Preparation of halogenated indigo

Details Preparation of halogenated indigo Preparation of halogenated indigo

1993-09-14

1995-01-10

Springer, David B.

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

548458, C07D40504

Patent

active

053808711

DESCRIPTION:

BRIEF SUMMARY
Of the multiplicity of halogen compounds of indigo described in the literature, only 5,7,5',7'-tetrabromoindigo finds industrial utility (Vat Blue 5, C.I. 73065). Vat Blue 5 is an important dye for dyeing and printing textiles in brilliant blues.
In industry this dye is synthesized by brominating indigo in glacial acetic acid as reaction medium (Ullmann's Encyclopedia of Industrial Chemistry, fifth edition: Vol. A 14, p. 155; Indigo and Indigo Colorants) in the presence of a hydrogen halide acceptor salt, e.g. sodium acetate (DR-C-237 262). It is also known to brominate indigo in nitrobenzene (DR-C-193 438) and in di- or trichlorobenzene (DR-C-208 471).
The halogenation of indigo in toxic solvents such as nitrobenzene or chlorobenzene is problematical with regard to occupational hygiene, clean air and clean water.
Unless the industrially practiced process involving glacial acetic acid as reaction medium is carried out with an uneconomical regeneration of the glacial acetic acid, the production of 1 kg of Vat Blue 5 tips about 5 kg of organically bound carbon into the waste water, so that considerable water treatment costs become unavoidable.
It is an object of the present invention to provide a process for preparing Vat Blue 5 that is free of the disadvantages of existing processes.
German Patents 226,319, 239,314 and in particular 229,304 likewise disclose processes for preparing indigo-halogen compounds. The nearest art is the disclosure in DE-C-229 304 of the reaction of indigo with bromine in sulfuryl chloride, which, however, does not give uniform indigo derivatives and, what is more, leads to the formation of by-products.
We have found, surprisingly, that halogenated indigo can be prepared in high yield and quality in an economically and ecologically advantageous manner by reacting indigo with bromine and/or chlorine in thionyl chloride as reaction medium.
As in the literature, only about half the theoretical amount of bromine is used for preparing tetrabromoindigo, since the resulting hydrogen bromide is converted back into bromine by passing chlorine into the reaction mixture. Excess chlorine eventually oxidizes the brominated indigo to its yellow dehydro form. ##STR1##
The dehydrotetrabromoindigo is finally reduced with a reducing agent, e.g. sodium thiosulfate, to tetrabromoindigo.
Depending on the reactor type employed, it has been found to be advantageous to introduce indigo into thionyl chloride in a weight ratio of from 1:1 to 1:10, preferably from 1:3 to 1:7, and to react it at from 0.degree. to 75.degree. C., preferably at from 20.degree. to 40.degree. C., with bromine in a molar ratio of from 1:0.5 to 1:3.0, preferably from 1:2.2 to 1:2.6, over 1-4 hours, preferably over 1.5-2.5 hours. The subsequent introduction of chlorine in a molar ratio of from 1:1.5 to 1:8, preferably from 1:5.5 to 1:7, at 0.degree.-75.degree. C., preferably at 20.degree.-40.degree. C., over 8-16 hours, preferably over 11-13 hours, in the presence or absence of iodine as catalyst gives the dehydroform of Vat Blue 5. The reaction mixture can then be introduced into water and the dehydroindigo reduced to Vat Blue 5 in a known manner. The bromine content of the resulting dye, which is of high quality in coloristic and fastness properties, is 50.5-53.5% coupled with a chlorine content of 0.5-2.5%.
If the halogenation is carried out in thionyl chloride using chlorine only in an indigo to chlorine molar ratio of from 1:5 to 1:10, preferably 1:8, then reduction of the dehydrotetrachloroindigo intermediate gives 5,7,5',7'-tetrachloroindigo with a chlorine content of 33.0-35.5%.
If indigo is reacted in thionyl chloride with less bromine than required for preparing Vat Blue 5, for example in a molar ratio of 1:1, then further reaction with chlorine and reduction of the dehydrotetrahaloindigo intermediate will give 5,5'-dibromo-7,7'-dichloroindigo.
It is particularly advantageous to halogenate indigo in thionyl chloride in a paddle dryer or in a rotating tube reactor made of a special steel, since in that case the thionyl chlor

REFERENCES:
patent: 2862930 (1958-12-01), Caliezi et al.
patent: 3530151 (1970-09-01), Hoare

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