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oxidation inhibitors, natural or synthetic substances capable of inhibiting oxidation of organic compounds.
The oxidation of hydrocarbons, alcohols, acids, fats, and so on by atmospheric oxygen is a chain process. The changes involved in the transformation are produced with the participation of active free radicals—that is, peroxide (), alkoxy (RO·), and alkyl (R·) radicals. Oxidation reactions are characterized by an increase in rate during the transformation (autocatalysis). This is due to the formation of free radicals during the decomposition of intermediate products such as hydroperoxides. The action of the most common antioxidants—for example, aromatic amines, phenols, and naphthols—consists of breaking the reaction chain: antioxidant molecules interact with active radicals to form relatively inactive radicals.
Oxidation also slows down in the presence of agents that decompose hydroperoxide—biakyl sulfides, for example. The role of such antioxidants is to decrease the rate of formation of free radicals. Effective antioxidants added in small amounts (0.1–0.001 percent) decrease the oxidation rate, and therefore the oxidation products are not detected for a certain period of time (inhibition or induction). The phenomenon of synergism, consisting of a mutual enhancement of the effective action of antioxidants in their mixture or in the presence of substances that are not antioxidants, is of great importance in the inhibition of oxidation processes.
Antioxidants have broad practical application. Oxidation leads to spoilage of valuable food products (rancidity of fats, destruction of vitamins); loss of mechanical strength; change in the color of polymers (rubber, plastics, fiber); tarring of fuel; formation of acids and sludge in turbine and transformer bowls; and so on. To increase the stability of food products that contain fats and vitamins, natural antioxidants—such as tocopherols (vitamin E) and nordihy-droguaiaretic acids—and synthetic antioxidants—such as propyl and dodecyl gallates and butyloxytoluene (ionol)—are used. Tarring of fuels is sharply inhibited by the addition of slight amounts of antioxidants (0.1 percent or less)—such as paraoxydiphenolamines, alpha-naphthol, and various fractions of tree resins. Antioxidants are added to lubricating oils and greases (1–3 percent)—for example, paraoxydiphenolamines, ionol, tributyl phosphate, zinc (or barium) dialkyldithiophosphate, and zinc dialkylphenol-dithiophosphate.
REFERENCESEmanuel’, N. M., and Iu. M. Liaskovskaia. Tormozhenie protses-sov okisleniia zhirov. Moscow, 1961.
Emanuel’, N. M., E. T. Denisov, and Z. K. Maizus. Tsepnye reaktsii okisleniia uglevodorodov v zhidkoifaze. Moscow, 1965.
Ingold, C. “Ingibirovanie avtookisleniia organicheskikh soedinenii v zhidkoi faze.” (Translated from English.) Uspekhi khimii,1964, vol. 33, no. 9.
A. B. GAGARINA and N. G. PUCHKOV