Chemistry behind soap making and glycerol



Can glycerol and lye make soap? The answer is no. This is due to glycerin is a by-product of soap making process.  The process (saponification/hydrolysis) takes place as follows: hydroxyl group (-OH)-nucleophile of the base (lye) which can  either be sodium hydroxide (NaOH) or potassium hydroxide (KOH) attacks the carbonyl of TAG(triglyceride) thus cleaving the ester bonds between the glycerol carbon number 1-3 and long-chain of carboxylic acid. This lead to formation of alkoxide and carboxylic acid (let R, R1represent long hydrocarbon chain of the acid and alkoxide respectively, that isRCO2H and R(O-)3)but since the alkoxide is stronger base than the hydroxyl group of carboxylic acid this enhance proton (H+) transfercarboxylicacid to alkoxide. This results in formation glycerol (R1 (OH) 3)and carboxylate ion (RCO2-) which then attacks cation (Na+ or K+) hence yielding soap. Glycerol is just an additive in soap if it isnot removed, but in commercialmanufacturersremoves it to capitalize on it by producing other products like lotions.
Glycerol (trihydric alcohol) is also a by-product of biodiesel production in a process known as transesterification (of replacing organic group of ester with organic group of alcohol). A process same as saponification but differs in that in transesterification base (NaOH and KOH) acts as a catalyst rather than a reagent in the process. The base deprotonates alcohol yielding a nucleophile (methoxide/ethoxide) which attacks carbonyl group of TAG (ester), cleaving as discussed before to give alkyl ester (biodiesel). Alkoxide abstracts a proton from water forming glycerol and releasing (-OH) which combine with cation(Na+) resulting to NaOH. In the all process base is not used up but acts like a catalysts. This reaction can also be achieved using acid as catalyst.
Chemical reactions for soap making
Sodium hydroxide/potassium (lye) dissociates when dissolve in water as follows:
NaOH + H2O = Na+ + -OH, hydroxyl group now adds to carbonyl group of TAG  leading the formation of the components as explained above.
TAG + -OH = Carboxylic acid + alkoxide, then there is a transfer of proton (H+) to alkoxide forming glycerol as shown:
H+ + R1OO- = R1 (OH) 3
Carboxylate ion attacks Na+ forming soap.
ROO- + Na+ = R2OONa+ (soap) - plus additives.
Chemical reactions for biodiesel production
In this case, base (NaOH) is dissolve in alcohol (let’s use methanol) to give nucleophile (methoxide) as follows:
Hydroxide dissociates: NaOH = Na+ + -OH
-OH (hydroxyl group) + CH3OH (methanol) = CH3O-(methoxide)+ H2O (water) + Na+ (sodium ion)
CH3O- now adds to carbonyl group of TAG resulting to alkoxide and biodiesel (methyl ester if methanol is used). Alkoxide then abstracts proton from water forming glycerol and the process releasing –OH.
R (O-) 3 + H2O = OH + R (OH) 3 + -OH,
-OH (hydroxyl ion) + Na+ (sodium ion) = NaOH(hence base  acts as a catalyst)
In conclusion glycerol and lye can’t yield soap, in soap making there is a nucleophile attacks which lead to cleavage of the ester bonds thus liberating glycerol. It is not logical for such reaction to lead to formation of soaps. The question you should ask is this, “what is saponification”; if you know the answer then you are in the right position to ignore such information. 

The picture below shows how the glycerol is attached to three fatty acids. Glycerol is represented by green color while the three fatty acids are represented by yellow color.

                                                                                                                                

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