Aldehyde Reactions: Formation of Alcohol from Aldehyde, Ketone using LiAlH4

Primary and secondary alcohols can be formed from aldehydes and ketones (respectively) using lithium aluminum hydride (LiAlH4):

This is a straightforward reaction. Visually, change the double bond to a single bond, add a H atom to the O, and the reaction is complete. If studying for a test or the MCATs, this reaction is the same as NaBH4/MeOH (methanol) and represents one of two ways to reduce aldehydes/ketones:

LiAlH4/NaBH4 Example

The reaction mechanism is depicted below:

In the first step, the hydrogen-aluminum bond breaks forming a hydride ion, which attacks the carbon atom that is part of the aldehyde/ketone functional group. This breaks the double bond between carbon and oxygen, sending the electrons to the oxygen atom.

In the second step, the newly acquired lone pair electrons on the oxygen atom attack a nearby H3O+ molecule, stripping away its proton, finishing the reaction.

The reagents used for this reaction are LiAlH4 and usually some sort of acid workup (H+, H3O+, H2SO4).