Thioindigo is an

organosulfur compound Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...

that is used to dye polyester fabric. A synthetic dye, thioindigo is related to the plant-derived dye

indigo InterGlobe Aviation Limited (d/b/a IndiGo), is an India, Indian airline headquartered in Gurgaon, Haryana, India. It is the largest List of airlines of India, airline in India by passengers carried and fleet size, with a 64.1% domestic market ...

, replacing two NH groups with two sulfur atoms to create a shade of

pink Pink is a pale tint of red, the color of the Dianthus plumarius, pink flower. It was first used as a color name in the late 17th century. According to surveys in Europe and the United States, pink is the color most often associated with charm, p ...

. Thioindigo is generated by the

alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...

of the sulfur in thiosalicylic acid with

chloroacetic acid Chloroacetic acid, industrially known as monochloroacetic acid (MCA), is the organochlorine compound with the formula . This carboxylic acid is a useful building block in organic synthesis. It is a colorless solid. Related compounds are dichlo ...

. The resulting thioether cyclizes to 2-hydroxy thianaphthene, which is easily converted to thioindigo.Elmar Steingruber "Indigo and Indigo Colorants" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2004, Wiley-VCH, Weinheim. The related compound 4,7,4',7'-tetrachlorothioindigo, also a commercially important dye (Pigment Red 88), can be prepared by chlorination of thioindigo.

References

{{reflist Indigo structure dyes Organosulfur compounds