Redox Reactions of Aldehydes and Ketones

Reduction of aldehydes with lithium aluminum hydride or sodium borohydride will yield primary alcohols. Reduction of ketones with lithium aluminum hydride or sodium borohydride will yield secondary alcohols.

There are several reactions (Clemmensen, Wolff-Kishner, desulfurization) that will reduce aldehydes and ketones to an alkane. However, alkanes are mostly nonreactive. Therefore, there is a need for a reaction that will reduce the carbonyl group but leave behind a functional group with more reactivity than an alkane. Metal hydride reductions of aldehydes and ketones will generate primary alcohols and secondary alcohols, respectively. Metal hydrides are compounds such as lithium aluminum hydride (commonly referred to as LAH) and sodium borohydride.

While the reaction conditions for using lithium aluminum hydride and sodium borohydride are not identical, the mechanism by which they react with carbonyls is fundamentally the same. The nucleophilic hydride ion donates its electron pair to the electrophilic carbon of the carbonyl. This donation causes the $\pi$ bond between the carbon and the oxygen to break, which results in the shared electrons moving to the oxygen of the carbonyl and the formation of an alkoxide intermediate. Protonation of the alkoxide in a separate step generates the final alcohol product. It is important to note that hydride ions (H) are negatively charged and behave as nucleophiles, while protons (H+) are positively charged and behave as electrophiles.

Metal Hydride Reactions

Another important nucleophilic addition reaction is the Baeyer-Villiger oxidation reaction. The Baeyer-Villiger oxidation is a reaction using peroxy acids or peroxides as the oxidant that cleaves the ${ \rm{C{-}C}}$ bond next to a carbonyl, converting aldehydes to carboxylic acids and ketones to esters. This reaction uses peroxy acids to convert aldehydes or ketones to carboxylic acids or esters, respectively. The unique feature of peroxy acids is they contain a ${-}{\rm{C(O){-}O{-}O{-}H}}$ group. The mechanism by which the reaction proceeds is very similar to the acid-catalyzed nucleophilic addition.