Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

Hydride Reductions of Aldehydes and Ketones
(review of Chapter 15)

Reaction type:  Nucleophilic Addition

Summary

• Aldehydes and ketones are most readily reduced with hydride reagents.
• The reducing agents LiAlH₄ and NaBH₄ act as a source of 4 x H^- (hydride ion).
• Overall 2 H atoms are added across the C=O to give H-C-O-H.
• Hydride reacts with the carbonyl group, C=O, in aldehydes or ketones to give alcohols.
• The substituents on the carbonyl dictate the nature of the product alcohol.
• Reduction of  methanal (formaldehyde) gives methanol.
• Reduction of other aldehydes gives primary alcohols.
• Reduction of ketones gives secondary alcohols.
• The acidic work-up converts an intermediate metal alkoxide salt into the desired alcohol via a simple acid base reaction.

• Reaction of RLi and RMgX with aldehydes and ketones

NUCLEOPHILIC ADDITION OF LiAlH4 TO AN ALDEHYDE
Step 1: The nucleophilic H in the hydride reagent adds to the electrophilic C in the polar carbonyl group in the aldehyde, electrons from the C=O move to the O creating an intermediate metal alkoxide complex.  (note that in principle all 4 of the H atoms can react)
Step 2: 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