Saponification-The process of Making Soap

Our Objective

To study the saponification reaction for preparation of soap.

The Theory

Soaps and detergents are essential to personal and public health. They safely remove germs, soils and other contaminants and help us to stay healthy and make our surroundings more pleasant.   Soaps are made from fats and oils or their fatty acids.

 

What are fatty acids?

Fatty acids are merely carboxylic acids consisting of a long hydrocarbon chain at one end and a carboxyl group (-COOH) at the other end. They are generally represented as RCOOH. They are an important component of plants, animals and other microorganisms. They are found in various parts of the body, such as cell membranes, the nervous system and as lung surfactant.
There are two groups of fatty acids: saturated fatty acids and unsaturated fatty acids.

Saturated fatty acids:Fatty acids contain carbon-carbon single bonds called saturated fatty acids.
Examples: stearic acid (C17H35COOH) & palmitic acid (C15H31COOH)

 

 

Unsaturated fatty acids:Unsaturated fatty acids contain one or more double bonds between carbon atoms.
Example: Oleic acid (C17H33COOH)

If the fatty acid has a single carbon-carbon double bond in the molecule, it is known as a mono-unsaturated fatty acid. Oleic acid is a mono-unsaturated fatty acid.

If a fatty acid has two or more carbon-carbon double bonds in the molecule, it is known as poly-unsaturated fatty acid.

Linoleic acid { CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH }  is a poly-unsaturated fatty acid. It contains two double bonds.

Long chain fatty acids always exist as triglycerides and are found in fats and oils. Triglycerides are esters of fatty acids and are formed by combining fatty acids with glycerol.

 

Glycerol has three alcohol functional groups (-OH group) and fatty acids have the carboxyl group (-COOH group).  Since glycerol has three –OH groups, three fatty acids must react with one glycerol molecule to make three ester functional groups and form triesters of glycerol or triglyceride. During this process three molecules of water are eliminated. The three fatty acids may or may not be identical.

 

The long chain fatty acids can be of either plant origin (linseed oil, castor oil, soya bean, coconut oil) or animal origin (tallow from cattle and sheep). In general, fats/oils from plant origin are high in unsaturated and low in saturated fatty acids. Fats/oils from animal source are high in saturated and low in unsaturated fatty acids.

SOAP

Soaps are sodium or potassium salts of long chain fatty acids. When triglycerides in fat/oil react with aqueous NaOH or KOH, they are converted into soap and glycerol. This is called alkaline hydrolysis of esters. Since this reaction leads to the formation of soap, it is called the Saponification process.

The soap molecule has two parts: a polar group (-COO-Na+) and a non-polar group (R-hydrocarbon part). The polar group is called the head and the non-polar group is called the tail. Thus, the soap molecule has a polar head and a non-polar hydrocarbon tail. The polar head is hydrophilic in nature (water loving) and the non-polar tail is hydrophobic (water repelling) in nature.

The saponification  reaction is exothermic in nature, because heat is liberated during the process. The soap formed remains in suspension form in the mixture. Soap is precipitated as a solid from the suspension by adding common salt to the suspension. This process is called Salting out of Soap.

Types of Soap

Depending upon the nature of alkali used in the production of soap, they are classified into two types.

The sodium salt of long chain fatty acid is known as hard soap. It is difficult to dissolve in water. It is used as laundry soap.

The potassium salt of long chain fatty acid is known as soft soap, as it produces more lather. It is used as toilet soap and shaving soap.

  • Hard soap    
  • Soft soap

In aqueous solution, soap ionises to form alkali ions.

Since soaps have free alkali ions, they are alkaline in nature. Hence, the soap solutions are slippery to the touch.

Learning Outcomes

  • Students understand the terms: soap, saponification, salting out, hard soap and soft soap.
  • Students identify the materials which are required for the preparation of soap.
  • Students understand the use of common salt in saponification process.
  • Students understand the alkalies required for the preparation of hard and soft soaps.
  • Students understand the procedure of saponification process.
  • Students acquire the skill to perform the preparation of soap in a real lab.

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