Hydrolysis

HYDROLYSIS (from Greek hydor-water and lysis-separation), processes of splitting molecules of complex chemical compounds through the addition of elements of water. One should distinguish between hydrolysis of electrolytes (hydrolytic dissociation) and hydrolysis of organic compounds. Hydrolytic dissociation. When salts of strong acids with strong bases (e.g., NaCl, K₂SO₄) are dissolved in water, the resulting solution shows a neutral reaction, whereas when salts of weak acids or weak bases are dissolved, the reaction changes to an alkaline or acidic side. Thus, e.g., a solution of potassium cyanide (K-strong base, and HCN-very weak acid) shows an alkaline reaction, a solution of aluminum sulfate (salt of a strong acid and weak base)-has an acidic reaction. This phenomenon, called hydrolysis, is explained as follows. The mentioned salts of weak bases or acids, like all electrolytes, in aqueous solution dissociate into ions, almost completely. On the other hand, water, although to a very small degree, is also dissociated into H" and OH' ions. Thus, in a solution, e.g., KCN, along with undissociated molecules, there will be ions: K', CN', H' and OH'. Since HCN is a very weak acid, it in aqueous solution dissociates into H' and CN' ions only to a very small degree, while the main mass of it remains in the form of undissociated HCN molecules, and conversely-if in aqueous solution there are H' and CN' ions, they in their main mass combine into undissociated HCN molecules. This is precisely the case when dissolving KCN: the CN' ions, appearing as a result of dissociation of KCN, meet with the H" ions formed due to dissociation of water and combine with them into undissociated HCN molecules. The resulting decrease in H' ions is compensated by dissociation of new water molecules until a state of some equilibrium is established. At this point, free OH' ions remain in the solution, which give it an alkaline reaction. When dissolving a salt of a weak base, e.g., Al, the latter's ions combine with OH' ions of water into undissociated molecules, while the remaining excess H' ions give the solution an acidic reaction. Thus, in hydrolysis of electrolytes, there is as it were a splitting of water molecules, during which one of the ions is bound into an undissociated molecule, while the second gives the solution one or another reaction. The degree of hydrolysis, i.e., the ratio of the number of hydrolyzed molecules to the number of non-hydrolyzed ones, increases with rising temperature. In contrast to processes of hydrolytic dissociation, which practically occur instantaneously, hydrolysis of organic compounds in most cases proceeds very slowly, and to accelerate it one has to resort to the participation of various catalysts. Such are primarily the elements of water itself, i.e., H and OH ions. Under the influence of acids or alkalis, many complex organic compounds, by adding elements of water, break down into simpler constituent parts. Processes of this kind include, e.g., hydrolytic splitting of esters (since the production of soap from fats is also based on hydrolysis of complex esters, other hydrolytic splittings are often called 'saponification'). Saponification of esters proceeds according to the scheme: R.CO.O.R' + H₂O = R.COOH + HO.R'. As another example, one can cite saponification of organic derivatives of hydrocyanic acid, so-called nitriles; acid halohydrates and some amides are already decomposed simply by water. Reactions of hydrolysis are widely used both in preparative chemistry and in technology. The most important technical applications of hydrolysis are soap-making and starch syrup production. In the first case, fats, when boiled with alkali, undergo hydrolysis and break down into their constituent parts-glycerin and fatty acids, with which the latter, with excess alkali, give salts, i.e., soap. The production of syrup is based on hydrolysis of the high-molecular carbohydrate-starch, which when heated with sulfuric acid turns into the monosaccharide glucose, the syrup-like solution of which is syrup. The application of hydrolysis in biological chemistry played an exceptionally important role for studying the structure of the most important constituent parts of animal and plant organisms-proteins and carbohydrates. It became possible to approach the elucidation of the structure of these extremely complex compounds only after hydrolysis was found to be a method for breaking them down into simpler structural units, already accessible to direct chemical analysis. Usually, protein hydrolysis is performed by boiling with strong acid or alkali for a rather long time. In this process, the protein breaks down into its main structural elements-individual amino acids. Recently, Zelinsky showed that at somewhat elevated pressure, protein hydrolysis already occurs under the influence of even such weak acids as oxalic and formic. For hydrolysis of complex carbohydrates, they are heated with dilute acid, with the result that free lower carbohydrates-monosaccharides-are obtained. Hydrolysis is the basis, e.g., for the determination of glycogen: the latter, when boiled with hydrochloric acid, breaks down to glucose, which is then determined. Reactions of hydrolysis, along with oxidation processes, are the most important processes in the economy of a living organism. The latter possesses powerful catalysts for various hydrolytic processes. These catalysts are enzymes. The very same splittings of complex high-molecular compounds, which are achieved by using strong acids and alkalis at high temperature, proceed in the organism under the influence of hydrolytic enzymes. The biological significance of hydrolysis reactions is twofold: first, the predominant part of substances taken with food represents such complex compounds that they, due to the size of the molecule giving them a colloidal character (proteins, starches), or due to physical properties (insolubility of fats in water), cannot penetrate, diffuse through the walls of the digestive canal. As a result of hydrolysis, conditioned by the enzymes contained in digestive juices, complex molecules break down into low-molecular, easily diffusing components, freely entering the blood and used by the cells and tissues of the organism. In this utilization, the circumstance that all hydrolysis processes are reversible is of great importance, i.e., under certain conditions, from simple substances, by elimination of elements of water, more complex substances are formed. This makes it possible, e.g., for the construction of tissue proteins from amino acids formed during hydrolysis of food proteins, or of glycogen from monosaccharide molecules. The second task which hydrolysis reactions perform in the organism is the preparation of substances for utilizing the chemical energy contained in them. The energy necessary for the organism to perform its life functions, for mechanical work, maintaining temperature, work of growth and reproduction, is drawn practically exclusively from processes of aerobic oxidation or anaerobic breakdown of organic molecules. However, these oxidation and breakdown processes in the overwhelming majority of cases occur only on the simplest compounds, whereas in the organism the entire reserve of potential chemical energy is deposited in the form of more stable, high-molecular substances. In the tissues, under the influence of tissue enzymes, gradual hydrolysis of these reserve substances occurs, and the products of hydrolytic breakdown are then utilized as substrate for oxidation and breakdown reactions. Thus, hydrolytic processes, themselves almost neutral in thermochemical relation, also participate in the energy economy of the organism.