Hydrolysis

Hydrolysis is a chemical reaction in which water (H₂O) reacts with another substance to split it into two or more new substances. Examples of the process of hydrolysis include the conversion of starch to glucose in water under the action of a suitable catalyst; conversion of sucrose to fructose and glucose in water in the presence of an acidic or enzyme catalyst; conversion of fats into fatty acids and glycerol in water in soap manufacture; and reactions of dissolved salts in water to form various products. Hydrolytic reactions are important in many biochemical processes, particularly in enzyme-catalyzed reactions such as those used in digestion. The processes of hydrolysis and condensation are often considered opposite processes since the later process brings together (rather than separates) two substances while involved with water.

In each reaction, a water molecule separates into H⁺ and OH^-, and the compound being hydrolyzed splits into two fragments, A⁺ and B^-, where AB represents the original (generic) compound. The products of hydrolysis are AOH and HB. If, for example, ordinary sodium chloride (table salt) is placed in water, then some of the salt will form sodium and chlorine ions. These ions will then combine with the hydrogen and hydroxide ions in water to form ionized sodium hydroxide (Na⁺ + OH^-) and ionized hydrogen chloride (H⁺ + Cl^-). Sodium hydroxide and hydrogen chloride essentially exist as ions in solution because they are a strong base and strong acid, respectively. Therefore, they are neutralized by the hydroxide and hydrogen ions, and the resultant solution is neutral.

Whenever water reacts with another chemical compound, the process is called hydrolysis. Hydrolysis differs somewhat from the process called hydration, although the two can occur together. Hydration is the bonding of whole water molecules to an ion (a charged atom or molecule), usually a metal ion. Hydrolysis, on the other hand, involves an actual chemical reaction of the water molecule itself with another reactant. Aluminum ion, for example, can bond with six water molecules to form the hydrated aluminum ion. In water, however, the hydrated ion can undergo hydrolysis; some of the hydrated molecules contribute a hydrogen ion to the solution, making the solution acidic.

Solutions of non-hydrated ions often become either acidic or basic because of hydrolysis, too. In general, negative ions (anions) form basic solutions if they hydrolyze, because the negative charge on the ion attracts the positively charged hydrogen ion (H+) away from water, leaving the basic hydroxide ion (OH-) behind. Similarly, positive ions (cations) form acidic solutions if they hydrolyze, because the positive charge on the ion attracts the negatively charged hydroxide ion away from water, leaving the acidic hydrogen ion behind. Hydrolysis of these ions only occurs, however, if the ion originally came from a weak acid or base, or the salt of a weak acid or base. (A salt is an ionic chemical compound derived from an acid or base, often as the result of a neutralization reaction.) Ions do not hydrolyze if they are from strong acids or bases—such as chloride ion from hydrochloric acid or sodium ion from sodium hydroxide (a base)—or their salts.

In biochemistry, hydrolysis often involves the decomposition of a larger molecule. If a fat undergoes hydrolysis, for example, it reacts with water and decomposes to glycerol and a collection of fatty acids. Similarly, complex sugars can hydrolyze to smaller sugars, and nucleotides can hydrolyze to a five carbon sugar, a nitrogenous base, and phosphoric acid.