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Piece of indigo plant dye from India, c. 2.5 inches (6.35 cm) square.
Lump of Indian indigo dye

Skeletal formula of indigo dye

Ball-and-stick model of the indigo dye molecule
2,2'-Bis(2,3-dihydro-3- oxoindolyliden), Indigotin
|Section1=Chembox Identifiers
chemspider It is also used as a food colorant, and is listed in the United States as FD&C+ Blue No. 2.

A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus ''Indigofera+'', which are native to the tropics+. The primary commercial indigo species in Asia was true indigo (''Indigofera tinctoria+'', also known as ''I. sumatrana''). A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is ''Strobilanthes cusia+''. In Central+ and South America, the two species grown are ''I. suffruticosa+'' (''añil'') and dyer's knotweed+ (''Polygonum tinctorum''), although the ''Indigofera'' species yield more dye. In Europe woad+ containing the same dye was used for blue-dying. Several plants contain indigo, but low concentrations make them difficult to work with and the color is then more easily tainted by other dye substances, typically leading to a greenish tinge.

Natural sources also include mollusks, the Murex+ sea snails produce a mixture of indigo and dibromoindigo (red) which together produce a range of purple hues known as Tyrian purple+. Light exposure during part of the dying process can convert the dibromoindigo into indigo resulting in blue hues known as royal blue or hyacinth purple.

The precursor to indigo is indican+, a colorless, water-soluble derivative of the amino acid tryptophan+. Indican readily hydrolyzes+ to release β-D-glucose+ and indoxyl+. Oxidation+ by exposure to air converts indoxyl to indigo. Indican was obtained from the processing of the plant's leaves, which contain as much as 0.2–0.8% of this compound. The leaves were soaked in water and fermented+ to convert the glycoside+ indican present in the plant to the blue dye '''indigotin'''. The precipitate from the fermented leaf solution was mixed with a strong base such as lye+, pressed into cakes, dried, and powdered. The powder was then mixed with various other substances to produce different shades of blue and purple.

Indigo was used in India, which was also the earliest major center for its production and processing.Kriger and Connah, page 120 The ''I. tinctoria+'' species was domesticated in India. Indigo, used as a dye, made its way to the Greeks+ and the Romans+, where it was valued as a luxury product.

Indigo is among the oldest dyes to be used for textile dyeing and printing. Many Asian countries, such as India, China, Japan, and Southeast Asia+n nations have used indigo as a dye (particularly silk+ dye) for centuries. The dye was also known to ancient civilizations in Mesopotamia+, Egypt+, Britain+, Mesoamerica+, Peru+, Iran+, and Africa. The oldest known fabric dyed indigo dating to 6,000 years ago was discovered in 2009 at Huaca Prieta+, Peru.

India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the dye, ''indikón'' (ινδικόν, Indian). The Romans latin+ized the term to ''indicum'', which passed into Italian dialect+ and eventually into English as the word indigo.

In Mesopotamia, a neo-Babylonian cuneiform+ tablet of the seventh century BC gives a recipe for the dyeing of wool, where lapis+-colored wool (''uqnatu'') is produced by repeated immersion and airing of the cloth. Indigo was most probably imported from India. The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.

Indigo remained a rare commodity in Europe throughout the Middle Ages. A chemically identical dye derived from the woad plant ''(Isatis tinctoria+)'', was used instead. In the late 15th century, the Portuguese+ explorer Vasco da Gama+ discovered a sea route to India. This led to the establishment of direct trade with India, the Spice Islands+, China, and Japan. Importers could now avoid the heavy duties imposed by Persia+n, Levant+ine, and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly. Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England. Spain imported the dye from its colonies in South America. Many indigo plantations were established by European powers in tropical climates; it was a major crop in Jamaica+ and South Carolina+, with much or all of the labor performed by enslaved Africans and African Americans. Indigo plantations also thrived in the Virgin Islands+. However, France and Germany outlawed imported indigo in the 16th century to protect the local woad dye industry.

Indigo was the foundation of centuries-old textile traditions throughout West Africa. From the Tuareg+ nomads of the Sahara+ to Cameroon+, clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the Yoruba+ of Nigeria+ and the Mandinka+ of Mali+ particularly well known for their expertise. Among the Hausa+ male dyers, working at communal dye pits was the basis of the wealth of the ancient city of Kano+, and they can still be seen plying their trade today at the same pits.

In Japan, indigo became especially important in the Edo period+, when it was forbidden to use silk, so the Japanese began to import and plant cotton. It was difficult to dye the cotton fiber except with indigo. Even today indigo is very much appreciated as a color for the summer Kimono+ Yukata, as this traditional clothing recalls Nature and the blue sea.

Newton+ used "indigo" to describe one of the two new primary color+s he added to the five he had originally named, in his revised account of the rainbow in ''Lectiones Opticae'' of 1675.

In North America indigo was introduced into colonial South Carolina by Eliza Lucas Pinckney+, where it became the colony's second-most important cash crop (after rice). As a major export crop, indigo supported plantation slavery there. When Benjamin Franklin+ sailed to France in November 1776 to enlist France's support for the American Revolutionary War+, 35 barrels of indigo were on board the ''Reprisal''+, the sale of which would help fund the war effort. In colonial North America, three commercially important species are found: the native ''I. caroliniana'', and the introduced ''I. tinctoria'' and ''I. suffruticosa''.

Because of its high value as a trading commodity, indigo was often referred to as blue gold.

Peasants in Bengal+ revolted against unfair treatment by the East India Company+ traders/planters in what became known as the Indigo revolt+ in 1859, during the British Raj+ of India+. The play ''Nil Darpan+'' by Dinabandhu Mitra+ is based on the slavery and forced cultivation of indigo.

The demand for indigo in the 19th century is indicated by the fact that in 1897, were dedicated to the cultivation of indican-producing plants, mainly in India. By comparison, the country of Luxembourg+ is .

In 1897, 19,000 tons of indigo were produced from plant sources. Largely due to advances in organic chemistry+, production by natural sources dropped to 1,000 tons by 1914 and continued to contract. These advances can be traced to 1865 when the German chemist Adolf von Baeyer+ began working on the synthesis of indigo. He described his first synthesis of indigo in 1878 (from isatin+) and a second synthesis in 1880 (from 2-nitrobenzaldehyde+). (It was not until 1883 that Baeyer finally determined the structure of indigo.) The synthesis of indigo remained impractical, so the search for alternative starting materials at BASF+ and Hoechst continued. Johannes Pfleger and Karl Heumann+ eventually came up with industrial mass production synthesis. The synthesis of N-(2-carboxyphenyl)glycine from the easy to obtain aniline+ provided a new and economically attractive route. BASF developed a commercially feasible manufacturing process that was in use by 1897. In 2002, 17,000 tons of synthetic indigo were produced worldwide.

Indigo is a challenging dye because it is not soluble+ in water. To be dissolved, it must undergo a chemical change (reduction+). Reduction converts indigo into "white indigo" (leuco+-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines with oxygen+ in the air and reverts to the insoluble, intensely colored indigo. When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and had many opportunities to injure workers. In the 19th century, English poet William Wordsworth+ referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem "The Prelude". Speaking of their dire working conditions and the empathy that he feels for them, he wrote,

''Doubtless, I should have then made common cause''

''With some who perished; haply perished too''

''A poor mistaken and bewildered offering''

''Unknown to those bare souls of miller blue''

A preindustrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine. A more convenient reductive agent is zinc. Another preindustrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of thermophilic+, anaerobic+ bacteria was maintained. Some species of such bacteria generate hydrogen+ as a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of ''shibori+'' (tie-dye), ''kasuri+, katazome+'', and ''tsutsugaki+''. Examples of clothing and banners dyed with these techniques can be seen in the works of Hokusai+ and other artists.

Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as 'pencil blue' because it was most often applied by pencil or brush, could be used to achieve dark hues. Arsenic trisulfide+ and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.

The second method was known as 'China blue' due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths of iron(II) sulfate+, with air-oxidation between each immersion. The China blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.

Around 1880, the 'glucose process' was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the China blue method.

Since 2004, freeze-dried+ indigo, or instant indigo, has become available. In this method, the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.

Indigo is a dark blue crystalline powder that sublimes+ at . It is insoluble in water, alcohol+, or ether+, but soluble in DMSO+, chloroform+, nitrobenzene+, and concentrated sulfuric acid+. The chemical formula+ of indigo is C+16H+10N+2O+2.

The molecule absorbs light in the orange part of the spectrum (λmax = 613 nm). The compound owes its deep color to the conjugation of the double bond+s, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is nonplanar.

Given its economic importance, indigo has been prepared by many methods. The Baeyer-Drewson indigo synthesis+ dates back to 1882. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. This route is highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but was impractical for industrial-scale synthesis. Johannes Pfleger and Karl Heumann+ (de+) eventually came up with industrial mass production synthesis. The first commercially practical route is credited to Pfleger in 1901. In this process, ''N''-phenylglycine is treated with a molten mixture of sodium hydroxide+, potassium hydroxide+, and sodamide+. This highly sensitive melt produces indoxyl+, which is subsequently oxidized in air to form indigo. Variations of this method are still in use today. An alternative and also viable route to indigo is credited to Heumann in 1897. It involves heating ''N''-(2-carboxyphenyl)glycine to in an inert atmosphere with sodium hydroxide. The process is easier than the Pfleger method, but the precursors are more expensive. Indoxyl-2-carboxylic acid is generated. This material readily decarboxylates to give indoxyl, which oxidizes in air to form indigo. The preparation of indigo dye is practiced in college laboratory classes according to the original Baeyer-Drewsen route.

The benzene rings in indigo can be modified to give a variety of related dyestuffs. Thioindigo+, where the two NH groups are replaced by S atoms, is deep red. Tyrian purple+ is a dull purple dye that is secreted by a common Mediterranean snail. It was highly prized in antiquity. In 1909, its structure was shown to be 6,6'-dibromoindigo. It has never been produced on a commercial basis. The related Ciba blue+ (5,7,5′,7′-tetrabromoindigo) is, however, of commercial value. Indigo and its derivatives featuring intra- and intermolecular hydrogen bonding have very low solubility in organic solvents. They can be made soluble using transient protecting group+s such as the tBOC group+, which suppresses intermolecular bonding. Heating of the tBOC indigo results in efficient thermal deprotection and regeneration of the parent H-bonded pigment.

Treatment with sulfuric acid converts indigo into a blue-green derivative called indigo carmine+ (sulfonated indigo). It became available in the mid-18th century. It is used as a colorant for food, pharmaceuticals, and cosmetics.

Indigo and some of its derivatives are known to be ambipolar organic semiconductors+ when deposited as thin films by vacuum evaporation.

Indigo has a low oral toxicity, with an LD50: of 5000 mg/kg in mammals. In 2009, large spills of blue dyes had been reported downstream of a blue jeans manufacturer in Lesotho+.

* Nilbidraha+ (Indigo Revolt)



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