Basic hydrolysis of esters (saponification)
Description: Esters can be converted to carboxylic acids through treatment with strong base.
Notes: This is not an equilibrium. The first product formed in step 1 is a carboxylate salt. Addition of acid in step 2 then gives the neutral carboxylic acid. The name “saponification” comes from the fact this method is used to make soaps from fats – many soaps are carboxylate salts.
Notes: Note that the last example is opening of a ring to give a linear molecule (these examples often give students trouble).
Mechanism: Addition of hydroxide to the ester (Step 1, arrows A and B) results in a tetrahedral intermediate, which then eliminates
alkoxide (Step 2, arrows C and D). This results in a carboxylic acid; however, since it is present in strongly basic solution,
the carboxylic acid is deprotonated to give a carboxylate (Step 3, arrows E and F). Finally, addition of acid (H+) gives the
neutral carboxylic acid.
Notes: be alert that some sources may not show the third and fourth steps(deprotonation / protonation) and will (for simplicity) simply show the results of Steps 1 and 2.