What is Saponification

The process of converting fats, oil, and lipid into soap using an aqueous alkali is called saponification. Vegetable oil and animal fats are triesters or triglycerides that can be saponified in one or two steps. During this process, the triglyceride reacts with an aqueous hydroxide ion to form a mixture of glycerol and fatty acid derivative. The sodium or potassium salts of long-chained fatty acids are a significant component of soap ^[1-6].

General Equation for Saponification

During saponification, esters are cleaved in the presence of aqueous alkali to form an alcohol and an alkali-metal salt of carboxylic acid. The alkali used in this process is sodium hydroxide (NaOH) or lye for hard soap and potassium hydroxide (KOH) for soft soap.

The general reaction, using NaOH, is given by:

RCOOR’ + NaOH → ROH + R’COO^– Na⁺

Here is a simple video explaining the saponification process:

Examples of Saponification ^[2-6]

1. Sodium Stearate

Sodium stearate (C₁₈H₃₅NaO₂) is the sodium salt of stearic acid (C₁₈H₃₆O₂). It is used as a soap and a detergent. It is a significant component of soaps and contains both a hydrophobic and a hydrophilic part. It is produced when glyceryl tristearate ((C₁₈H₃₅O₂)₃C₃H₅), the triglyceride of stearic acid, is hydrolyzed with aqueous sodium hydroxide (NaOH).

(C₁₈H₃₅O₂)₃C₃H₅ + 3 NaOH → C₃H₅(OH)₃ + 3 C₁₈H₃₅O₂Na

2. Sodium Palmitate

Sodium palmitate (C₁₆H₃₁NaO₂) is the sodium salt of fatty palmitic acid (C₁₆H₃₂O₂). It is found in soaps and detergents. It can be derived by the saponification of glyceryl palmitate ((C₁₆H₃₁O₂)₃C₃H₅) using sodium hydroxide (NaOH) in the form of caustic soda, lye, or lime.

(C₁₆H₃₁O₂)₃C₃H₅ + 3 NaOH → C₃H₅(OH)₃ + 3 C₁₆H₃₁O₂Na

3. Methyl Salicylate

Methyl salicylate (HOC₆H₄COOCH₃) reacts with sodium hydroxide (NaOH) to form a thick white solid of sodium salicylate (HOC₆H₄COO^–Na⁺).

HOC₆H₄COOCH₃ + NaOH → HOC₆H₄COO^–Na⁺ + CH₃OH

4. Methyl Acetate

Methyl acetate (CH₃COOCH₃) saponifies in the presence of sodium hydroxide (NaOH) to sodium acetate (CH₃COO⁻Na⁺).

CH₃COOCH₃ + NaOH → CH₃COO⁻Na⁺ + CH₃OH.

5. Methyl Benzoate

Methyl benzoate (C₈H₈O₂) reacts with aqueous sodium hydroxide (NaOH) to give sodium benzoate (water-soluble) and methanol (miscible with water).

C₈H₈O₂ + NaOH → C₇H₅O₂Na + CH₃OH

Saponification Mechanism

The mechanism of saponification is a nucleophilic carbonyl substitution process, which is explained in the following steps ^[4-6].

Step 1: The nucleophilic hydroxide ion attacks the ester group and forms an intermediate.

Step 2: The intermediate splits by releasing the leaving group to form a carboxylic acid and an alkoxide.

Step 3: Deprotonation removes the hydrogen from the carboxylic acid resulting in a carboxylate ion and alcohol.

Saponification Value

Saponification value (SV) or saponification number (SN) represents the number of milligrams of potassium hydroxide (KOH) required to saponify one gram of fat under a specific condition. The saponification value is a significant parameter used to characterize and evaluate the quality of edible fats and oils. Also, the saponification number provides information about the fatty acids’ average molecular weight. The higher the number, the lower the molecular weight of all fatty acids ^[7].

Unit: mg KOH/g

The following table gives the saponification values of various oils and fats.

Oil/ fat	Saponification value (mg KOH/ g)
Canola oil	182 – 193
Sunflower oil	189 – 195
Olive oil	184 – 196
Soyabean oil	187 – 195
Coconut oil	248 – 265
Cottonseed oil	189 – 207
Palm kernel oil	230 – 254
Palm oil	190 – 209
Castor oil	176 – 187

Applications of Saponification

The main application of saponification is in the manufacture of soaps. Different kinds of soaps serve different purposes like laundry, cleaning, and lubrication. Soaps may be precipitated by salting them out with saturated sodium chloride. Saponification also works with fire extinguishers. Fire extinguishers use it to convert burning fats and oil into non-combustible soap, which helps to reduce fire^[7].

Saponification is significant in the food industry because it helps to know the amount of free fatty acid in a food item. The amount of free fatty acid can be distinguished by determining the quantity of alkali added to the fat or oil to make it neutral.

Esterification vs. Saponification^[8]

Why is Saponification the Reverse of Esterification

During esterification, acid and alcohol combine to form an ester and release water. On the other hand, saponification breaks the bonds in an ester to form alcohol and long-chained fatty acid derivative.

Example of Esterification

C₂H₅OH + CH₃COOH → CH₃COOC₂H₅OH + H₂O

The following table shows the difference between saponification and esterification.

	Esterification	Saponification
Definition	Process of preparing ester	Process of preparing soap
Process	Acid reacts with alcohol in the presence of concentrated sulfuric acid	Ester reacts with aqueous alkali
Heat	Yes	No
Catalyst	Acid	Base
Byproduct	Water	Alcohol

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