CHAIN REACTIONS

From Big Medical Encyclopedia

CHAIN REACTIONS — chemical and nuclear processes, in to-rykh transformation of mother substances into end products it is carried out by regular alternation of several consecutive reactions (links) with participation of atoms, free radicals or neutrons. C. rubles are called (are initiated) by free radicals (see Radicals free) or neutrons (see. Neutron emission) also support -

the CERVICITIS 175


are howled by the radicals regenerated on each unit step (link) of process (chain), e.g.:



where p — a neutron, v — number of secondary neutrons.

Depending on that how many new free radicals (active centers) are formed in one chain link, C. rubles divide on unbranched (one radical is formed) and branched out (two-three radicals). Reactions of initiation (origin), development, or continuation, and break of chains are characteristic of any chain process. At a stage of initiation of C. the river occurs disintegration of molecules on atoms and radicals that demands receipt of energy from the outside. Heating, a shockwave, ionizing radiation, light, etc. can be an energy source. If mother substances do not absorb light, initiation can be carried out by means of photosensitizers (see Fotosensibilization). E.g., at illumination of vapors of mercury there are excited Hg\atoms to-rye. reacting with other molecules, free radicals form. The same radicals are formed at illumination of solutions of porphyrine in the presence of oxygen as a result of reaction of the excited atoms O with organic molecules.

There are substances, addition to-rykh in stationary system causes education in it free radicals and start of C. river. Such substances are called initiators. In chain reactions with participation of organic matters as initiators peroxides often act (see). Dissociation of molecules and formation of active centers can happen due to energy of adsorption of molecules on a surface of a solid. C. the river of compound of H2 hydrogen and oxygen 02 are initiated in the presence of platinum sponge, on a surface the cut occurs dissociation of H2 on atoms. After formation of active centers chain process develops spontaneously.

Development or continuation of a reaction chain consists of alternation of unit steps of an expenditure of mother substances and formation of products and new radicals. Chain length is defined by the number of links falling on average on one free radical formed in reaction of initiation. Reaction photo chemical educations of HC1 hydrogen chloride (see. Photochemical reactions) has quantum you -


the course 106, i.e. one chlorine atom gives rise to the chain consisting of 1 million links. Examples of such chain processes are formation of HC1 from hydrogen and chlorine, chlorination of methane and ethylene, oxidation of benzaldehyde, polymerization of chloroprene, etc. If one radical reacts, and two or more radicals are formed, the chain branches, and reaction gains the avalanche self-accelerating character. Usually oxygen takes part in such reactions:


At the end of one development cycle of a chain the same active centers, as at the beginning of process appear. Examples of branched C. rubles can serve oxidation of hydrogen, phosphine, silane, carbon sulfur, carbon oxide and phosphorus in a gas phase. Branched out by C. the river is uranium fission (see).

Theory of branched C. the river was created by H. N. Semenov and scientists of his school. The chain has final length owing to death of active centers (breaks) as a result of a recombination of radicals or education of low-active radicals. The recombination of active centers can happen because of existence in reaction mixture of the inert impurity which are selecting excess of energy at collision of radicals, and also at adsorption of radicals on a vascular wall. Low-active radicals are formed at interaction of active centers with inhibitors of chain reaction. In live organisms there are enzymes interfering development of chain processes, napr, the superoxide scavenger (peroxide-dismutase; KF 1.15.1.1) catalyzing dismutation of radicals 0 ~ 2 and interfering transformation superoxidic radical anion in a cytotoxic hydroxylic radical. Development of chain processes in animals and plants (as a rule, undesirable) is promoted aging (see the Old age, aging), by nek-ry diseases and impact of ionizing radiation. The chain mechanism characterizes process of peroxide oxidation of lipids (see). Increase in contents in an organism of free radicals and an intensification of free radical processes is observed in an initial stage of a leukosis, and also at atherosclerosis, a radial illness and many other diseases. For detection of free


radicals, definition of their concentration and speed of C. rubles apply physical methods: spectroscopy -

(see), a mass spectral analysis (see Mass spectrometry), a method элект^ a ronny paramagnetic resonance (see), a catalytic recombination, etc., and also the chemical methods based on property of radicals or atoms to form steady molecules, concentration to-rykh and measure,

See also Kinetics of biological processes.

Bibliography: Vladimirov Yu. A. and

Archakov A. I. Peroxide oxidation of lipids in biological membranes, M., 1972; N and to and t and K. N. Kinetik's N of chain reactions, M., 1972; Semenov H. H. Chain reactions, JI., 1934; Chemical: kinetics and chain reactions, under the editorship of V. N. Kondratyev, M., 1966;

D an i n-t about n F. S. Chain reactions, L. — N. Y. 1966. century A. Pekkel.

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