Chemistry of soap making

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Feb 1, 2021
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I'm researching the chemistry of soap making and I would like to know if I have the reactions correct.
Step 1.
Adding NaOH (sodium hydroxide) to H2O (water).

The reaction: Breaking the bond between Sodium (Na) and hydroxide (OH) with water.

The sodium molecule (Na) is positively charged and the hydroxide molecule (OH) is negatively charged.
Because they have opposite charges they have a ionic bond (like two magnets stuck to one another).

The H2O molecule will break the ionic bond between Na+OH- and release heat.
The Na+ and the H in the OH- will attract to the negatively charged oxygen molecules in the H2O.
The O- in the OH- will attract to the positively charged hydrogen molecules in the H2O.

(Na+OH-) + (H2O) = Na+(aq) and (OH-(aq)

(aq)= aqueous= dissolved in water

In silly terms: the lye is broken apart and arrested by the water.

Step 1.jpg

Step 2.
Adding the Na+(aq) OH-(aq) (lye water solution) to triglycerides (fat/oil)

The reaction: Saponification (Base promoted ester hydrolysis)

Stage 1:
The OH- will attack the carbon with the double bond (nucleophilic attack).
In the attack the OH- will attach it's self to the carbon (nucleophilic addition).

The addition of OH- will kick the negative charge on the OH though the carbon to the double bonded oxygen. Making that oxygen O-.

Because of the movement of the negatively charged ion and the addition of OH, the double bond between the oxygen and the carbon will break making a single bond.
Making a tetrahedral intermediate.

Step 2 stage 1.jpg

Stage 2:
The newly made O- will move the negatively charged ion through the carbon to the oxygen that is connected to the triglyceride backbone (RO group).

Because of the movement of the negatively charged ion the double bond reforms and the triglyceride backbone (RO group) gets cut away from the rest of the molecule.

The triglyceride backbone (RO group) now has a negatively charged ion on the oxygen in the RO.

Making carboxylic acid (fatty acid) and the RO- (which is strong base)

Step 2 stage 2.jpg

Stage 3:
O- in the RO- group will kick the negatively charged ion to the oxygen on the original OH in the carboxylic acid (fatty acid).
Making a acid based proton transfer.

The oxygen in the OH will exchange its' hydrogen for the negatively charged ion.

Making a carboxylic anion and glycerol.

Step 2 stage 3.jpg

Stage 4.
The lonely sodium (Na+) hasn't reacted though all these reactions so far.
But now the sodium (Na+) will form an ionic bond with the carboxylic anion to make a molecule that has a sodium head and fatty acid tail (soap).

Step 2 stage 4.jpg

In simple terms: The hydroxide (OH) in the NaOH (sodium hydroxide) will attack the fat/oil and after a bunch of back and forth with some negative charges you eventually cut the fat/oil into glycerin and soap.
Did I get it right?
Here is where I got most of the info from:
"...Did I get it right? ..."

Probably, but I'm not absolutely sure, to give you an honest answer. You've listed all the intermediary reactions, and I don't retain that level of detail about the saponification reaction.

The overview reaction is the level of detail that most people want and need to know, so that's the most I normally cover --



I applaud your determination to fully understand the chemistry -- kudos to you!
Last edited:
@Kat852 Wow, that looks great! I appreciate the details. And so far I don't see any error, I just want to comment on some details.
  • Nature of NaOH and breakdown (lye solution): NaOH is a ionic compound, and I am a bit uncomfortable to call the forces between the sodium cation and the hydroxide anion a “bond”, likewise to call it a chemical reaction at all. That “magnet” metaphor is better suited here: magnetic forces might stabilise a rigid (crystal) lattice like in solid NaOH, or a dynamically bonded random network of Na⁺, OH⁻, and H₂O (lye solution). From the perspective of Na, the hydroxide ion doesn't look that much different than a water molecule (and in fact there is an equilibrium proton exchange between hydroxide ions and water, the equilibrium that makes defining a pH value in aqueous solutions sensible.)
  • Also, take a not-so-unusual lye concentration of 35.7%, that has a molar ratio of 4:1 water:NaOH, i. e. both the Na⁺ and OH⁻ together have to share mere four water molecules. That means your lye will be a 3-dimensional water network with one Na⁺ per 5 water molecules (with one proton missing at one of these). You have got the molecule orientation right (dipole moment alignments towards cations and anions), though it is of course difficult to draw 3-D sketches.
  • Step 2 appears correct to me. Just notice that stage 3 is only one of (at least) two reactions that will happen with some probability: the proton source to neutralise the glycerol alcoholate might well be the carboxylic acid, but it might also be water, creating a hydroxide ion. Since, in the end, this will neutralise the carboxylic acid, the sum reaction still holds. It'd need some fancy NMR spectroscopy and/or tracer studies to find out which process dominates.
  • It is unusual/outdated to write chemical reactions with “=”, the equal sign. Everyone uses reaction arrows “→”, or in case of equilibria “” (step 2 stages 1 and 2).
  • Again, just like with the NaOH, the resulting soap is not a simple “compound” of Na⁺ and the fatty acid anion, but both coexist in solution. Sodium doesn't “attack” the carboxylate like hydroxide attacks an ester bond. To make things worse, there occurs some micro-phase separation (alkylic ends of the fatty acid molecules will cluster to form micelles, lamellae or network structures – the reason for the detergent action of soap!), and the watery parts will hold the carboxylate groups, sodium ions, water, and glycerol. Soap solidifies/hardenes up with time since the fatty acids themselves rearrange into more ordered structures (crystals) – the “bonds” between the alkylic groups are more important in this than the electrostatic connections to the Na⁺ cations; at least as long as there is enough water and glycerol “in the way” to keep things soft, waxy and slightly soluble, rather than hard and brittle.
  • And, finally, I dare to claim that a majority of the folks around here agree that glycerol is a fundamental part of soap, much more than a mere by-product. It will stay within the soap for good reasons, not just because it's difficult for artisans/hobbyists to remove it. This is not an “error”, but a conflict of terms between the household definition of soap (that waxy block that I wash my hands with) and a chemical/reductionist/industry/legal point of view (that type of molecule that is used to disperse dirt).