldehydes and ketones
• Reaction of RLi and RMgX with epoxides
• Reaction of RLi and RMgX with carbon dioxide

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 an intermediate metal alkoxide complex.
Step 2: The tetrahedral intermediate collapses and displaces the alcohol portion of the ester as a leaving group, this produces a ketone as an intermediate.
Step 3:  Now we are reducing an aldehyde. 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.

© Dr. Ian Hunt, Department of Chemistry