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the transformation of one or more substances into one or more new substances, differing from the original in chemical composition or structure. Both the total number of atoms of any given element and the chemical elements making up the substances remain unchanged during a chemical reaction. In this regard, chemical reactions are distinguished from nuclear reactions. Chemical reactions are brought about by the interaction of substances or by the effect of such external influences as temperature, pressure, and electric and magnetic fields. During the course of these reactions, certain substances—the reactants—are changed into new substances —the products—and the reaction is expressed in the form of a chemical equation. Each chemical reaction is characterized by a stoichiometric relationship between the substances and by a reaction rate. The series of steps constituting a reaction is either determined experimentally or derived from theory, and the entire series is known as the reaction mechanism.
Every chemical reaction is reversible, although the rates of the forward and reverse reactions may differ markedly. When the two rates are equal, the system is said to be in chemical equilibrium. The behavior of a system at or near equilibrium is described by the laws and relationships of chemical thermodynamics. The study of the mechanisms and rates of both reversible and practically irreversible chemical reactions as a whole constitutes the subject of chemical kinetics, and when also taking into account such physical processes as diffusion and heat transfer in a system, the subject of macrokinetics.
The investigation of chemical reactions on the molecular level involves the use of concepts describing the interaction of atoms and molecules upon collision with each other, with electrons, and with other particles, and the molecular transformations that occur upon the absorption and emission of photons. This approach is based, as a rule, on quantum theory and is connected with the study of the elementary event of a chemical reaction, that is, the individual process wherein the molecules of the substances involved in the reaction collide with each other. The quantum-mechanical description of the elementary event is based on one of two approaches. According to the temporal approach, the elementary event is regarded as a process of dispersion of subsystems (atoms, molecules, ions) upon collision. In accordance with the stationary approach, the motion of the configuration point (representing the nuclear configuration of the entire system of substances involved in the reaction) is studied along a potential surface. This surface is determined by the interaction of the subsystems of the substances, in particular, the nuclei of molecules in a neutralized field of electrons. The development of the stationary approach began with the introduction of the concept of the activated complex. A comparative study of reactions, especially in organic chemistry, usually employs the concepts of the most probable reaction mechanisms and of the activity of the substances in specific classes of reactions. These concepts include reactivity, orientation rules, nucleophilic and electrophilic reagents, and the principle of the preservation of orbital symmetry.
Chemical reactions are highly dependent on the nature of the substances involved and the external conditions. Many reactions are possible only upon the action of external sources of energy, for example, thermal energy, energy in the form of electromagnetic waves (photochemical reactions), and electric energy (electrochemical reactions). Furthermore, the chemical reaction itself can serve as a source of energy. The quantitative experimental study of chemical reactions has established a number of basic chemical laws, which reflect both the stoi-chiometry and the energetics of reactions. Examples are the law of definite proportions and Hess’ law.
Chemical reactions are classified according to various characteristics, and the method of classification differs with the field of chemistry in which the reactions are being studied. Thermodynamic classification employs as characteristics the energetics of reactions (exothermic, that is, proceeding with the liberation of heat, and endothermic, that is, proceeding with the absorption of heat) and the number of phases involved in the reaction (homogeneous and heterogeneous reactions). For example, reactions occurring in bulk are distinguished from those occurring at the boundary between phases.
Kinetic classification selects as characteristics the rates of the forward and reverse reactions (reversible and irreversible reactions), the number of interrelated reactions in a system (simple reaction, which is a single, practically irreversible reaction, and complex reaction, which may be subdivided into several simple reactions), the molecularity of reactions (the number of molecules whose simultaneous interaction brings about the elementary event of a chemical transformation), and the order of the reaction for each reactant and as a whole. Complex chemical reactions, depending on the relationship to the simple reactions, can be classified as side, consecutive, coupled, or reverse reactions. The broad class of reactions involving catalysis forms a separate group. Depending on the particles involved in the elementary event of a reaction, reactions can be grouped into such categories as molecular, ionic, or photochemical or can be considered as reactions involving radicals or chain reactions. There is also a detailed classification of reactions based on reaction mechanism.
A classification widely used for the reactions in inorganic chemistry depends on the type of compounds taking part in the reactions and the nature of the interactions of these compounds. These interactions include reactions of formation and decomposition, hydrolysis, neutralization reactions, and oxidation-reduction reactions. The various reactions forming complex compounds constitute a large group of chemical reactions.
The two large groups into which the reactions in organic chemistry are divided are the heterolytic, where the breaking of a bond in a molecule proceeds asymmetrically and the electrons remain paired, and the homolytic, where the bond breaking is symmetrical and leads to the formation of radicals. Heterolytic reactions can be either nucleophilic (denoted by the symbol N) or electrophilic (symbol E), depending on the type of attacking reactant. The three main classes of organic reactions include substitution (symbol S with subscripts N or E), addition (symbol A), and elimination (symbol E). Depending on the mechanism, each of these reactions can proceed as a nucleophilic, electrophilic, or radical process. Cycloaddition reactions form a special class of reactions. Reactions are classed as uni-molecular, for example, SE 1, or bimolecular, for example, SE 2, when the molecularity of the limiting step is considered. Aside from the mechanisms given above, addition and substitution reactions can occur as a result of the oxidation-reduction interaction of reactants. Many organic reactions involve a number of consecutive steps, including reverse steps. General reversibility is characteristic of such reactions as metalation and aromatic sulfonation. Reactions may occur in which the intermediate compounds enter into side reactions; this leads to the formation of a mixture of products. Among the numerous transformations of organic molecules are processes that, while not effecting a change in composition, alter the chemical structure of a compound. These processes include various types of isomerization, molecular rearrangements, and tautomeric transformations.
The concept of chemical reactions is to a certain extent arbitrary. For example, chemical reactions do not usually include such processes as the formation of associated molecules in solutions and the electron excitation of molecules, even when the excitation causes a marked change in the geometric equilibrium configuration.
REFERENCESEmanuel’, N. M., and D. G. Knorre. Kurs khimicheskoi kinetiki, 2nd ed. Moscow, 1969.
Kurs fizicheskoi khimii, 2nd ed., vol. 2. Editor in chief, Ia. I. Gerasimov. Moscow, 1973.
Mathieu, J., and R. Panico. Kurs teoreticheskikh osnov organicheskoi khimii. Moscow, 1975. (Translated from French.)
N. F. STEPANOV