Ch15: Reduction Of Aldehydes And Ketones - Chemistry

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Chapter 15: Alcohols, Diols and Thiols

Reductions of Aldehydes and Ketones

reduction of aldehydes and ketones using LiAlH4 or NaBH4 Reactions usually in Et2O or THF followed by H3O+ work-ups

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.

Related Reactions

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 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.