ChemTalk

Saponification

Core Concepts

Saponification is the reaction where an ester is hydrolyzed into an alcohol and a carboxylic acid salt upon the addition of an aqueous base. In this article, we will explain the saponification reaction and its mechanism, as well as its applications to science and beyond!

Topics Covered in Other Articles

Saponification Overview

Saponification is an umbrella term for a general reaction of an ester and an aqueous base to form a carboxylic acid salt and an alcohol.

As a review, an ester is a general functional group that describes a carbon group (denoted “R”) bonded to a carbonyl (C=O), bonded to another oxygen, bonded to another carbon group. A carboxylic acid salt is a carboxylic acid ((C=O)OH) that has been deprotonated and bound to some kind of positive counterion, usually sodium. An alcohol is the functional group (O-H). Although it’s not mandatory to know these terms to understand saponification, we recommend learning the fundamental functional groups if you have not done so already. This is extremely useful in organic chemistry to recognize patterns for how particular groups react!

The general saponification reaction is illustrated below:

general reaction scheme for saponification

Saponification Mechanism

Now that you have learned the general reaction scheme, let’s dive in to the step by step reaction mechanism for saponification!

Step 1: Nucleophilic Attack

The hydroxide ion (OH-) is an excellent nucleophile (electron donor) due to a lone pair on the oxygen. Furthermore, carbonyls (C=O) are excellent electrophiles (electron acceptors) because the electronegative oxygen will draw electron density towards it and leave the carbon center with a partial positive charge. Thus, the hydroxide will attach the carbonyl center. However, this would make 5 bonds on the carbon, and carbon can only have 4 bonds. So, the electrons from one of the bonds in the C=O double bond will jump up to the oxygen, as shown below!

Step 2: Loss of Leaving Group.

After the tetrahedral intermediate is formed, the electrons on the negatively charged oxygen will move back down and reform the C=O double bond (which is a more stable configuration). However, because we added another bond to the carbon center to make 5 bonds, we need to kick something else off. From the three options (R, OR, and OH) the best leaving group is OR (see this article for how to identify the best leaving group).

Step 3: Deprotonation

The carboxylic acid formed is a relatively strong acid which gives up its proton fairly easily. The negatively charged O-R group is a Lewis base that is looking to stabilize its negative charge. So, a lone pair on the oxygen on OR will deprotonate the acidic H on the carboxylic acid. This produces an alcohol and a carboxylate anion, as shown below. The carboxylate anion will typically be stabilized by some sort of counterion from the base to form a carboxylic acid salt. The counterion does not participate in the reaction and thus it is a spectator ion. In this case, it is drawn with sodium as the counterion, but other counterions are also possible!

Saponification step 3: deprotonation

Saponification Practical Applications

Saponification is a very useful reaction with many real world applications. One notable example is the metabolization of fat in the human body. In the body, fats are molecules that contain an ester group. (see below for a picture of oleic acid, a fatty acid found in olive oil!) They need to be broken down so that energy is released for us to function! Another example is the production of soap. In industrial production, soap is commonly made by combining a mixture of fats with either NaOH or KOH base. In fact, soap is really just a carboxylic acid salt!

Oleic Acid Structure (makes up olive oil)

The soap molecule, in fact, is able to function as soap because of its structure. It has a polar end (the side with the negatively charged oxygen and positively charged sodium) and a nonpolar end. The nonpolar end is the carbon group “R”. Many fats in real life have a very long carbon chain, which would be nonpolar. So, when you wash your hands with soap, the nonpolar end sticks to any nonpolar dirt and grime. (Remember, like attracts like!) Then, when you rinse with water, the polar end of the molecule enables it to be cleanly washed away. A picture of a soap micelle is shown below! As you can see, the soap molecules arrange themselves into a circle, with the nonpolar tails surrounding the dirt/grease (grey sphere), with the polar heads facing the polar water.

File:Oleic acid flat.svg
Chemical Structure of Oleic Fatty Acid
File:Micelle.png
Soap Micelle Structure

Another fun fact is that the reaction name, “saponification”, actually comes from its practical application in soap making! In Latin “sapo” means soap and “ficare” means “to make”. So, the word saponification literally translates to “make soap”!

Further Reading