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Ester Reactions: Ester Reduction to form Primary Alcohols using LiAlH4

Primary alcohols can be formed from esters using a strong reducing agent called Lithium aluminum hydride (abbreviated as LiAlH4):

Ester Reactions: Ester Reduction to form Primary Alcohols using LiAlH4 image2.png

This reaction is known as a reduction reaction. The ester molecule is cleaved at the “ether” oxygen, resulting in the formation of a primary alcohol. If the starting ester is part of a ring, the ring will be hydrolyzed and the resulting molecule will be have both two alcohol groups :

Intramolecular Reduction

Ester Reactions: Ester Reduction to form Primary Alcohols using LiAlH4 image3.png

The reaction mechanism is depicted below:

Ester Reactions: Ester Reduction to form Primary Alcohols using LiAlH4 image1.png

In the first step, the hydride from the LiAlH4 molecule attacks the ester-carbon, breaking the double bond and sending those electrons to the oxygen.

In the second step, the free electrons from the negatively charged oxygen come back down and reform the carbon-oxygen double bond. The ether carbon-oxygen bond breaks allowing the ether-oxygen group to leave the molecule.

In the third step, a second equivalent of LiAlH4 approaches and again, the hydride from LiAlH4 attacks the now aldehyde-carbon, sending the electrons to the oxygen atom.

In the fourth step, the lone pair electrons attack a nearby H3O+ molecule, removing the proton and ending the reaction.

This reaction is conducted using a reducing agent known as LiAlH4 in an acidic environment (H3O+, H2SO4/H2O).