transesterification
Summary
Transesterification is the conversion of one ester into another by swapping the alkoxy group (OR′) for a different alcohol's OR″, under acid or base catalysis. It is a nucleophilic acyl substitution, and an excess of the new alcohol drives the equilibrium toward the new ester.
Detail
Meaning: An ester RCOOR′ reacts with a different alcohol R″OH, and the OR′ group is replaced by OR″. The mechanism is nucleophilic acyl substitution: the alcohol (or alkoxide) attacks the carbonyl carbon, forming a tetrahedral intermediate, which then collapses and expels the original alcohol as the leaving group. Acid catalysis protonates the carbonyl oxygen to make the carbon more electrophilic. Base catalysis uses alkoxide, a stronger nucleophile.
Equation: RCOOR′ + R″OH ⇌ RCOOR″ + R′OH (acid or base catalyst). The reaction is reversible, so the position of equilibrium follows Le Châtelier's principle. Using the incoming alcohol as solvent, or removing the displaced alcohol (for example by distilling off a low-boiling one such as methanol), pushes it toward products.
On the MCAT: A passage may show a triglyceride reacting with methanol and a catalyst to give fatty acid methyl esters (biodiesel) plus glycerol. You may be asked to identify the products, the nucleophile, the leaving group or the tetrahedral intermediate, or to predict how excess alcohol shifts the equilibrium.
Common confusion: Students call it hydrolysis or esterification. Water is not a reactant, and no carboxylic acid is formed. The acyl group stays attached to oxygen and only the alkoxy part changes. In base, using an alkoxide that matches the ester's own alkoxy group avoids a mixture of products.
Sources
- Organic Chemistry by Paula Yurkanis Bruice
- Organic Chemistry by John McMurry
- Organic Chemistry by Francis Carey
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.