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Encyclopedia Britannica - Main :: PRE-PYR |
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PYRAZINES, PIAZINES , Or PARADIAZINES, in organic chemistry , a group of compounds containing a ring system
sugar
Morin , Comptes rendus, 1888, 106, p. 36o). They were first prepared synthetically by reducing the isonitrosoketones. They may also be prepared by the inner condensation of a-aminoaldehydes or a-aminoketones in the presence of a mild oxidizing agent, such as mercuric chloride or copper sulphate in boiling alkaline solution (L. Wolff, Ber., 1893, 26, p. 183o; S. Gabriel, ibid. p. 2207); and by the action of ammonia on a-halogen ketonic compounds (W. Staedel and L. Rugheimer, Ber., 1876, 9, p. 563; V. Meyer and E. Braun, Ber., 1888, 21, p. 19). They are also formed when grape sugar
prakt. Chem., 1895 (2), 51, p. 450; 1896 (2), 54, p. 481). They are feeble basic compounds which distil unchanged. They are mostly soluble in water and somewhat hygroscopic in character. Their salts are easily dissociated. They form characteristic compounds with mercuric and auric chlorides. Their alkyl derivatives readily oxidize to pyrazine carboxylic acids. Pyrazine, C4H4N2, crystallizes from water in prisms, which have a heliotrope odour. It melts at 55 C. and boils at 115 C. It may also be obtained by elimination of carbon dioxide from the pyrazine dicarboxylic acid formed when quinoxaline is oxidized with alkaline potassium permanganate (S. Gabriel). 2.5-Dimethylpyrazine, or ketine, C4H2(CH3)2N2, is obtained by reducing isonitrosoacetone, or by heating
Two classes of dihydropyrazines are known, namely the 1.4 and 2.3 dihydro-compounds, corresponding to the formulae II. and III., pyrazine being I.: HCNCH HCNHCH HCNCH2 H2C.NH.CH2 HCNCH HCNHCH HC]VCH2 H2CNHC. H2 I (Pyrazine) II (1.4 dihydro) III (2.3 dihydro) IV (Piperazine). Those of the former type are obtained by condensing a-bromketones with primary amines (A. T. Mason, Journ. Chem. Soc., 1893, 63, p. 1355) ; the latter type result on condensing alkylene diamines with a-diketones. The 2.3 derivatives are somewhat unstable compounds, since on heating
formula
diphenyl
zine. It is a strong base, melting at 104 and boiling at 145146 It is used in medicine
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