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Originally appearing in Volume V22, Page 687 of the 1911 Encyclopedia Britannica.
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PYRAZOLES, in organic chemistry, a series of heterocyclic compounds containing a five-membered ring consisting of three carbon atoms united to two nitrogen atoms, thus: the derivatives are orientated from the 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 (Ber., 1890, 23, p. 1103), who condensed epichlorhydrin with hydrazine hydrate in the presence of zinc chloride: C3H6OC1+2N2H4 =C 3H4N2+N2H4•HCl+H2O +H2. It may also be prepared by the union of diazomethane with acetylene (H. v. Pechmann, Ber., 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 homologues of pyrazole may be obtained by digesting 0-diketones or 13-keto-aldehydes with phenylhydrazine; by heating the phenylhydrazones of some monoketones with acetic anhydride; 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) and have it replaced by hydrogen. On reduction, the pyrazoles with a free :NH group are scarcely affected, whilst the N-phenyl 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 readily prepared from the pyrazolones by the action of phosphorus oxychloride. The pyrazole carboxylic acids may also be obtained by condensing'3-diketone or oxymethylene ketone carboxylic esters with hydrazines, or the diazo fatty esters with acetylene dicarboxylic esters: N2CH.CO2R+C2(CO2R)2=C3HN2(CO2R)a[3.4-5]; by heating ,3-diketones and diazo-acetic ester with sodium hydroxide (A. Klages, Ber., 1903, 36, p. 1128), and from the diazo-anhydrides of #-diketones or '3-ketonic acids. These acids all split CO2 readily when heated, most easily 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, Ber., 189o, 23, p. 703; Ann., 1895, 284, p. 212; H. v. Pechmann, Bee., 1894, 27, p. 1891), and by rearrangement of the hydrazones of a-olefine aldehydes 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 trimethylene 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 hydrazones of 13-ketonic acids; or on the oxidation of the pyrazolidones with ferric chloride. Three types are possible with the formulae: H2C•CO HC.CO HC:CH ~NH H NNH I NNH HC:N / HC•NH" OC•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-methylpyrazolone-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 13-halogen butyric acid. When methylated it yields isoantipyrine, an isomer of antipyrine, which is more poisonous. CH:CH ~NH(1) CH=N' The pyrazolidines are tetrahydropyrazoles. The N-phenyl derivative, from sodium phenylhydrazine and trimethylene bromide, is an oil which readily oxidizes to phenylpyrazoline on exposure. The corresponding keto-derivatives, or pyrazolidones, are produced by the action of hydrazines on the $-haloid acids or a$-olefine dicarboxylic acids. Isomeric compounds may arise here when phenylhydrazine is used, the keto-group taking either the 3 or 5 position; thus with $-iodopropionic acid I-phenylpyrazolidone-5 is formed, whilst potassium $-iodopropionate gives the 3-compound. Isomers of this type may be distinguished by the fact that the pyrazolidone-5 compounds are basic, whilst the 3-compounds are acidic. The simplest member of the series, pyrazolidone-5, is a liquid which is formed by the action of hydrazine on acrylic acid. The 3.5-pyrazolidones are the cyclic hydrazides of the malonic acid series. Thiopyrazoles have been obtained by A. Michaelis (Ann., 1904, 331, p. 197; Bee., 1904, 37, p. 2774) by the action of an aqueous or alcoholic solution of the methyl chloride or iodide of phenylmethylchlorpyrazole on a solution of an alkaline hydrosulphide into which carbon bisulphide has been passed ; or by the action of sodium thiosulphate on antipyrine hydrochloride or a similar compound. The simplest member of the group is probably to be represented as HC: C(SH)\ H2C—CS N•C6H6 or I >N•C6H6.
End of Article: PYRAZOLES

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