SAPONIFICATION: REACTION AND SUBSTANCES THAT CAN BE OBTAINED - CHEMISTRY - 2025

The saponification is the basic hydrolysis of an ester. This means that the ester reacts with a base (NaOH or KOH) irreversibly, producing alcohol and sodium or potassium carboxylates. The word means "soap making" and, in fact, it is one of the oldest chemical reactions used by mankind.

In Babylonian times, with the help of ashes collected from wood and plants and animal fats, they perfected the art of soap-making. Why animal fat? The reason is because it is rich in glycerol triesters (triglycerides), and wood ashes are a source of potassium, the basic metal.

Otherwise, the reaction proceeds at a lower yield, but enough to reflect its effects on paints and some surfaces. Such is the case of oil paints, where the pigments are mixed with oil (source of esters).

Saponification reaction

Mechanism

Esters have an acyl group (O = C – R), which is susceptible to nucleophilic attacks such as OH ^-.

Because oxygen atoms "steal" electron density from the carbon atom, it finds itself with a partially positive charge, even more so in the case of esters.

Consequently, this positive charge attracts negative species capable of contributing electrons to the carbon atom, causing nucleophilic attack (the left side of the image). As a result, a tetrahedral intermediate is formed (the second molecule from left to right).

The negative charge on the oxygen of the tetrahedral intermediate is a product of OH ^- surrounding. Then, this negative charge is delocalized to give rise to the carbonyl group, then "forcing" the C – OR 'bond to break. Also, this delocalization produces a carboxylic acid RCOOH and an alkoxide ion R'O ^-.

Finally, as the reaction medium is basic, the alkoxide deprotonates a water molecule and the carboxylic acid reacts with another OH ^- from the medium, generating the saponification products.

Kinetics

The speed of the saponification reaction is proportional to the concentrations of the reagents. In other words, increasing either the concentration of the ester (RCOOR ') or the base (NaOH), the reaction will proceed faster.

This also translates as follows: the saponification rate is first order relative to the ester, and first order relative to the base. The above can be expressed with the following mathematical equation:

Speed ​​= k

Where k is the constant or coefficient of velocity, which varies as a function of temperature or pressure; that is, the higher the heat, the higher the saponification rate. For this reason the medium is subjected to boiling.

Since both reactants are of the first kinetic order, the overall reaction is of the second order.

In the reaction mechanism of saponification, the formation of the tetrahedral intermediate requires nucleophilic attack, which involves both the ester and the base.

Thus, second order kinetics is reflected in this fact, because they intervene in the determining (slow) step of the reaction.

Substances that can be obtained by saponification

The main products of saponification are alcohols and salts of carboxylic acids. In an acid medium, the respective RCOOH are obtained, obtained by saponification of fats and oils, which are known as fatty acids.

Thus, soaps consist of salts of the fatty acids produced by saponification. Do you come out with what cations? They can be Na ⁺, K ⁺, Mg ²⁺, Fe ³⁺, etc.

These salts are soluble in water, but are precipitated by the action of NaCl added to the mixture, which dehydrates the soap and separates it from the aqueous phase. The saponification reaction for a triglyceride is as follows:

Glycerin is the alcoholic "E", and soap is all the salts of the resulting fatty acids. Here, each -R side chain has different lengths and degrees of unsaturation. Consequently, these chains make the difference between vegetable fats and oils.

The key to making soaps then lies in the selection of the best fats and oils, or more specifically, in the selection of different sources of triglycerides.

This soapy white mass can contain colorants and other organic compounds in its structure, giving it pleasant aromas and bright colors. From here, the range of possibilities is tamed by art and vocation in this trade.

However, the saponification reaction is also a synthetic route of carboxylic acids and alcohols that does not necessarily have to do with glycerin or soaps.

For example, basic hydrolysis of any ester, such as simple ethyl acetate, will produce acetic acid and ethanol.

Solvent action of soaps

Fatty acid salts are soluble in water, but not in the same way as ions are solvated; that is, surrounded by a watery sphere. In the case of soaps, their -R side chains theoretically prevent them from dissolving in water.

Therefore, to counteract this energetically uncomfortable position, they are oriented in such a way that these chains come into contact, forming an apolar organic nucleus, while the polar heads, the end (–COO ^- Na ⁺), interact with the molecules of water and create a “polar shell”.

The above is illustrated in the upper image, where this type of structure known as a micelle is shown.

The "black tails" correspond to the hydrophobic chains, entangling themselves in an organic nucleus protected by the gray spheres. These gray spheres make up the polar shell, the –COO ^- Na ⁺ heads.

So, the micelles are clusters (agglomerations) of the salts of fatty acids. Inside these they can contain fat, which is insoluble in water due to its nonpolar character.

How do they do that? Both fat and -R chains are hydrophobic, so they both have a high affinity for each other.

When the micelles enclose the fats, the water interacts with the polar shell, allowing the solubility of the soap. Also, the micelles are negatively charged, causing repulsions to each other and, therefore, generating the dispersion of fat.

References

1. Anne Marie Helmenstine, Ph.D. (October 03, 2017). Saponification Definition and Reaction. Retrieved on April 24, 2018, from: thoughtco.com
2. Francis A. Carey. Organic Chemistry. Carboxylic acids. (sixth ed., pages 863-866). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. Organic Chemistry. Lipids (10th edition., Pages 1056-1058). Wiley Plus.
4. Wikipedia. (2018). Saponification. Retrieved on April 24, 2018, from: en.wikipedia.org
5. Boyd C. (February 27, 2015). Understand the chemistry and history of soap. Retrieved on April 24, 2018, from: chemservice.com
6. Luca Laghi. (March 27, 2007). Saponification. Retrieved on April 24, 2018, from: commons.wikimedia.org
7. Amanda Crochet. (May 12, 2015). Micelle (grayscale). Retrieved on April 24, 2018, from: commons.wikimedia.org