Reduction of Aldehydes and Ketones to Alcohols
Reduction of Aldehydes and Ketones to Alcohols: Selectivity
Since sodium borohydride is a milder reducing agent than lithium aluminum hydride, only aldehyde or ketone can be reduced selectively (Luche reduction).
Aldehydes are more reactive, because transition of the hybridized carbonyl carbon to the hybridized alkoxy form is sterically less hindered. Therefore, at low temperatures only the carbonyl group of aldehydes is reduced. Because of the ,+I- effect of the two alkyl groups, the oxygen of the keto carbonyl group is a stronger Lewis base than that of the aldehyde. The addition of Lewis acids, such as CeCl3 , generates a more stable complex of the ketone carbonyl resulting in a stronger activated keto group.