Isatin: Indigo Oxidised with Nitric Acid (Erdmann, Cohen 1920)

Indigo boiled with dilute nitric acid breaks into isatin, which dissolves in the hot water and crystallises out red, leaving tar behind. 100 g of indigo gives about 10 g of isatin. Isatin forms red prisms melting at 201 °C, soluble in hot water and alcohol. With strong sulphuric acid and benzene it turns blue if thiophene is present. Not given: the strength and amount of the potash, and how much acid the purification takes. Source: J. B. Cohen, Practical Organic Chemistry (1920), Preparation 111, after Erdmann (1841). Picture: our drawing.