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PYRAZOLES

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Originally appearing in Volume V22, Page 687 of the 1911 Encyclopedia Britannica.
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PYRAZOLES , in organic See also:

chemistry, a See also:series of heterocyclic compounds containing a five-membered See also:ring consisting of three See also:carbon atoms See also:united to two See also:nitrogen atoms, thus: the derivatives are orientated from the imino See also:group, the second position being at the other nitrogen See also:atom. Pyrazole, C3H4N2, was obtained by E. See also:Buchner (Ber., 1889, 22, p. 2165) by See also:heating pyrazole 3.4.5.-tricarboxylic See also:acid; and by L. Balbiano (Ber., 1890, 23, p. 1103), who condensed epichlorhydrin with See also:hydrazine See also:hydrate in the presence of See also:zinc chloride: C3H6OC1+2N2H4 =C 3H4N2+N2H4•HCl+See also:H2O +H2. It may also be prepared by the See also:union of diazomethane with See also:acetylene (H. v. Pechmann, Ber., 1897, 31, p. 2950), and by warming the acetal of propargyl aldehyde with an aqueous See also:solution of hydrazine sulphate (Ber., 1903, 36, p. 3662). It crystallizes in colourless needles, is very See also:stable and behaves as a weak See also:base. It does not combine with the alkyl iodides.

Ammoniacal See also:

silver nitrate gives a precipitate of pyrazole silver. The homologues of pyrazole may be obtained by digesting 0-See also:diketones or 13-keto-See also:aldehydes with phenylhydrazine; by heating the phenylhydrazones of some monoketones with acetic anhydride; by elimination of See also: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 See also:form See also:double salts with mercuric and platinic chlorides. On oxidation with See also: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 See also: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 See also:halogens, See also:bromine being the most effective. The chloro-derivatives are most readily prepared from the pyrazolones by the See also:action of See also:phosphorus oxychloride. The pyrazole carboxylic acids may also be obtained by condensing'3-diketone or oxymethylene ketone carboxylic See also: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 See also:sodium hydroxide (A. Klages, Ber., 1903, 36, p. 1128), and from the diazo-anhydrides of #-diketones or '3-ketonic acids. These acids all split See also: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; See also:

Ann., 1895, 284, p. 212; H. v. Pechmann, See also:Bee., 1894, 27, p. 1891), and by rearrangement of the hydrazones of a-See also:olefine aldehydes or See also:ketones on warming or on See also: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 See also: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 See also:POLYMETHYLENES). Pyrazoline is a colourless liquid which boils at 144° C. It may be prepared by the action of diazomethane on See also: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 See also:

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: See also:H2C•CO HC.CO HC:CH ~NH H NNH I NNH HC:N / HC•NH" OC•NH" Pyrazolone-5 See also: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-See also:

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. See also: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•See also:C6H6 or I >N•C6H6.

End of Article: PYRAZOLES

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