PHTHALAZINES (benzo-orthodiazines or benzopyridazines)  , in organic chemistry a group of heterocyclic compounds containing the ring complex shown in formula I . They are isomeric with the cinnolines (q.v.) . The parent substance of the group, phthalazine, CBH6N2, is best obtained from the condensation of w-tetrabromorthoxylene with hydrazine (D . Gabriel, Ber., 1893, 26, p . 2210), or by the reduction of chlorphthalazine with phosphorus and hydriodic acid (Ber., 1897, p . 3024) . It possesses basic properties and forms addition products with alkyl iodides . On oxidation with alkaline potassium permanganate it yields pyridazine dicarboxylic acid . Zinc and hydrochloric acid decompose it with formation of orthoxylylene diamine . The keto-hydro derivative phthalazone, C8H6ON2, (formula II.), is obtained by condensing hydrazine with orthophthalaldehydoacid . On treatment with phosphorus oxychloride it yields a chlorphthalazine which with zinc and hydrochloric acid gives isoindole, C8H,N, and with tin and hydrochloric acid phthalimidine, C8H2ON, the second nitrogen atom being eliminated as ammonia . \I NIN I \I CO NH \/\/ N I\/\CH/N I .

Phthalazine . II . Phthalazone .