PYRAZOLES, in organic chemistry a series of compounds containing a five-membered ring with three carbon atoms united to two adjacent nitrogen atoms arranged as in formula (I.), which represents the structure of pyrazole itself.
4-aminopyrazole, which resembles the aromatic bases; it gives a colour reaction with bleaching powder solution and its hydro chloride is readily diazotised. Pyrazole-4-diazonium chloride couples with phenols and naphthols to form azo-dyes after the manner of benzenoid diazo-compounds.
Pyrazoline or 4 :5-dihydropyrazole (formula II.), is formed from acrolein and hydrazine hydrate and has also been obtained by the interaction of diazomethane and ethylene. It is a colour less liquid boiling at C. The pyrazolines are unstable oxidis able substances resembling dihydrobenzene and its homologues; they are generally prepared by reducing the corresponding pyra zoles with sodium in alcoholic solution. On further reduction they yield pyrazolidines or tetrahydropyrazoles and when oxidised they develop blue or red colorations. This reaction, described by L. Knorr as the "pyrazoline reaction," is used for the detection of pyrazole and pyrazoline bases derived from phenylhydrazine. The pyrazolones (ketodihydropyrazoles), first prepared by Knorr in 1883, result from the elimination of the elements of alcohol from the hydrazones of (3-ketonic acids. The most important member of this group is I-phenyl-3-methyl-5-pyrazolone, the parent substance of antipyrine and an intermediate in the manu facture of certain pyrazolone dyes (Eriochrome Red, etc.). I-Phenyl-3-methyl-5-pyrazolone, prepared by condensing to gether phenylhydrazine and ethyl acetoacetate, crystallises in colourless prisms melting at 127° C, and although it is only known in this form, nevertheless it exhibits twofold tautomerism (see IsomERIsm) and behaves as if it had one or another of the three following structures When condensed with aldehydes, water is eliminated between aldehydic oxygen and the two hydrogens of the methylene group, thus indicating the methylene form. When methylated with methyl iodide in methyl alcohol to form the hydriodide of anti pyrine (I-phenyl-2 :3-dimethylisopyrazolone), the pyrazolone re acts in its imino form, the methyl group replacing the hydrogen of the imino group. Antipyrine (q.v.) and its therapeutically im
portant derivatives and analogues are all constituted on the imino type. When in the production of synthetic dyes the pyra zolone is coupled with diazonium salts, it reacts in its third phe nolic form, the diazo-group replacing the hydrogen of the CH residue adjacent to the hydroxyl-group.
Picrolonic acid, The application in medicine of such pyrazole derivatives as antipyrine (q.v.) and pyramidone, and the production of syn thetic dyes (q.v.) of the pyrazolone type, lends a special interest to this chapter. of organic chemistry.
The simplest synthesis of pyrazole is effected by heating to gether acetylene and diazomethane (H. von Pechmann, 1897), this process corresponding with the synthesis of pyrrole (q.v.) from acetylene and ammonia. The earliest synthesis of pyrazole was accomplished by E. Buchner in 1889, who condensed ethyl diazoacetate with diethyl acetylenedicarboxylate, thus obtaining triethylpyrazole-3 :4 : 5-tricarboxylate. The free pyrazoletricar boxylic acid on prolonged heating lost carbon dioxide and yielded pyrazole, which crystallises in colourless needles melting at 70° C (boiling point, 185° C), and giving well-defined salts with acids. Pyrazole has certain chemical properties in common with pyridine and benzene ; it exhibits an aromatic character and can be sul phonated or nitrated. Its 4-nitro-compound can be reduced to or 4-nitro-I-p-nitrophenyl-3-methyl-5-hydroxypyrazole, is also a pyrazolone if phenolic type. It closely resembles picric acid and is used to identify alkaloids and other bases owing to the formation of very sparingly soluble picrolonates. For benzo pyrazoles, see INDAZOLES.
Schmidt, A Text Book of Organic Chemistry (translated by H. G. Rule) (1926) ; W. Meijer, Pyrazolones (1925); G. Cohn, Tabillarische Obersicht der Pyrazolderivate (Brunswick, 1897). (G. T. M.)