What is Saponification?

The process of formation of soap from oil, fats and lipids by the action of heat in presence of aqueous alkali is defined as saponification. Soap is a fatty acid salt in which the fatty acids are monomers of lipids having long carbon chains. The reaction of saponification is generally exothermic and irreversible

The word saponification comes from ''sapo'' means soap. In this, the reaction is carried out between the ester group with water and a base resulting in the formation of alcohol and a carboxylate ion.

Reaction of Saponification

This reaction involves the production of glycerol and fatty acid salt of sodium or potassium by carrying out a reaction between sodium or potassium hydroxides with glycerides. Soap is a fatty acid salt. Salt of hydroxide anion reacts with the carbonyl group of ester. The product formed is an ortho-ester. Carboxylic acid is formed by the expulsion of alkoxide. It is a strong base.

The mechanism of saponification involves:

1. First step is the nucleophilic attack on the hydroxide group.
2. Second step involves the removal of the leaving group.
3. The final step is the deprotonating step.

Saponification Number/ Value

To saponify 1 gram of fat, the amount of base required under a particular condition is known as saponification value for particular fat. In terms of KOH or NaOH, the saponification value is expressed. 

Significance of saponification value:

• The length of the carbon chain in fatty acid used is assessed by saponification value.
• The value of saponification is low for those fatty acids which have a long carbon chain.
• The value of saponification is high for fats containing short carbon chains.
• The saponification value gives an idea about the average molecular weight of fat or oil used.

Uses of saponification

• Used in fire extinguishers. Non-combustible salts are formed from burning oil which helps in forming fire extinguisher.
• Oil paintings can be damaged by this reaction.
• Soaps are made by this reaction.

What is Soap?

The water-soluble compound prepared by the process of saponification. For preparing soap, the reaction between sodium hydroxide with vegetable oil is carried out. In this, hydrolysis of the base is done.

Characteristics of soap

• Hardness: The longevity of soap is determined by its hardness.
• Cleansing: Soap is generally used for cleaning purpose. Long chains of carbon atoms are present in which its one end attracts the oil and other end attracts water. 
• Conditioner: Emollients are also known as soap conditioner. 
• Lather: Generally soap that produces more lather is preferred by a person.
• Fragrance: Soap which produces good fragrance is preferred by people.

Process of making soap in a lab

• In a beaker, take 15 ml vegetable oil.
• To the vegetable oil, 10ml of ethanol and 20ml of 20% sodium hydroxide is added with constant stirring. 
• For 15-20 minutes, the mixture is heated till the formation of two separate layers. Overheating of the mixture is avoided.
• From the burner, the beaker is removed and 15 ml of saturated sodium chloride solution is added.
• The mixture is cooled till its solidification.
• Solid is taken and cut into the desired shape.

The cleansing action of soap 

The cleansing action of soap is dependent on the formation of an aggregate in water. The nature of dirt is oily and so it does not dissolve in water which is highly polar. Sodium or potassium salts of carboxylic acids are present in a molecule of soap. On coming in contact with the non-polar dirt, these long chains form an aggregate around the dirt in such a way that one end that is soluble in the water, which is, the ionic end, faces outwards and the other end which not soluble in the solvent, faces towards the non-polar dirt. This aggregate structure is termed as a micelle. Thus an emulsion is formed in water, thereby helping in the dissolution of dirt.

Critical micelle concentration is the concentration at which the formation of micelle starts. The aggregation number of a micelle represents the average number of monomers, that is, the carbon chains. Micelle molecular weight is represented by its relative micelle size.

Soft soaps v/s hard soaps 

Based on the nature of alkali used in the production of soap, soaps have different properties.

• Hard soap is produced from sodium hydroxide. It can be used in water in which Mg, Cl is dissolved.
• Soft soaps, on the other hand, are produced from potassium hydroxide. Soft soaps have low melting points and weaker intermolecular forces relatively.

What is Detergent?

Detergent is an amphipathic molecule containing charged hydrophilic group at the extreme of the lipophilic hydrocarbon group. The head is represented by charged hydrophilic group and the tail is represented by lipophilic hydrocarbon. Surfactants and detergents are the same. They decrease the surface tension of water. They are useful for cleaning purposes in case of hard water.

Saponification of Fats

The oldest material which is saponified is vegetable oil and animal oil. Triglycerides are greasy materials. It is a mixture derived from fatty acids. One or two-step process is carried out in conversion of triglyceride to soap. In the first step, a strong base is used to treat triglyceride resulting in the cleavage of ester bond and causing the release of fatty acid and glycerol. For the production of glycerol at the industrial level, this procedure is used.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry and science courses at high school level.