Reduction (or restoration, is a chemical process)

Reduction, or restoration, is a chemical process consisting either in the removal of oxygen from a given substance, or in the replacement of oxygen with hydrogen, or in the addition of hydrogen to this substance. If the reacting substance is in the form of ions, then reduction is understood as the transition of this ion from a higher degree of positive valence to a lower one, i.e., a decrease in its positive charge; for example, the reduction of ferric oxide to ferrous oxide is expressed by the following reduction scheme: Fe+++ + H2 = Fe++ + 2H+. The majority of reduction processes, with the exception of hydrogenation reactions (see), are inextricably linked with oxidation phenomena, because in the reaction system one of the substances, at the expense of which the reduction proceeds, itself becomes oxidized. In some cases, especially with respect to organic compounds, it is possible to speak of reduction (or oxidation) of individual carbon atoms of a chain or cycle. Thus, rearrangements associated with the movement of H or O2 atoms within a molecule can be considered as redox phenomena. These intermolecular and intramolecular phenomena play a huge role in the processes of living nature. The assimilation of CO2 by plants, respiration, metabolism, and many other vital functions of a living organism are fundamentally based on redox phenomena. The study of these phenomena, their mechanisms, and the conditions under which they occur is one of the essential problems of biochemistry (for the theory of these processes, see Oxidation). Reduction plays no less important a role in questions of synthetic organic chemistry and in the methodology for studying the structure of organic substances. Methods of reduction can be divided into three main groups: 1) purely chemical, 2) catalytic, and 3) electrolytic. Chemical methods are most often used in synthetic work. Their peculiarity lies in the fact that for the purposes of reduction, special reducing agents are used, which either by themselves or in combination with other substances under certain conditions are capable of donating H or accepting O from the compound being reduced. Reducing agents include: 1) many metals, such as sodium, zinc, iron, tin, magnesium, aluminum, their amalgams and alloys, which upon contact with acids, alkalis, or water (as well as with alcohols) release H in an active (atomic) form, possessing a particularly strong reducing action; 2) a large number of mineral salts and acids (tin chloride, ferrous salts, titanium chloride, sulfurous and hydrosulfurous acids and their salts, hydrogen sulfide and sulfides of metals, hydroiodic, arsenious, and phosphorous acids), as well as some bases (hydrazine, hydroxylamine); 3) individual metalloids (phosphorus, sulfur); numerous organic compounds (methyl, ethyl alcohols and their alkoxides, formaldehyde, acetaldehyde, formic acid, glucose, etc.). To obtain the desired reducing effect, in addition to the choice of reducing agent, the nature of the solvent or the character of the medium in which the reaction is carried out is also of great importance. Thus, in the reduction of nitrobenzene in acidic media, aniline is obtained, in alkaline media (depending on the experimental conditions) various azo compounds are obtained: azoxybenzene, azobenzene, or hydrazobenzene. In some cases, the solvent promotes the isomerization of the primarily formed substance or interacts with the products of reduction. There is no general theory that would allow one to predict in advance the reducing agent and the conditions of reduction for a given substance, and therefore in each individual case one must rely only on a series of experimental observations. The reduction of alcohols to hydrocarbons, R.OH → RH, is carried out by heating alcohols with hydroiodic acid and red phosphorus, or they are first converted into iodides, which are then easily reduced by hydrogen "in statu nascendi". The reduction of some alcohols can be achieved by boiling them with zinc dust or by the action of metallic sodium or sodium amalgam on their alcoholic solutions. Catalytic methods of reduction have recently been widely used both in laboratories and in technology. Their peculiarity lies in the fact that in all cases the reducing agent is H. The latter by itself does not act on most organic compounds. In the presence of special substances - catalysts - it is activated, i.e., it acquires the ability to react (see Catalysis). There are two main methods of catalytic reduction: 1) at elevated t°, with the substance to be reduced being used in the form of vapor (Sabatier), and 2) at ordinary or low temperature in solution (Willstatter, Paal, Fokin, Skita). In reduction by the first method, the catalysts are metals - cobalt, iron, copper, and especially nickel - in a finely divided state or deposited on some porous (pumice) or fibrous (asbestos) material. By this method, the most diverse reduction effects can be achieved: alcohols are reduced to hydrocarbons, fatty and aromatic aldehydes to alcohols, ketoacids to ketols, ketones to hydrocarbons, acids to aldehydes, acid amides to amines, unsaturated compounds to saturated ones, aromatic nitro compounds to amines. The Sabatier method is suitable for reducing only those compounds that do not decompose at temperatures above their boiling point. This disadvantage is eliminated by the method of reduction in solution. Here, finely divided or colloidal palladium, platinum, and nickel are used as catalysts. By regulating the amount of hydrogen absorbed, different degrees of reduction can be achieved. Since the process takes place at low temperatures, the possibility of side reactions such as isomerization, decomposition, etc., is almost completely excluded, for which reason this method is especially suitable for research related to the study of the structure of organic substances. In some cases, reduction has to be carried out under increased pressure. By the method of reduction in solution, aldehydes and ketones can be converted to alcohols and hydrocarbons, amino ketones and amino aldehydes to keto- and aldehyde alcohols, acid chlorides to aldehydes, alcohols to hydrocarbons, nitro compounds to amines. Both methods of catalytic reduction are especially suitable for the hydrogenation (see) of unsaturated, aromatic, and heterocyclic compounds. Electrolytic (cathodic) reduction is the basis for the electrolytic deposition of metals from their salts (see Electrolysis). Despite the very great advantages of electrolytic reduction over purely chemical reduction (convenience of regulating conditions, control, and isolation of reduction products), it has not yet found wide application in organic synthesis. With the exception of very rare individual cases, the cathodic method of reduction is not used either in laboratory practice or in technology.