Both crystallize in the ditrigonal pyramidal (hemimorphic-hemihedral) class of the rhombohedral system, possessing the same degree of symmetry as tourmaline. Crystals are perfectly developed and are usually prismatic in habit; they are frequently attached at one end, the hemimorphic character being then evident by the fact that the oblique striations on the prism faces are directed towards one end only of the crystal. Twinning according to several laws is not uncommon. The angles are nearly the same in the two species; the rhombohedral angle rr' being 71° 22' in pyrargyrite and 72° 12' in proustite. The hexagonal prisms of pyrargyrite are usually terminated by a low hexagonal pyramid (310) or by a drusy basal plane. The colour of pyrargyrite is usually greyish-black and the lustre metallic-adamantine; large crystals are opaque, but small ones and thin splinters are deep ruby-red by transmitted light, hence the name, from Gr. πῦρ (fire) and ἄργυρος (silver), given by E. F. Glocker in 1831. The streak is purplish-red, thus differing markedly from the scarlet streak of proustite and affording a ready means of distinguishing the two minerals. The hardness is 2½, and the specific gravity 5.85: the refractive indices and birefringence are very high, ω=3.084, ε=2.881. There is no very distinct cleavage and the fracture is conchoidal. The mineral occurs in metalliferous veins with calcite, argentiferous galena, native silver, native arsenic, &c. The best crystallized specimens are from St Andreasberg in the Harz, Freiberg in Saxony, and Guanajuato in Mexico. It is not uncommon in many silver mines in the United States, but rarely as distinct crystals; and it has been found in some Cornish mines.
Although the “ red silver ores ” afford a good example of isomorphism, they rarely form mixtures; pyrargyrite rarely contains as much as 3% of arsenic replacing antimony, and the same is true of antimony in proustite. Dimorphous with pyrargyrite and proustite respectively are the rare monoclinic species pyrostilpnite or fireblende (Ag3SbS3) and xanthoconite (Ag3AsS3): these four minerals thus form an isodimorphous group. (L. J. S.)
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, Bef., 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, Bef., 1876, 9, p. 563; V. Meyer and E. Braun, Bef., 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, J ourn. pralzt. 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 pyrazinc dicarboxylic acid formed when quinoxaline is oxidized with alkaline potassium permanganate (S. Gabriel). 2-5~-Dimethylpyrazfine, or ketine, C¢l°l2(CH3)2N2, 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-4 and 2~3- dihydro-compounds, corresponding to the formulae II. and Ill., pyrazine being l. 1- '-HC-N'CH
HC~NH-CH HC-N~CH2 HzC-NH-CHz
(Pyrazine) ll (1-4 dihydro) Ill (2~3 dihydro) IV (Piperazine). Those of the former type are obtained by condensing a-bromketones with primar amines (A. T. Mason, fourn. Chem. Soc., 1893, 63, p. 1355); the flatter type result on condensing alkylene diamines with a.-dike tones. 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, Bcr., 1891, 24, 956 R). Hexahydropyrazfine 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 piperazine. It is a strong base, melting at 104° and boiling at 145°-146°. It is used in medicine on account of the high solubility of its salt with uric acid.
PYRAZOLES, in organic chemistry, a series of hetero cyclic compounds containing a five-membered ring consisting of three carbon atoms united to two nitrogen atoms,
CH:CHNH(1) thus: the derivatives are orientated from the éH=N/ imino group, the second position being at the other nitrogen atom. Pyrazole, C3H4N2, was
obtained by E. Buchner (Ber., 1889, 22, p. 2165) by heating pyrazole 3.4.5.-tricarboxylic acid; and by L. Balbiano (Ben, 1890, 2 3, p. 1103), who condensed epichlorhydrin with hydrazine hydrate in the presence of zinc chloride:
It may also be prepared by the union of diazomethane with acetylene (H. v. Pechmann, Bef., 1897, 31, p. 2950), and by Warming the acetal of propargyl aldehyde with an aqueous solution of hydrazine sulphate (Ber., 1903, 36, p. 3662). It crystallizes in colourless needles, is Very stable and behaves as a weak base. It does not combine with the alkyl iodides. Ammoniacal silver nitrate gives a precipitate of pyrazole silver. The homologies of pyrazole may be obtained by digesting Bdiketones or B-keto-aldehyde's with phenylhydrazine; by heating the phenylhydrazones of some monoketones with acetic an hydride; by elimination of hydrogen from pyrazolines, and by distilling pyrazolones and pyrazolidones over zinc dust. They are all weak bases, which combine directly with the alkyl iodides and form double salts with mercuric and platinic chlorides. On oxidation with potassium permanganate the C-alkyl-derivatives give carboxylic acids, whilst the N-phenyl derivatives frequently split off the phenyl group (especially if it be amidated) andhave it replaced by hydrogen. On reduction, the yrazoles with a free:NH group are scarcely affected, whilst the Aiiphenyl derivatives give pyrazolines, or by the use of very strong reducing agents the ring is ruptured and trimethylenediamine derivatives are formed. They yield substitution derivatives with the halogens, bromine being the most effective. The chloro-derivatives are most readil prepared from the pyrazolones by the action of phosphorus oxychlloride. The pyrazole carboxylic acids may also be obtained by condensing /S-diketone or oxymetliylene ketone carboxylic esters with hydra zines, or the diazo fatty esters with acetylene dicarboxylic CSlZ€I'SI N2CH'CO2R+C2(CO2R)2 =C3HN2(CO2R) 3[3'4'5]§ by l1€2l.til'lg /S-dike tones and diazo-acetic ester with sodium hydroxide (A. Klages, Bef., 1903, 36, p. 1128), and from the diazo-anhydrides of B~dike tones or 13-ketonic acids. These acids all split CO2 readily when heated, most easil from the carboxyl group in position 3, and with most difficulty from the group in position 4. The dihydropyrazoles or pyrazolines are less stable than the pyrazoles and are more like unsaturated compounds. They may be obtained by the reduction of pyrazoles (especially N-phenyl derivatives) with sodium in alcoholic solution; by condensing diazo-acetic ester or diazomethane with ethylenic compounds (fumaric ester, &c.) (E. Buchner, Bef., 1890, 23, p. 703; Ann., 1895, 284, p. 212; H. v. Pechmann, Bef., 1894, 27, p. 1891), and by rearrangement of the hydra zones of a-olefine aldehyde's or ketones on warming or on distillation. They are weak bases which are only soluble in concentrated acids. On reduction they yield pyrazolidines, or the ring is broken; and when oxidized they form blue or red colouring matters. The carboxylic acids show a remarkable behaviour on heating, the nitrogen is entirely eliminated, and trim ethylene carboxylic acids are obtained (see POLYMETHYLENES). Pyrazoline is a colourless liquid which boils at 144° C. It may be prepared by the action of diazomethane on ethylene (E. Azzarello, Gazz., 1906, 36, (i.), p. 628).
The pyrazolones (ketodihydropyrazoles), first prepared by L. Knorr in 1883, result from the elimination of the elements of alcohol from the hydra zones of B-ketonic acids; or on the oxidation of the pyrazolidones with ferric chloride. Three types are possible with the formulae:
HQC-CO HC~CO HC:CH
| NH 4) NH é NH
HC:N/ H -NH/ o -NH/
Pyrazolone- 5 Antipyrine type Pyrazolone-3
They form salts with both acids and bases, and yield benzylidine and isonitroso derivatives. Pyrazolone is obtained by the condensation of hydrazine with formylacetic ester. It is a colourless crystalline solid which melts at 164° C. I-Phenyl-3-methylpyrazulone-5 is antipyrine (q.v.). The isomeric I~phenyl-5-methylpyrazolone-3 is formed by condensing aceto-acetic ester with acetophenylhydrazine in the presence of phosphorus oxychloride, or by the action of ferric chloride on the corresponding pyrazolidone, which is produced by condensing phenylhydrazine with a B-halogen butyric acid. When methylated it yields fisoantipyrine, an isomer of antipyrine, which is more poisonous.