Isatin

Isatin, C8H5NO2, in chemistry, a derivative of indol, interesting on account of its relation to indigo; it may be regarded as the anhydride of ortho-aminobenzoylformic or isatinic acid. It crystallizes in orange red prisms which melt at 200-201° C. It may be prepared by oxidizing indigo with nitric or chromic acid (O. L. Erdmann, Jour. prak. Chem., 1841, 24, p. 11); by boiling ortho-nitrophenylpropiolic acid with alkalis (A. Baeyer, Ber., 1880, 13, p. 2259), or by oxidizing carbostyril with alkaline potassium permanganate (P. Friedlander and H. Ostermaier, Ber., 1881, 14, p. 1921). P. J. Meyer (German Patent 26736 (1883)) obtains substituted isatins by condensing para-toluidine with dichloracetic acid, oxidizing the product with air and then hydrolysing the oxidized product with hydrochloric acid. T. Sandmeyer (German Patents 113981 and 119831 (1899)) obtained isatin-α-anilide by condensing aniline with chloral hydrate and hydroxylamine, an intermediate product isonitrosodiphenylacetamidine being obtained, which is converted into isatin-α-anilide by sulphuric acid. This can be converted into indigo by reduction with ammonium sulphide. Isatin dissolved in concentrated sulphuric acid gives a blue coloration with thiophene, due to the formation of indophenin (see Abst. J.C.S., 1907). Concentrated nitric acid oxidizes it to oxalic acid, and alkali fusion yields aniline. It dissolves in soda forming a violet solution, which soon becomes yellow, a change due to the transformation of sodium N-isatin into sodium isatate, the aci-isatin salt being probably formed intermediately (Heller, Abst. J.C.S., 1907, i. p. 442). Most metallic salts are N-derivatives yielding N-methyl ethers; the silver salt is, however, an O-derivative, yielding an O-methyl ether (A. v. Baeyer, 1883; W. Peters, Abst. J.C.S., 1907, i. p. 239).