ISATIN, in chemistry, an oxygenated derivative of indole (q.v.) prepared by oxidizing indigo with nitric or chromic acid. It crystallizes in orange-red prisms melting at 2oo-2oi° C. Isatin, which furnishes indirubin, the red pigment of indigo, on con densation with indoxyl, is of practical importance in the manu facture of vat colours (see DYES, SYNTHETIC). For example, it yields Ciba red G (thioindigo scarlet G) by condensation with a-hydroxythio-naphthene and subsequent bromination of the con densation product. Isatin is also of outstanding scientific interest because it affords a noteworthy example of tautomerism (see ISOMERISM). It is a chemical individual with two personalities. Certain of its reactions suggest the lactam constitution (I.) whereas others indicate the lactim configuration (II.).
Although itself existing in only one form, isatin has two methyl ethers : pseudomethylisatin (III.) made from dibromomethyl oxindole, and methylisatin (IV.) prepared by the interaction of methyl iodide and silver isatin. The absorption curves of isatin and the pseudomethyl ether are practically identi cal and differ considerably from that of the methyl ether. Accord ingly, isatin is regarded as having the lactam structure, although its silver derivative corresponds with the lactim formulation.
Isatin and its homologues are produced by condensing aniline and its homologues with hydroxylamine sulphate and chloral hydrate; an intermediate isatinimide is formed which on hy drolysis furnishes the corresponding isatin.