Лако-красочные материалы — производство

Although the Sandmeyer indigo synthesis is no longer used, it is such an interesting example of the combined efforts of science and in­
dustry that it deserves a place in this book. The process involves the following steps:

(a) Aniline is converted to thiocarbanilide (A. W. Hoffmann) by heating with carbon bisulfide:

2 (/’4’j NH2 + CS, /-NH / 4/ CS—NH—^ + HjS

Thiocarbanilide
m. p. 151°

(b)

Sulfur is split from the thiocarbanilide by treatment with basic lead carbonate, and simultaneously hydrocyanic acid is added, resulting in the formation of Laubenheimer’s hydrocyanoearbodiphenylimide:

(c) The hydrocyanoearbodiphenylimide is converted to thiooxami — dodiphenylamidine (“thioamide”) by treatment with yellow ammonium sulfide:

Y~NH

./ C « N-</

s=c

I

nh2

“Thioamide,” m. p., 161-2°

(d)

The “thioamide” is converted smoothly to a-isatinanilide by the action of concentrated sulfuric acid:

alsatinanilide,
m. p. 126°

(e)

The a-isatinanilide can be converted to indigo in various ways. It can be reduced in alcoholic solution with dilute ammonium sulfide solution, or converted to a-thioisatin which immediately goes to indigo when treated with alkali. The latter method is shown here because it was used industrially:

+ Na. CO,

A mixture of 186 grams of pure aniline and 100 grams of pure car­bon bisulfide is boiled under reflux until evolution of hydrogen sulfide ceases, which requires about 2 days. The temperature of the oil bath is then raised to 160°C. and the excess carbon bisulfide is distilled off. The residue of melted thiocarbanilide is poured out into a flat dish and pulverized after cooling. It is desirable to recrystallize the crude prod­uct from alcohol which gives lustrous crystals melting at 151°. The yield is about 200 grams of purified material. (Recrystallization of the thiocarbanilide is usually omitted in industrial preparations, although such purification would prevent the formation of small amounts of oily by-products in subsequent steps.)