|| Chapter 17: Aldehydes
and Ketones. Nucleophilic Addition to C=O
Hydride Reductions of
Aldehydes and Ketones
(review of Chapter 15)
Reactions usually in Et2O or THF followed by H3O+
Reaction type: Nucleophilic
- 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)
- Reduction of other aldehydes
gives primary alcohols.
- Reduction of ketones gives secondary
- The acidic work-up converts an
intermediate metal alkoxide salt into the desired alcohol via a simple acid
ADDITION OF LiAlH4 TO AN ALDEHYDE
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)
This is the work-up step, a simple acid/base reaction. Protonation
of the alkoxide oxygen creates the primary alcohol product from the intermediate
Ian Hunt, Department
of Chemistry, University of Calgary