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

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

The reaction mechanism is depicted below:

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.

Esters