Hydrolysis is a chemical reaction in which water reacts with another substance to split it into two or more new substances. Examples 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.
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 compound. The products of hydrolysis are AOH and HB. If, for example, ordinary sodium chloride (table salt) is placed in water, some of the salt will form sodium and chlorine ions, and these ions will 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.
The solution formed by hydrolysis may be acidic, basic, or neutral. In the case of sodium carbonate dissolved in water, the sodium ions react with the hydroxide ions of water to form ionized sodium hydroxide, but the hydrogen ions and carbonate ions form a weak acid that is only slightly ionized. The resultant solution is basic. When zinc chloride is placed in water, the hydrogen ions and chlorine ions form a strong acid, while the zinc and hydroxide ions form a weak base. In this case, an acidic solution is produced. Finally, if ammonium carbonate is placed in water, the weak base ammonium carbonate and weak acid carbonic acid are produced. Because there is neither an excess of hydrogen nor hydroxide ions, the resulting solution is neutral.
In the hydrolysis of carboxylic esters of the form R-C(=O)- OR', the carboxylic ester reacts with water under strong acid catalysis to produce the carboxylic acid R-C(=O)-OH and alcohol or phenol R'OH from which the ester was originally formed. Esters can also be hydrolyzed with a basic catalyst, in which case the carboxylic ester converts into a carboxylate ion R-C(=O)-O- and an alcohol. (Following the hydrolysis, the carboxylate ion can be converted into a free acid by adding hydrogen chloride.) The advantage of the basic catalysis over the acidic catalysis is that the former is a non-reversible. This is because the carboxylic acid and alcohol produced under acid catalysis can recombine to give the ester, but no recombination is possible when the carboxylate ion is formed under basic conditions. The base- catalyzed hydrolysis of carboxylic esters is referred to as saponification. Although, the human body contains many carboxylic esters, which are constantly being formed and hydrolyzed, these reactions are neither acid- nor base- catalyzed, being rather catalyzed by enzymes.
The case of the hydrolysis of amides (having the form R- C(=O)-NR2') is similar to ester hydrolysis in that the hydrolysis requires a strong base or acid, but different in that neither the acid- catalyzed nor base-catalyzed reaction is reversible.
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