Thioindigo is an

organosulfur compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulf ...

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

indigo Indigo is a deep color close to the color wheel blue (a primary color in the RGB color space), as well as to some variants of ultramarine, based on the ancient dye of the same name. The word "indigo" comes from the Latin word ''indicum'', ...

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

pink Pink is the color of a namesake flower that is a pale tint of red. 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, politeness, ...

. Thioindigo is generated by the

alkylation Alkylation is the transfer of an alkyl group from one molecule to another. 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 effectin ...

of the sulfur in

thiosalicylic acid Thiosalicylic acid is an organosulfur compound containing carboxyl and sulfhydryl functional groups. Its molecular formula is C6H4(SH)(CO2H). it is a yellow solid that is slightly soluble in water, ethanol and diethyl ether, and alkanes, but more ...

with

chloroacetic acid Chloroacetic acid, industrially known as monochloroacetic acid (MCA), is the organochlorine compound with the formula ClCH2CO2H. This carboxylic acid is a useful building block in organic synthesis. It is a colorless solid. Related compounds a ...

. 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, can be prepared by chlorination of thioindigo.

References

{{reflist Indigo structure dyes Organosulfur compounds