Indazoles (Benzopyrazoles), organic substances containing the ring system 
The parent substance indazole, C7H6N2, was obtained by E. Fischer (Ann. 1883, 221, p. 280) by heating ortho-hydrazine cinnamic acid,
It has also been obtained by heating ortho-diazoaminotoluene with acetic acid and benzene (F. Heusler, Ber., 1891, 24, p. 4161).
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 
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.
