Answer to Question #171244 in Organic Chemistry for Tori

Question

Lithium aluminum hydride (LiAlH₄) readily reduces the C=O double bond of aldehydes and ketones to the corresponding alcohols whereas the reduction of alkenes is unfavorable. Briefly state the reason for the differential reactivity.

Answer 1

Mechanism of Reduction of Aldehydes or Ketones to 1⁰ or 2⁰ alcohols: Initially, a hydride ion is transferred onto the carbonyl carbon and the oxygen atom coordinates to the remaining aluminium hydride species to furnish an alkoxytrihydroaluminate ion, which can reduce the next carbonyl molecule. Thus three of the hydride ions are used up in reduction.

Lithium aluminium hydride, LAH reagent cannot reduce isolated non-polar multiple bonds like C=C because these cannot attack an electron-rich carbon-carbon double bond, or be attacked by a pair of negative electrons from a pi orbital. However, the double or triple bonds in conjugation with the polar multiple bonds can be reduced.