HYDROLYSIS: WHAT IT CONSISTS OF AND EXAMPLES OF REACTIONS - CHEMISTRY - 2025

The hydrolysis is a chemical reaction that can occur in molecules or ions both inorganic and organic, and involves the participation of water to the breaking of its bonds. Its name originates from the Greek, 'hydro' of water, and 'lysis' of rupture.

The water molecule, H ₂ O, establishes an equilibrium with the ions of salts of weak acids and bases, this concept appearing for the first time in general studies of chemistry and in analytical chemistry. It is therefore one of the simplest chemical reactions.

General equation for a hydrolysis reaction. Source: Gabriel Bolívar.

In several examples of hydrolysis, water alone is unable to break a certain covalent bond. When this occurs, the process is accelerated or catalyzed by acidification or alkalization of the medium; that is, in the presence of H ₃ O ⁺ or OH ^- ions, respectively. Also, there are enzymes that catalyze hydrolysis.

Hydrolysis occupies a special place with regard to biomolecules, since the bonds that hold their monomers together are susceptible to hydrolyzing under certain conditions. For example, sugars are hydrolyzed to break down polysaccharides into their constituent monosaccharides thanks to the action of glucosidase enzymes.

What is hydrolysis?

The image above explains what hydrolysis is. Note that not only the molecule or substrate (if enzymes mediate) breaks its bond, but also the water itself, which "fractures" into H ⁺ and OH ^-, where H ⁺ ends with A, and OH ^- with B. AB It therefore reacts with a water molecule, giving rise to two products, AH and B-OH.

Hydrolysis therefore is the opposite reaction to condensation. In condensation, two products, to say AH and B-OH, are united by releasing a small molecule: water. In hydrolysis a molecule is consumed, while in condensation it is released or produced.

Returning to the example of sugars, suppose that AB corresponds to a sucrose dimer, where A represents glucose, and B represents fructose. The glycosidic bond AB can be hydrolyzed to give rise to the two monosaccharides separately and in solution, and the same happens to oligo and polysaccharides if enzymes mediate such reactions.

Note that in this reaction, that of AB, the arrow has only one direction; that is, it is an irreversible hydrolysis. However, many hydrolysis are in fact reversible reactions that reach an equilibrium.

Examples of hydrolysis reactions

- ATP

ATP is stable between pH values ​​of 6.8 and 7.4. However, at extreme pH values ​​it hydrolyzes spontaneously. In living beings, hydrolysis is catalyzed by enzymes known as ATPases:

ATP + H ₂ O => ADP + Pi

This reaction is strongly exergonic, since the entropy of ADP is greater than that of ATP. The variation of the Gibbs free energy (ΔGº) is - 30.5 kJ / mol. The energy produced by the hydrolysis of ATP is used in numerous endergonic reactions.

Coupled reactions

In some cases, hydrolysis of ATP is used for the conversion of a compound (A) to a compound (B).

A + ATP + H ₂ O <=> B + ADP + Pi + H ⁺

- Water

Two water molecules can react with each other in apparent hydrolysis:

H ₂ O + H ₂ O <=> H ₃ O ⁺ + OH ^-

It is as if one of these water molecules fractured into H ⁺ and OH ^-, the H ⁺ going to bind to the oxygen atom of the other water molecule, which gives rise to the hydronium ion, H ₃ O ⁺. This reaction, rather than hydrolysis, is about the autoionization or autoprotolysis of water.

- Proteins

Proteins are stable macromolecules and to achieve their complete hydrolysis, in the amino acids that constitute them, extreme conditions are required; such as a concentration of hydrochloric acid (6 M) and high temperatures.

However, living beings are endowed with an enzymatic arsenal that allow the hydrolysis of proteins into amino acids in the duodenum. The enzymes involved in protein digestion are almost entirely secreted by the pancreas.

There are exopeptidase enzymes that degrade proteins, starting at their ends: aminopeptidase at the amino end, and carboxypeptidase at the carboxyl end. Endopeptidase enzymes exert their action inside the protein chain, for example: trypsin, pepsin, chymotrypsin, etc.

- Amides and esters

Amides, when heated in an alkaline medium, give rise to a carboxylic acid and an amine:

RCONH ₂ + H ₂ O => RCOO ^- + NH ₂

Esters in an aqueous medium are hydrolyzed to a carboxylic acid and an alcohol. The process is catalyzed by either a base or an acid:

RCO-OR '+ H ₂ O => RCOOH + R'OH

This is the famous saponification reaction.

- Acid-base

In water, several species are hydrolyzed to acidify or alkalize the aqueous medium.

Adding a basic salt

Sodium acetate, a basic salt, dissociates in water giving Na ⁺ (sodium) and CH ₃ COO ^- (acetate) ions. Its basicity is due to the fact that acetate is hydrolyzed to generate OH ^- ions, while sodium remains unchanged:

CH ₃ COO ^- + H ₂ O <=> CH ₃ COOH + OH ^-

OH ^- is responsible for the pH to rise and become basic.

Adding an acid salt

Ammonium chloride (NH ₄ Cl) is formed by the chloride ion (Cl ^-) from hydrochloric acid (HCl), a strong acid, and the ammonium cation (NH ₄ ⁺) from ammonium hydroxide (NH ₄ OH), a weak base. Cl ^- does not dissociate in water, but the ammonium cation is transformed in water in the following way:

NH ₄ ⁺ + H ₂ O <=> NH ₃ + H ₃ O ⁺

The hydrolysis of the ammonium cation produces protons that increase the acidity of an aqueous medium, for which it is concluded that NH ₄ Cl is an acid salt.

Adding a neutral salt

Sodium chloride (NaCl) is a salt product of the reaction of a strong base (NaOH) with a strong acid (HCl). By dissolving sodium chloride in water, the sodium cation (Na ⁺) and the anion (Cl ^-) are produced. Both ions do not dissociate in water, so they do not add H ⁺ or OH ^-, keeping their pH constant.

Therefore, sodium chloride is said to be a neutral salt.

References

1. Mathews, CK, van Holde, KE and Ahern, KG (2002). Biochemistry. (Third edition). Edit. Pearson-Addison Wesley.
2. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
3. Helmenstine, Anne Marie, Ph.D. (January 13, 2019). Hydrolysis: Definition and Examples. Recovered from: thoughtco.com
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5. The Editors of Encyclopaedia Britannica. (2016, November 16). Hydrolysis. Encyclopædia Britannica. Recovered from: britannica.com
6. Wikipedia. (2019). Hydrolysis. Recovered from: en.wikipedia.org