Hydrolysis is a chemical reaction in which a chemical compound decomposes by reaction with water. The reacting water molecules are split into hydrogen (H+) and hydroxide (OH−) ions, which react with and break up (or "lyse") the other reacting compound. The term hydrolysis is also applied to the electrolysis of water (that is, breaking up of water molecules by an electric current) to produce hydrogen and oxygen.
The hydrolysis reaction breaks down a variety of polymers, including proteins, carbohydrates, fats, and nucleic acids. Such reactions may be catalyzed by acids, bases, or enzymes (classified as hydrolases). Such reactions regularly occur during the process of digestion of food in living systems. In addition, hydrolysis is said to occur when a salt reacts with water to produce new ions or precipitates.
The hydrolysis reaction is distinct from a hydration reaction, in which water molecules attach to molecules of the other reacting compound without breaking up the latter compound. Also, hydrolysis should not be confused with hydrogenolysis, a reaction of hydrogen.
There are various types of hydrolysis reactions, as noted below.
In the hydrolysis of an amide link into a carboxylic acid and an amine or ammonia, the carboxylic acid has an hydroxyl group derived from a water molecule and the amine (or ammonia) gains the hydrogen ion.
A specific case of the hydrolysis of an amide link is the hydrolysis of peptides to smaller fragments or amino acids.
Many polyamide polymers such as nylon 6,6 are attacked and hydrolyzed in the presence of strong acids. Such attack leads to depolymerization and nylon products fail by fracturing when exposed to even small amounts of acid. Other polymers made by step-growth polymerization are susceptible to similar polymer degradation reactions. The problem is known as stress corrosion cracking.
In a polysaccharide molecule, monosaccharide subunits are linked together by glycosidic bonds. This bond can be cleaved by hydrolysis to yield monosaccharides. The best known disaccharide is sucrose (table sugar). Hydrolysis of sucrose yields glucose and fructose.
There are many enzymes that speed up the hydrolysis of polysaccharides. Invertase is used industrially to hydrolyze sucrose to so-called invert sugar. Invertase is an example of a glycoside hydrolase (glucosidase). Lactase is essential for digestive hydrolysis of lactose in milk. Deficiency of the enzyme in humans causes lactose intolerance. β-amylase catalyzes the conversion of starch to maltose. Malt made from barley is used as a source of β-amylase to break down starch into a form that can be used by yeast to produce beer. The hydrolysis of cellulose into glucose, known as saccharification, is catalyzed by cellulase. Animals such as cows (ruminants) are able to digest cellulose because of the presence of parasitic bacteria that produce cellulases.
Metal ions are Lewis acids, and in aqueous solution they form "aqua ions," of the general formula M(H2O)nm+. The aqua ions are hydrolyzed, to a greater or lesser extent. The first hydrolysis step is given generically as
Thus the aqua ion is behaving as an acid in terms of Brønsted-Lowry acid-base theory. This is easily explained by considering the inductive effect of the positively charged metal ion, which weakens the O-H bond of an attached water molecule, making the liberation of a proton relatively easy.
The dissociation constant, pKa, for this reaction is more or less linearly related to the charge-to-size ratio of the metal ion. Ions with low charges, such as Na+ are very weak acids with almost imperceptible hydrolysis. Large divalent ions such as Ca2+, Zn2+, Sn2+ and Pb2+ have a pKa of 6 or more and would not normally be classed as acids, but small divalent ions such as Be2+ are extensively hydrolyzed. Trivalent ions like Al3+ and Fe3+ are weak acids whose pKa is comparable to that of acetic acid. Solutions of salts such as BeCl2 or Al(NO3)3 in water are noticeably acidic; the hydrolysis can be suppressed by adding an acid such as nitric acid, making the solution more acidic.
Hydrolysis may proceed beyond the first step, often with the formation of polynuclear species. Some "exotic" species such as Sn3(OH)42+ are well characterized. Hydrolysis tends to increase as pH rises leading, in many cases, to the precipitation of an hydroxide such as Al(OH)3 or AlO(OH). These substances, the major constituents of bauxite, are known as laterites and are formed by leaching from rocks of most of the ions other than aluminum and iron and subsequent hydrolysis of the remaining aluminum and iron.
Ions with a formal charge of four are extensively hydrolyzed and salts of Zr4+, for example, can only be obtained from strongly acidic solutions. With oxidation states five and higher the concentration of the aqua ion in solution is negligible. In effect the aqua ion is a strong acid. For example, aqueous solutions of Cr(VI) contain CrO42-.
Note that reactions such as
are formally hydrolysis reactions as water molecules are split up yielding hydroxide ions. Such reactions are common among polyoxometalates.
Under physiological conditions (i.e. in dilute aqueous solution), a hydrolytic cleavage reaction, where the concentration of a metabolic precursor is low (on the order of 10-3 to 10-6 molar) and the reaction is catalyzed by an enzyme, is essentially thermodynamically irreversible. This theme of physiological irreversibility of hydrolysis is used consistently in metabolic pathways, since many biological processes are driven by the cleavage of anhydrous pyrophosphate bonds.
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