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 .