GB395183A - Improvements in the manufacture and production of stable reduction products of dyestuffs of the indigo series - Google Patents

Abstract

Stable indigo preparations.--Stable reduction products of halogen derivatives of indigo, indirubin and their homologues are prepared by heating the leuco compounds of the dyestuffs wholly or substantially in the free form, dispersed in aqueous or water-containing media until the leuco-compounds are converted wholly or substantially into compounds which are difficultly soluble or insoluble in cold aqueous solutions of caustic alkalies. The dyestuffs may be reduced in alkaline, neutral or acid media, and the free leuco compounds heated simultaneously or subsequently or after isolation either alone in aqueous or water-containing media, or with acid agents; further organic solvents may be present. The products may be obtained in one operation by reducing in the absence of alkalies, or in the presence of smaller amounts than are required for forming the mono-metallic salts of the leuco compounds, heating being continued until conversion is complete; a normally alkaline reducing bath is acidified by adding acids or acid salts, and the mixture then heated; a strongly acid solution is to be avoided. In examples (1) 5,5<1>:-7,7<1>-tetrachlorindigo is boiled in alcohol with ammonium formate and sulphur dioxide until a stable reduction product separates out; (2) 5,5<1> : 7,7<1>-tetrabromindigo is heated in aqueous-alcohol with sodium hydrosulphite, and after reduction the solution is boiled; or the dye is treated in very dilute alkali solution in presence of a nickel catalyst with hydrogen under pressure at about 90 DEG C.; or the dehydroacetate derivative is boiled in alcohol with ammonium formate and bisulphite solution; (3) 5,5<1>-dibromindigo is heated in dilute alkali solution with sodium hydrosulphite at 60 DEG C., and sulphur dioxide is passed in to precipitate the free leuco-compound which is then boiled in alcohol with exclusion of air and in presence of bisulphite; (4) tribromo-Indigo red and 4,4<1>-dichloro-5,5<1>-dibromindigo are treated as in (1); (5) 5,5<1>-dibromo-7-7<1>-dimethylindigo is reduced in alcohol by sodium hydrosulphite at 55 DEG C. and the solution boiled; 4,4<1>-dichlorindigo is similarly treated, the products thus obtained are stable in air and may be dried without special precautions; they dissolve in hot alkali solutions and in cold alcoholic alkali solutions to give the ordinary leuco-compounds into which they may also be converted by steaming (when worked up as printing pastes). Specifications 18023/08, [Class 2, Acids and salts, Organic &c.], 19848/14, 299,899, and 309,970, [all in Class 2 (iii), Dyes &c.], are referred to. The Specification as open to inspection under Sect. 91 relates to a heat treatment of leuco compounds of halogenated dyestuffs of the indigo series to give stable reduction products, but this subject-matter does not appear in the Specification as accepted.