What is saponification?

The saponification is the chemical process by which soap is made. In fact, saponification literally means soap making. This process consists of the alkaline hydrolysis, or in a strongly basic medium, of fatty acid esters such as those found in edible fats and oils.

Saponification has been known for centuries. It was originally used to determine the composition of some common triglycerides, such as stearin.

The chemist who first discovered the reaction used it to show that stearin was actually nothing more than the bond between an alcohol, in this case glycerin or glycerol, with three molecules of a fatty acid called stearic acid.

Saponification reaction

Saponification is a hydrolysis reaction of a fatty acid ester . This means that the ester bond is broken ( lysis -breaking) through the action of water ( hydro -water). This requires the use of a strong base such as sodium hydroxide or potassium hydroxide.

As products, a salt of the acid (which is the soap itself) and an alcohol are obtained, as shown below:

Mechanism of the saponification reaction

The mechanism of this reaction is a three-step process, beginning with an attack by the hydroxide ion, then the removal of the alcohol, and finally the formation of the salt.

  • Step 1: Hydroxide ion attack

The first step, and the reason why you need to add sodium hydroxide or some other strong base is the attack of the hydroxide ion on the ester:

  • Step 2: Removal of alcohol in the form of an alkoxide ion

In this stage, the pair of electrons from oxygen pushes the alcohol away from the ester as an alkoxide ion. This ion is a very strong base. In addition to the alkoxide, a carboxylic acid molecule (which is a fatty acid) is also formed.

  • Step 3: Salt formation

The alkoxide ion that forms in the previous step is a very strong base and easily removes the proton from the carboxylic acid. In doing so, it converts the acid into an anion which, together with the sodium in the original hydroxide, forms the fatty acid salt, that is, soap.

Triglyceride saponification

In general, the saponification reaction is carried out with either vegetable or animal fats or oils.

Vegetable oils, such as olive oil, are often used to make soaps by saponification

The esters that are present in these fats and oils are almost always triglycerides or fatty acid esters with a special alcohol called glycerol or glycerin.

In these cases, the saponification reaction looks like this:

1 , R 2 and R 3 represent the fatty acid chains. As there are many different fatty acids, saponification can produce different soaps with different properties.

Saponification products

Depending on the starting oil or fat and which base is used, soaps with different properties will be obtained.

  • For example , the use of sodium or potassium hydroxide will produce a sodium salt or a potassium salt (a sodium or potassium soap) as a product. In general, potassium soaps have a lower melting point than sodium soaps, and some are even liquid at room temperature.

Likewise, the characteristics of the fatty acid that is part of the ester will also affect the properties of the resulting soap. The longer the chain, the higher its melting point.

For this reason, when it is desired to prepare liquid soaps, for example, fatty acids with shorter chains are selected and hydrolyzed with potassium hydroxide instead of sodium.

Importance of saponification

Saponification is a very important reaction.

  • At an industrial level, it is the reaction that is used for the manufacture of all pasta soaps, such as hand soaps or bar soaps for washing clothes.
  • On the other hand, it is also frequently used as the first step in obtaining fatty acids, which are then used in a wide variety of industries such as in the manufacture of cosmetics.
  • In other cases, fatty acids are often used as solvents or dispersing agents in the manufacture of paints, although it is more common to obtain mineral oils in these cases.
  • In addition to this, this reaction is also used to clean any greasy surface such as the interior of ovens or the surface of kitchens.

In those cases in which the fats are very stuck to the surface or are charred and do not dissolve properly with any soap or detergent, a concentrated solution of sodium hydroxide is usually added to carry out the saponification reaction directly in the grease. In this way, the grease turns into soap, the soap dissolves in water and the surface is clean.

Examples of saponification reactions

  • Stearin saponification

Stearin is a triglyceride that contains three molecules of stearic acid linked to glycerin. Its saponification produces three soap molecules, in this case sodium or potassium stearate.

  • Oleic acid saponification

Olive and sunflower oils contain more than 80% oleic acid. This is an unsaturated fat that has chains of 18 carbons. The saponification of these oils produces, among other things, sodium oleate.

  • Saponification of palmitic acid

Palmitic acid is a long chain saturated fatty acid that contains 16 carbon atoms. This is the most important fatty acid in the human diet. It is the main saturated fatty acid found in our body.

In food, palmitic acid is one of the main components of animal fat, so soaps prepared from fats such as butter from cattle mainly contain sodium or potassium palmitate.

In addition, palmitic acid forms more than 40% of palm oil (hence its name). In fact, the saponification of palm oil is the main industrial source of palmitic acid worldwide.

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