PYRAZINES, PIAZINES, or PARADIAZINES, in organic chemistry, a group of compounds containing a ring system composed of 4 carbon atoms and 2 nitrogen atoms, the nitrogen atoms being in the para position. The di- and tri-methyl derivatives are found in the fusel oil obtained by fermentation of beetroot sugar (E. C. Morin, Comptes rendus, 1888, 106, p. 360). 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. 1830; 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 is heated with ammonia or when glycerin is heated with ammonium chloride and ammonium phosphate (C. Stoehr, Journ. prakt. Chen., 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, Cir^Nj, 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-$-Dimethylpyrazine, or ketine, C4H 2 (CH 3 ) 2 N 2 , is obtained by reducing isonitrosoacetone, or by heating glycerin with ammonium chloride and ammonium phosphate. It boils at 153 C.
Two classes of dihydropyrazines are known, namely the I -A and 2-3- dihydro-compounds, corresponding to the formulae II. and III., pyrazine being I.:
HC-N-CH HC-NH-CH HC-N-CH 2 H,C-NH-CH, HC-N-CH HC-NH-CH HC-N-CH S H 2 C-NH-CH, 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. Ghem. 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 they readily give up two hydrogen atoms. Tetrahydropyrazines of the 1-2-3-4 type have also been obtained (L. Garzini, Ber., 1891, 24, 956 R). Hexahydropyrazine or piperazine (formula IV. above), also known as diethylene diamine, may be prepared by reducing pyrazine, or, better, by combining aniline and ethylene bromide to form diphenyl diethylene diamine, the dinitroso compound of which hydrolyses to para-dinitrosophenol and pipera- zine. It is a strong base, melting at 104 and boiling at I45-I46. It is used in medicine on account of the high solubility of its salt with uric acid.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)