Carboxylic Acid Derivatives
Basic Hydrolysis of Esters – Saponification
Last updated: February 8th, 2023 |
Basic Hydrolysis of Esters (Saponification)
- When esters are treated with sodium hydroxide, they are converted into carboxylate salts, which upon neutralization yield carboxylic acids. This process is called basic hydrolysis of esters, or saponification.
- Hydrolysis occurs via a two step addition-elimination mechanism (nucleophilic acyl substitution)
- The carboxylic acid is then deprotonated to give a carboxylate.
- To get the final carboxylic product, acid is added during the workup step.
Table of Contents
- Basic Hydrolysis of Esters (Saponification)
- The Mechanism for Basic Hydrolysis
- Saponification is Irreversible Under Basic Conditions
- Basic Hydrolysis of Lactones
- Saponification of Fats
- Quiz Yourself!
- (Advanced) References and Further Reading
1. Basic Hydrolysis of Esters (Saponification)
When esters are treated with hydroxide ion, followed by neutralization with acid, they are converted into carboxylic acids.
This process is called basic hydrolysis of esters. Another name for it is saponification, since the carboxylate salts initially formed through hydrolysis are often used as soaps (sapon = soap in Latin).
Many different hydroxide salts can be used. In the laboratory, lithium hydroxide in a mixture of THF and water is commonly used. For soap making, lye (usually NaOH but sometimes KOH) from wood ashes is traditionally used.
Diagrams here will use Li(+) but the precise identity of the alkali counter-ion is not crucial.
2. The Mechanism for Basic Hydrolysis
The first step in saponification is nucleophilic addition of the hydroxide ion to the carbonyl carbon of the ester to form a tetrahedral intermediate. (See post: Nucleophilic Addition) This is followed by elimination of alkoxide (RO–) from the tetrahedral intermediate to give a carboxylic acid.
This two step addition-elimination process is an example of nucleophilic acyl substitution (See post: Nucleophilic acyl substitution)
Since base is present, however, and since carboxylic acids (pKa around 4-5) are much more acidic than alcohols (pKa around 15-16), the carboxylic acid is then quickly deprotonated to give the conjugate base of the carboxylic acid, called a carboxylate salt.
To obtain the neutral carboxylic acid, one generally adds strong acid to the aqueous solution of carboxylate until the carboxylic acid precipitates out, and we then perform an extraction with an organic solvent.
3. Saponification Is Irreversible Under Basic Conditions
If we can obtain carboxylic acids from esters under basic conditions, it’s worth asking if can we go in the opposite direction and obtain esters from carboxylic acids by using an alkoxide (RO(-) ?
The answer is no. Saponification of an ester with HO(-) is irreversible.
Getting the ester back is highly unfavorable under basic conditions. (Note 1– it can be done under acidic conditions, however, using the Fischer esterification).
It’s instructive to walk through why this is.
Under basic conditions, the first thing to happen will be an acid-base reaction between the alkoxide and the carboxylic acid to give the carboxylate salt. Alcohols are at least 10 pKa units less acidic than carboxylic acids, so this is a very favorable (and essentially irreversible) acid-base reaction.
The next step that would have to occur is addition of the alkoxide RO(-) to the carboxylate to give the tetrahedral intermediate RC(OR)(O2)2- . This is not a misprint – this species would bear two negative charges, making it a di-anion.
Generally, species become more unstable as the number of point charges increases, so we’d already expect this to be quite unfavorable relative to the mono-anionic carboxylate and alkoxide. (See article: 5 Key Factors That Influence Acidity)
Still, for the sake of argument, let’s allow that this might happen to a tiny extent.
What would have to happen next to get the ester from here? Well, the extremely strong di-anion O(2-) would have to be eliminated from the tetrahedral intermediate.
This is bad.
Remember the Principle of Acid-Base Mediocrity? (See post: How To Use a pKa Table). Strong acid plus strong base gives weaker acid and weaker base?
This reaction is disfavored because it would result in the conversion of a moderately strong base into an extremely strong base.
It’s like asking a river to run uphill!
Far, far more likely is elimination of RO(-) from the tetrahedral intermediate to regenerate the carboxylate and the alkoxide. And that is essentially what happens. No net reaction is observed, except for deprotonation of the carboxylic acid.
4. Basic Hydrolysis of Lactones
Any time you learn a new reaction it is worth the time to explore the intramolecular version. It involves no new concepts, but it looks weird, and for this reason intramolecular reactions make for good exam problems.
In this case a cyclic ester, called a lactone, can be converted into an acyclic hydroxy-acid (Note 2) through addition of hydroxide ion.
The reaction looks like this:
The mechanism is completely identical to that of the intermolecular version. The bonds that form and break are exactly the same. The only thing to remember is that since the OR group is attached to the carboxylic acid via a carbon chain, we end up with one molecule instead of two.
5. Saponification of Fats
The origin of the term saponification comes from the Latin sapo for soap.
Fats contain a molecule of glycerol attached to three long-chain carboxylic acids via ester linkages.
At some point in human history, some bright spark it discovered that lye (essentially NaOH) undergoes reaction with fats to give molecules that can act as detergents and soaps.
When these long chain fatty acid carboxylates are added to water, they spontaneously form spherical entities called micelles that organize themselves such that the hydrophilic carboxylate “heads” face out towards the water, and the hydrophobic alkyl tails are arranged inward.
These micelles are excellent at sequestering oils and other non-polar contaminants from skin and dishware in a way that pure water is not.
Note 1. The Fischer esterification is the conversion of a carboxylic acid to an ester under acidic conditions (See post: The Fischer Esterification)
Note 2. Sometimes called a “seco-acid” from the Latin seco, secaris, secare “to cut, sever”.
(Advanced) References and Further Reading
- On the mechanism of hydrolysis. The alkaline saponifications of amyl acetate
Polanyi and A. L. Szabo
Trans. Faraday Soc., 1934, 30, 508-512
One of the first mechanistic studying of saponification, or ester hydrolysis. These authors were the first to find that alkaline hydrolysis of amyl acetate occurs with acyl-oxygen bond fission. Also noted that amyl alcohol had not undergone O18-exchange with the solvent water after 2 days at 70°C.
- Mechanisms of Catalysis of Nucleophilic Reactions of Carboxylic Acid Derivatives.
Myron L. Bender
Chemical Reviews 1960, 60 (1), 53-113
This review describes evidence supporting kinetic studies that show a dependence on pH and isotopic labeling studies that prove it is the acyl-oxygen, not the alkyl-oxygen bond that is cleaved during hydrolysis.
- Rate-limiting deprotonation in tetrahedral intermediate breakdown
Robert A. McClelland
Journal of the American Chemical Society 1984, 106 (24), 7579-7583
This paper describes mechanistic studies that show that alkyl benzoate esters give only a small amount of exchange under basic hydrolysis conditions, indicating that reversal of OH- addition must be slow relative to the forward breakdown of the tetrahedral intermediate.
- Alkyl–oxygen heterolysis in carboxylic esters and related compounds
G. Davies and J. Kenyon
Q. Rev. Chem. Soc., 1955, 9, 203-228
This review describes studies that show that the mechanism of saponification can change based on the stability of the tertiary carbocation formed by alkyl-oxygen cleavage, when using an ester formed from a tertiary alcohol.
- General Base Catalysis of Ester Hydrolysis1
William P. Jencks and Joan Carriuolo
Journal of the American Chemical Society 1961, 83 (7), 1743-1750
- General base catalysis of ester hydrolysis
Dimitrios Stefanidis and William P. Jencks
Journal of the American Chemical Society 1993, 115 (14), 6045-6050
These two papers by Prof. William P. Jencks, an influential figure in Physical Organic Chemistry, show that general base catalysis has been observed in the case of esters in which the acyl group carries electron-attracting substituents.
- General Basic Catalysis of Ester Hydrolysis and Its Relationship to Enzymatic Hydrolysis1
Myron L. Bender and Byron W. Turnquest
Journal of the American Chemical Society 1957, 79 (7), 1656-1662
- Catalysis in ester cleavage. II. Isotope exchange and solvolysis in the basic methanolysis of aryl esters. Molecular interpretation of free energies, enthalpies, and entropies of activation
Carl G. Mitton, Richard L. Schowen, Michael Gresser, and John Shapley
Journal of the American Chemical Society 1969, 91 (8), 2036-2044
There has been a good deal of study of substituent effects, solvent effects, isotopic exchange, kinetics, and the catalysis of ester hydrolysis, as these two papers illustrate.