Transesterification is the process of exchanging the alkoxy group of an ester by another alcohol. These reactions are often catalyzed by the addition of an acid or base.
O O
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C - CH3 + ROH <---> C - CH3 + CH3OH
/ /
CH3O RO
(ester + alcohol <---> different ester + different alcohol)
Transestrification is crucial for producing biodiesel from vegetable oils.