Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

ISATIN 375
The beautiful preparation should be isolated by the former of
Kalb's two procedures. 1
The best known oxidative transformation of indigo is that into isatin.
This is a normal oxidation at a double bond :
CO CO CO CO
Isatin is the inner anhydride (lactam) of a y-amino-a-ketocarboxylic
acid, isatinic acid (A), and is converted into a salt of this acid by the
action of alkali. The keto-group in position 3 can condense with many
other substances, and for this reason isatin is manufactured on a technical
scale and converted into valuable indigoid vat dyes. The magnificent
thioindigo scarlet, which is obtained from isatin and a sulphur
analogue of indoxyl (the so-called hydroxythionaphthene), may be taken
as example: QQ
co co c=c/
Sandmeyer's valuable isatin synthesis involves the removal of
sulphur from diphenylthiourea (I) (p. 169) with basic lead carbonate.
Hydrogen cyanide is combined with the reactive diphenylcarbodiimide
(II) so obtained, and the nitrile (III) produced is converted by means
of hydrogen sulphide into the thioamide (IV). Concentrated sulphuric
acid brings about ring closure and the product is the a-anil of isatin (V).
Then, by hydrolysis with dilute sulphuric acid, aniline is removed :
CN
(C6H5NH)2.C8 —> C6H5.N=C=N.C6H5 >- [ IIIJ ^C=*
C=NC6H5.
Sandmeyer's synthesis may be recommended also as a preparative
method.
It may be recalled that the first synthesis of indigo, by Baeyer,
started from isatin chloride. (Formulate it.)