Basic hydrolysis of esters (saponification)

by James

Description: Esters can be converted to carboxylic acids through treatment with strong base.
[private_ReactionGuide]

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.

Examples


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.

[/private_ReactionGuide]

{ 2 comments… read them below or add one }

Lynne Dusseau March 16, 2015 at 5:03 pm

Is this really not an equilibrium? Because it says it is in my textbook for mechanism of Saponification of esters

Reply

James March 17, 2015 at 10:37 pm

It’s not an equilibrium if the ester is being treated with NaOH, no. Once the carboxylic acid forms, it will be deprotonated to give the carboxylate and it stays there.

Now, if instead of NaOH, an alkoxide is used (like NaOCH3 or NaOEt) THEN the reaction will be in equilibrium between two esters. That’s called, “transesterification” which is slightly different : – )

Reply

Leave a Comment