4943. Indigo

Nomenclature

CAS number: 482-89-3
2-(1,3-Dihydro-3-oxo-2H-indol-2-ylidene)-1,2-dihydro-3H-indol-3-one; [Δ2,2′-biindoline]-3,3′-dione; Δ2,2′-bipseudoindoxyl; indigotin; indigo blue; D & C Blue No. 6; C.I. Pigment Blue 66; C.I. Vat Blue 1; C.I. 73000.
C16H10N2O2; mol wt 262.26.
C 73.28%, H 3.84%, N 10.68%, O 12.20%.

Description and references

Probably the oldest known coloring matter. Originally obtained from various species of Indigofera, Leguminosae, indigenous to Bengal, Java, Guatemala, in which it occurs as a glucoside. First synthesis: Baeyer, Ber. 11, 1296 (1878); 12, 456 (1879). First commercial process: Heumann, Ber. 23, 3043, 3431 (1890). Newer prepns: Harley-Mason, J. Chem. Soc. 1950, 2907; Ziegler, Kappe, Monatsh. Chem. 96, 889 (1965); J. Gosteli, Helv. Chim. Acta 60, 1980 (1977). Indigo occurs in cis and trans forms, in solid state it is in the trans form: Posner, Ber. 56, 31 (1923); 57, 1313 (1924); 59, 1799 (1926). Reviews on indigo and indigoid dyes: Lubs, The Chemistry of Synthetic Dyes, ACS Monograph Series no. 127 (Reinhold, New York, 1955) pp 551-576; Schweizer, Künstliche Organische Farbstoffe und ihre Zwischenprodukte (Springer Verlag, 1964) pp 320-334; Colour Index vol. 4 (3rd ed., 1971) p 4595.

Chemical structure

Properties

Dark-blue powder with coppery luster. Sublimes at about 300°; dec 390°. Practically insol in water, alcohol, ether, and dil acids. Dissolves in nonpolar solvents with red and in polar solvents with blue color. With fuming H2SO4 it forms a sol sulfonic acid.

Use

As textile dye. In sutures.