Reduction to Hydrocarbons
(review of Chapter 12)

• Zn(Hg) in HCl reduced the C=O into -CH₂-

• NH₂NH₂ / KOH / ethylene glycol (a high boiling solvent)  reduces the C=O into -CH₂-

• These reduction methods do not reduce C=C , CºC or -CO₂H
• The choice of method should be made based on the tolerance of other functional groups to the acidic or basic reaction conditions.

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

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

Summary

• Aldehydes and ketones are most readily reduced with hydride reagents.
• The reducing agents LiAlH₄ and NaBH₄ act as a source of 4H^- (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 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.