Saponification
What the heck are soaps?
Soaps are defined as salts of fatty acids, and they’re the most straightforward way to produce surfactants from natural ingredients.
Soaps are made from triglycerides – fat molecules composed of one glycerol molecule and three fatty acid molecules. The glycerol and fatty acids are held together by ester bonds.
When a strong base (typically sodium hydroxide or potassium hydroxide, both known as lye) is mixed with water and applied to a triglyceride, it breaks the triglyceride’s ester bonds, neutralizes the fatty acids, and forms fatty acid salts with a hydrophilic head and a hydrophobic tail. These fatty acid salts are our naturally derived surfactants.
This chemical process is known as saponification, the chemistry behind soap making!
Saponificitation 101
Let’s step through the process with, say, a triglyceride made from stearic acid and sodium hydroxide.
Step 0: Lye Disassociation
When sodium hydroxide is dissolved into water, it disassociates into Na⁺ and OH⁻ ions.
NaOH + H2O → H2O + Na⁺ + OH⁻
A Brief Detour — Why do salts dissociate in water?
Salts dissolve in water not because they are salts (the byproduct of an acid reacting with a base), but because they are ionic compounds. All salts are ionic compounds. Ionic compounds are created when a highly electronegative element encounters a highly electropositive element. The highly electropositive element will “donate” an electron to the highly electronegative element, creating a cation and an anion.
These ions are attracted to one another, forming ionic bonds. Zooming out, each cation will attract multiple anions, and vice versa. This results in the ions arranging themselves into a lattice, forming a rigid, tightly organized structure.
When an ionic compound is mixed into water, the anions and cations are attracted to the dipoles of the water molecules and, depending on a couple of factors, may be able to break the ionic bonds holding the lattice together. The water molecules then stabilize the separated ions in solution.
The thermodynamic concept of Gibbs free energy can be applied to understand whether a process will occur spontaneously under constant temperature and pressure. It is described mathematically as:
ΔG = ΔH − TΔS
Where:
- ΔG is the change in Gibbs free energy
- ΔH is the change in enthalpy (energy absorbed or released)
- T is the temperature ΔS is the change in entropy (disorder)
A process will take place if ΔG < 0.
Applying the Gibbs free energy equation to dissolving lye in water at constant temperature:
- ΔH corresponds to the energy required to break the lattice structure of the ionic compound (moderately negative)
- ΔS corresponds to the increase in disorder when the polar water molecules stabilize the freed ions (strongly positive)
This results in ΔG < 0, meaning the process is favorable and the lye will dissolve in the water.
In short: breaking the ionic lattice costs some energy, but the water molecules stabilizing the ions release even more, making the overall process spontaneous.
Step 1: Ester Attack
The triglyceride is combined with the dissolved lye in water. Again, a triglyceride is a glycerol molecule bonded to three fatty acids by ester bonds.
When hydroxide (OH⁻) ions are present in the solution, they attack the carbonyl carbon of the ester bond that holds the fatty acids to the glycerol.
Why? Hydroxide is highly reactive. Some contributing factors are:
- Hydroxide is small, meaning it can more easily approach other molecules.
- The oxygen in hydroxide has multiple lone pairs available for donation.
The hydroxide targets that particular carbon because:
- Hydroxide is nucleophilic, meaning it is attracted to electron-deficient (partially positively charged) atoms. It readily donates a lone pair of electrons.
- Carbonyl carbons (C=O) are electrophilic. Oxygen is much more electronegative than carbon, so it pulls electron density toward itself, leaving the carbon partially positive.
These properties make hydroxide a perfect candidate to attack the electron-poor carbon in the ester
This nucleophilic attack results in the following intermediate.
Step 2: Leaving Group Removal
This molecule is not very stable. The newly bonded carbon atom is now surrounded by oxygen atoms, each of which has multiple electron pairs (including the extra electron on the oxygen ion). This creates a region of very high electron density, which is energetically unfavorable.
A double bond to oxygen is more stable, so the carbon reestablishes its carbonyl double bond. As this happens, what is known as the leaving group, the —OR′ group, is expelled from the molecule.
This results in a fatty acid (a carboxylic acid bonded to a long hydrocarbon chain) and an alkoxide.
Step 3: Deprononation
Note: an acid and a base have just been created!
The carboxylic acid is… an acid! The proton on its hydroxyl group can be donated to another molecule due to its polarity. Oxygen is highly electronegative, so it pulls electron density away from the hydrogen, making the hydrogen partially positive. This makes the hydrogen electrophilic, encouraging it to leave as a proton.
Additionally, the carboxylic acid can stabilize itself after deprotonation. The resulting carboxylate ion is stabilized by resonance, meaning the molecule can be represented by multiple equivalent Lewis structures that distribute the negative charge.
On the other hand, the alkoxide is a base. The negatively charged oxygen is nucleophilic and readily accepts a proton.
Acids and bases will therefore react with one another via proton transfer.
At this point, an alcohol group has formed on the glycerol backbone, resulting in a complete glycerol molecule. The other product (the neutralized acid) remains negatively charged due to its carboxylate head.
Step 4: Ionic Bonding
The final reaction to take place is the Na⁺ ions from the lye forming ionic bonds with the carboxylate ions.
And here is soap! A naturally derived byproduct of lye and triglycerides — a fatty acid salt.
Again, soap’s head is hydrophilic, and its tail is hydrophobic. The sodium (or potassium)–oxygen head is ionically bonded, with electrons unevenly shared between the two atoms. This allows the head to interact with water through ion–dipole interactions.
As a side note: ionic compounds, despite their uneven electron distribution, are not considered polar. They are considered charged, whereas the term polar is reserved for uneven electron sharing within a covalent bond.
The hydrocarbon tail of soap is considered nonpolar due to the very similar electronegativities of carbon and hydrogen. These tails interact with other nonpolar substances (oils and grease) via London dispersion forces.
Below is a (slightly different) diagram showing each step of saponification as a recap.
