Addition followed by elimination
An ester contains an acyl carbon bonded to an alkoxy group. In nucleophilic acyl substitution, a nucleophile attacks that carbon and opens the carbonyl π bond, forming a tetrahedral intermediate. The carbonyl then reforms as a leaving group departs.
The net result replaces a group attached to the acyl carbon. Unlike simple addition to an aldehyde or ketone, the final product retains a carbonyl group.
Example: changing the alcohol-derived group
Acid-catalyzed transesterification can convert methyl acetate and ethanol into ethyl acetate and methanol. Proton transfers activate the carbonyl and allow an alcohol to leave. The methyl-to-ethyl change occurs on the oxygen-linked group; it does not add a new carbon to the acyl carbon skeleton.
This exchange is reversible. Excess incoming alcohol or removal of the departing alcohol can favor the desired ester. A catalyst speeds equilibration but does not by itself change the equilibrium constant.
Distinguish substitution from hydrolysis
Hydrolysis replaces the ester alkoxy group with hydroxyl, producing a carboxylic acid under acidic conditions or a carboxylate under basic conditions. The nucleophile, leaving group, proton transfers, and final charge should all be tracked separately when drawing the mechanism.
Related question
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What Type of Esters Can Undergo Claisen Reactions?Sources
These references support the core concepts and interpretation boundaries explained above.