Ester hydrolyseras
We have seen that all the steps in the Fischer esterification are reversible and the equilibrium is shifted toward the ester product by using an excess of alcohol.
Ester Hydrolysis
This nature of the reaction allows to hydrolyze esters back into a carboxylic acid and alcohol when the water is now used in a large excess :. The reaction works even better by base catalysis saponification because it makes the process irreversible. We will see why this happens when discussing the mechanism of each hydrolysis. Essentially, we are drawing the reverse order of Fischer esterification so, in the first step the ester is protonated promoting the nucleophilic attack of water:.
Notice that just like the Fischer esterification, the process is an equilibrium which makes the reaction a little challenging as it may require higher temperatures and removal of the alcohol as it is formed to push the equilibrium forward. And this is when the base-catalyzed ester hydrolysis turns to be more beneficial.
Hydrolysis of Esters and Amides - Dalal Institute
The carboxylic acid formed during the reaction is deprotonated by the alkoxide or the hydroxide ions making the overall reaction irreversible. After the addition of the — OH and elimination of the — OR, a carboxylic acid is formed. The base-catalyzed ester hydrolysis is also known as saponification because it is used in the production of soaps from fats.
Remember, soap is a salt of a fatty acid and can be formed when a fat an ester derived from a glycerol and three molecules of fatty acid is hydrolyzed by base catalysis:. How do we know this is the correct mechanism of base-catalyzed ester hydrolysis? Can this S N 2 reaction be a suitable alternative to the addition-elimination mechanism we discussed above?
We have a primary carbon and the acetate is not the worst leaving group — certainly better than the ethoxide. So, does the ethoxide attack the carbonyl carbon or the CH 2 of the ethyl group? This can be tested by isotope labeling. We take an ester containing 18 O isotope in the alkoxy part and react it with a hydroxide. Now, if the reaction was S N 2, then the 18 O oxygen should still appear in the carboxylate ion:.
However, experimental studies have indicated that the reaction goes by addition-elimination mechanism as the oxygen in the carboxylate comes from the hydroxide ion:. The products are a carboxylic acid and alcohol just as expected.
However, the mechanism is a little different. After the protonation of the carbonyl, instead of the nucleophilic addition to the carbonyl, we have a loss of a leaving group by an S N 1 mechanism :. This C-O bond cleavage occurs because it produces a relatively stable tertiary alkyl group just like we have seen in the S N 1 mechanism.
Basic Hydrolysis of Esters – Saponification
To summarize, yes, the nucleophilic addition-elimination mechanism predominates in ester hydrolysis, however, you should not exclude the possibility of S N 2 and S N 1 reactions depending on the structure of the ester. Need some good practice on the reactions of carboxylic acids and their derivatives? Check this question, Multiple-Choice Quiz with a min Video Solution covering the reactions of acids, esters, lactones, amides, acid chlorides and etc.
Notify me of followup comments via e-mail. You can also subscribe without commenting. Have we been helpful? Please let us know in the Reviews section here. This nature of the reaction allows to hydrolyze esters back into a carboxylic acid and alcohol when the water is now used in a large excess : The reaction works even better by base catalysis saponification because it makes the process irreversible.
Essentially, we are drawing the reverse order of Fischer esterification so, in the first step the ester is protonated promoting the nucleophilic attack of water: Notice that just like the Fischer esterification, the process is an equilibrium which makes the reaction a little challenging as it may require higher temperatures and removal of the alcohol as it is formed to push the equilibrium forward.
ester hydrolyseras4
Base-Catalyzed Hydrolysis of Esters And this is when the base-catalyzed ester hydrolysis turns to be more beneficial. Remember, soap is a salt of a fatty acid and can be formed when a fat an ester derived from a glycerol and three molecules of fatty acid is hydrolyzed by base catalysis: More About the Ester Hydrolysis Mechanism One interesting question here; How do we know this is the correct mechanism of base-catalyzed ester hydrolysis?
And that is the acid-catalyzed hydrolysis of esters containing a tertiary alkyl group: The products are a carboxylic acid and alcohol just as expected.