Radicals

RADICALS (residues), stable groupings of atoms in a substance molecule. Radicals pass from one compound to another in an unchanged form in a whole series of chemical transformations. The concept of radicals first appears with Lavoisier (Lavoisier, 1787). The works of Gay-Lussac on cyanogen (1815), Liebig and Wöhler on benzoyl (1832) served as the foundation for the creation of the radical theory, the main position of which was the idea that organic substances are composed of radicals, just as inorganic substances are composed of atoms. Berzelius, a zealous defender of the radical theory, even proposed to denote radicals by a single symbol, just as individual atoms are denoted. The recognition of the absolute invariability of radicals was the weak point of the old radical theory, as a result of which it was later replaced by the type theory (see Organic Chemistry), however the concept of radicals has been preserved to the present time and greatly facilitates both the nomenclature of organic compounds and the consideration of transformations of organic substances. - According to the number of free valences, one-valent, two-valent and three-valent radicals are distinguished; for example, from methane CH₄ successive removal of hydrogen produces radicals: one-valent methyl CH₃-, two-valent methylene CH₂= and three-valent methenyl or methin CH=. The most common are one-valent radicals - residues of saturated hydrocarbons with the general formula C_nH_2n+1, so-called alkyls (see). Residues of aromatic hydrocarbons containing a benzene ring received the name aryls or aromatic radicals. The following radicals are often encountered: CH₃-methyl; C₂H₅-ethyl; C₃H₇-propyl; C₄H₉-butyl; C₅H₁₁-amyl, representing the residues of the corresponding hydrocarbons; CH₂=CH- vinyl; CH₃-C- acetyl - residue of acetic (CH₃CO-) acid; -C- acyl - the general name of the residue of an acid; C₆H₆-phenyl - residue of benzene; C₆H₅-C- benzoyl; CN- cyano; -C- carbonyl; -C-OH- carboxyl; C₆H₅-SO₂-sulfonyl and others. The reaction of introducing a methyl radical into an organic substance molecule received the name methylation, and analogously the terms acylation, acetylation, etc. are used. Radicals, entering into a given compound, can impart to it characteristic biochemical action. Radicals, entering into the composition of a complex organic molecule, influence the atoms and groupings associated with them, for example, radicals C₆H₅-phenyl, CH₃-C- acetyl, C₆H₅-C- benzoyl impart acidic properties to the substance, i.e., make the hydrogen atom mobile in neighboring groupings. Such radicals received the name negative atomic groups, negative radicals. The influence of negative radicals can be illustrated by the following examples: 1) in alcohols (C₂H₅OH and others) the hydrogen of the hydroxyl group does not possess acidic properties, alcohols are substances of a neutral character; when the hydroxyl group is connected to a negative radical, then clearly expressed acidic properties appear: C₆H₅OH- carbolic acid; CH₃-C-OH acetic acid; 2) the residue of ammonia - the amino group NH₂ - loses its alkaline properties upon connection with a negative radical: CH₃-NH₂-methylamine - a substance of sharply basic character; C₆H₅NH₂-aniline - only weak basic properties; CH₃-C-NH₂-acetamide - a neutral body. The negative influence of radicals is closely connected with the unsaturation of radicals, i.e. with the presence of double or triple bonds in them. The first attempts to obtain free radicals (Frankland, Kekulé, 1848; Butlerov, 1859) led to negative results due to the ease with which individual radicals combine with each other. In 1890 Nef again raised the question of free radicals, showing the existence of a series of compounds with a two-valent carbon, and in 1900 Gomberg first discovered the free radical - triphenylmethyl (C₆H₅)³C-, formed at high temperature due to the dissociation of hexaphenylethane (C₆H₅)₃C-C(C₆H₅)₃ ± 2(C₆H₅)₃C-. Later it was possible to obtain a whole series of free radicals, relatively stable at normal temperature (the works of Schlenk, Ziegler, Wieland). As a result of these studies, the modern conception of radicals as freely existing, unsaturated complexes with a valence of the central atom deviating from the norm (Wieland) was worked out.

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