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):
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 :
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.