A. o-Hydrazinobenzoic acid hydrochloride. In a 2-l. beaker, provided with a stirrer and a low-temperature thermometer, and cooled in an ice-salt bath, are placed 42 g. (0.31 mole) of anthranilic acid and 300 ml. of water. The stirrer is started, and 340 ml. of concentrated hydrochloric acid (sp. gr. 1.18) is added in one portion; the anthranilic acid dissolves, and its hydrochloride begins to separate almost immediately. After the mixture has been cooled to 0°, a solution of 21.6 g. (0.31 mole) of technical sodium nitrite in 210 ml. of water is added from a dropping funnel, the tip of which extends below the surface of the suspension, at such a rate that the temperature never rises above 3°. The addition requires about 30 minutes; stirring is continued for 15 minutes longer, and at the end of this period a positive test with starch-iodide paper should be obtained (Note 1). The clear brown solution is then diluted with 150 ml. of ice water.

In a 12-l. flask, equipped with a low-temperature thermometer and surrounded by an ice-salt bath, a solution of sulfurous acid is prepared by saturating 2.4 l. of water at 0–5° with sulfur dioxide from a cylinder. A brisk stream of the gas is continued (Note 2) while the cold diazonium salt solution is added in about 150-ml. portions over a 30-minute period and the temperature is maintained at 5–10°; the reaction mixture assumes a dark orange color (Note 3). The cooling bath is removed, but sulfur dioxide is passed into the mixture for an additional 30 minutes. After the mixture has been allowed to stand for 12 hours at room temperature, 3 l. of concentrated hydrochloric acid (sp. gr. 1.18) is added; the o-hydrazinobenzoic acid hydrochloride separates at once. The mixture is chilled to 0–5° and filtered through a precooled Büchner funnel; the product is washed with two 50-ml. portions of ice-cold dilute (1:1) hydrochloric acid. The yield is 50–51 g. (86–88%); the salt melts at 194–195° with decomposition (Note 4) and is suitable for the next step without further purification (Note 5).

B. Indazolone. In a 2-l. round-bottomed flask to which a reflux condenser is attached are placed 47.1 g. (0.25 mole) of o-hydrazinobenzoic acid hydrochloride, 1.25 l. of water, and 12.5 ml. of concentrated hydrochloric acid (sp. gr. 1.18). The mixture is refluxed for 30 minutes. The resulting pale yellow solution is transferred in two portions to a 23-cm. evaporating dish and concentrated on the steam bath to about one-fourth its original volume. The indazolone separates at an early stage of the evaporation but redissolves as the concentration of acid increases. Sodium carbonate is added to the warm solution in small portions until the acid is neutralized (Note 6), and the suspension is allowed to stand for 2 hours. The nearly colorless indazolone is removed by filtration, washed with two 25-ml. portions of cold water, and air-dried. The yield of product, m.p. 246–249°, is 30–33 g. (90–98%) (Note 7).

C. 3-Chloroindazole. In a 200-ml. flask connected by a glass joint to an air condenser protected by a drying tube are placed 26.8 g. (0.2 mole) of dry indazolone and 15.8 g. (16 ml., 0.2 mole) of dry pyridine (Note 8); 46.1 g. (27.6 ml., 0.3 mole) of phosphorus oxychloride is then added, with shaking, over a 10-minute period. Heat is evolved, and acid fumes are generated. The mixture is heated with occasional shaking in an oil bath, which is maintained at 128–130° for 1 hour and at 130–140° for 4 hours. The clear brown solution is then cooled to 70° and poured, with hand stirring, upon 500 g. of cracked ice. This mixture is allowed to stand for 24 hours. The pale buff solid is removed, washed on the filter, first with 100 ml. of 0.5 N hydrochloric acid and then with 40 ml. of cold water, and air-dried (Note 9). The 3-chloroindazole is crystallized from 3 l. of 20% ethanol. The yield is 21–22.5 g. (68–74%) of material melting at 148–148.5° (Note 10).

D. Indazole. In a 300-ml. flask are placed 15.3 g. (0.1 mole) of 3-chloroindazole, 18.6 g. (0.15 mole) of red phosphorus, and 100 ml. of constant-boiling hydriodic acid (sp. gr. 1.7) (Note 11). The mixture is refluxed for 24 hours (Note 12), cooled, and filtered through a sintered-glass funnel (Note 13) to remove the phosphorus; the flask and the solid are washed with two 20-ml. portions of water. The clear filtrate is transferred to a 300-ml. Claisen flask and concentrated to about 40 ml. by heating in a water bath at a reduced pressure. The residue is washed into a 250-ml. beaker with 70–80 ml. of hot water, and the clear solution is cooled in an ice bath and made strongly alkaline with concentrated ammonium hydroxide (about 80 ml. is required). The next day, the indazole is collected and dried; the white solid melts at 143–145° (Note 14).

The product is added to 75 ml. of benzene, and the suspension is boiled until the frothing has ceased, the benzene lost being replaced (Note 15); the resulting suspension is filtered to remove the insoluble material. The clear filtrate is heated to boiling, diluted with 25 ml. of petroleum ether (b.p. 70–90°), and allowed to cool slowly. The yield of white product, m.p. 145–146.5°, is 9.7–10.2 g. (82–86%). The over-all yield from anthranilic acid is 43–55%.