1911 Encyclopædia Britannica/Indazoles

INDAZOLES (Benzopyrazoles), organic substances containing the ring system

	∕CH	NH.
	|
	N

The parent substance indazole, C₇H₆N₂, was obtained by E. Fischer (Ann. 1883, 221, p. 280) by heating ortho-hydrazine cinnamic acid,

C6H4	CH = CH·COOH	=C2H4O2+C7H6N2.
	NH·NH2

It has also been obtained by heating ortho-diazoaminotoluene with acetic acid and benzene (F. Heusler, Ber., 1891, 24, p. 4161).

C6H4	CH3	=C7H7NH2+C7H6N2.
	N:N·NHC7H7

It crystallizes in needles (from hot water), which melt at 146.5° C. and boil at 269°-270° C. It is readily soluble in hot water, alcohol and dilute hydrochloric acid. Nitrous acid converts it into nitrosoindazole; whilst on heating with the alkyl iodides it is converted into alkyl indazoles.

A series of compounds isomeric with these alkyl derivatives is known, and can be considered as derived from the ring system

	∕NH	N.
	CH

These isomers are called isindazoles, and may be prepared by the reduction of the nitroso-ortho-alkylamino-acetophenones with zinc dust and water or acetic acid. The indazoles are weak bases, which crystallize readily. Phenyl indazole, on reduction with sodium and absolute alcohol, gives a dihydro derivative (K. L. Paal, Ber., 1891, 24, p. 963).

For other derivatives, see E. Fischer and J. Tafel, Ann. 1885, 227, p. 314.