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Ester Reactions: Ester Reduction to form Aldehydes using DIBAH

Aldehydes can be formed from esters using a strong reducing agent called Diisobutylaluminum hydride (abbreviated as DIBAH or DIBAL):

Ester Reactions: Ester Reduction to form Aldehydes using DIBAH image1.png

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

Intramolecular Reduction

Ester Reactions: Ester Reduction to form Aldehydes using DIBAH image2.png

The reaction mechanism is depicted below:

Ester Reactions: Ester Reduction to form Aldehydes using DIBAH image3.png

In the first step, the free electrons from the double bonded oxygen atom bind to the aluminum atom on the DIBAH molecule..

In the second step, the carbon-oxygen double bond is broken, sending the free electrons to the oxygen. This leaves the carbocation available for attack; a hydride shift occurs and the H atom bonded to the aluminum shifts to the carbon atom..

In the third step, the lone pair electrons from a nearby water molecule attack the aluminum and bond to it..

In the fourth step, the oxygen-aluminum bond is broken, sending the aluminum moiety off the alkane chain.

In the fifth step, the now negative oxygen atom reforms the double bond with carbon, breaking the oxygen-carbon ether bond, removing that group from the molecule.

This reaction is conducted using a reducing agent known as DIBAH or DIBAL. Sometimes the reaction will specify the temperature:

If the temperature is specified as a low temperature (-°C), this reaction will proceed as described above.

If the temperature is not specified (or specified as a hotter temperature), this reaction can further reduce the aldehyde to an alcohol, similar to LiAlH4.