The method was discovered by an accident by Karl Heumann in Zurich which involved a broken thermometer. Johannes Pfleger and Karl Heumann eventually came up with industrial mass production synthesis from aniline by using mercury as a catalyst. This route was highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but proved impractical for industrial-scale synthesis. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. The Baeyer-Drewson indigo synthesis dates back to 1882. Given its economic importance, indigo has been prepared by many methods. Light exposure during part of the dyeing process can convert the dibromoindigo into indigo, resulting in blue hues known as royal blue, hyacinth purple, or tekhelet. Natural sources of indigo also include mollusks the Murex genus of sea snails produces a mixture of indigo and 6,6′-dibromoindigo (red), which together produce a range of purple hues known as Tyrian purple. The powder was then mixed with various other substances to produce different shades of blue and purple. They precipitate from the fermented leaf solution when mixed with a strong base such as lye, pressed into cakes, dried, and powdered. The leaves were soaked in water and fermented to convert the glycoside indican present in the plant to indigotin. Indican was obtained from the processing of the plant's leaves, which contain as much as 0.2–0.8% of this compound. Oxidation by exposure to air converts indoxyl to indigotin, the insoluble blue chemical that is the endpoint of indigo dye. Indican readily hydrolyzes to release β- D- glucose and indoxyl. The precursor to indigo is indican, a colorless, water-soluble derivative of the amino acid tryptophan. Several plants contain indigo, which, when exposed to an oxidising source such as atmospheric oxygen, reacts to produce indigo dye however, the relatively low concentrations of indigo in these plants make them difficult to work with, with the color more easily tainted by other dye substances also present in these plants, typically leading to a greenish tinge. In Europe, Isatis tinctoria, commonly known as woad, was used for dyeing fabrics blue, containing the same dyeing compounds as indigo, also referred to as indigo. In Central and South America, the species grown is Indigofera suffruticosa, also known as anil, and in India, an important species was Indigofera arrecta, Natal indigo. Until the introduction of Indigofera species from the south, Polygonum tinctorum (dyer's knotweed) was the most important blue dyestuff in East Asia however, the crop produced less dyestuff than the average crop of indigo, and was quickly surpassed in favour of the more economical Indigofera tinctoria plant. A common alternative used in the relatively colder subtropical locations such as Japan's Ryukyu Islands and Taiwan is Strobilanthes cusia. The primary commercial indigo species in Asia was true indigo ( Indigofera tinctoria, also known as I. Sources Natural sources Ī 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, notably the Indian Subcontinent. It is also used as a food colorant, and is listed in the United States as FD&C Blue No. 2. About 20,000 tonnes are produced annually, again mainly for the production of blue jeans. Indigo carmine, also known as indigo, is an indigo derivative which is also used as a colorant. Smaller quantities are used in the dyeing of wool and silk. The primary use for indigo is as a dye for cotton yarn, mainly used in the production of denim cloth suitable for blue jeans on average, a pair of blue jeans requires just 3 grams (0.11 oz) to 12 grams (0.42 oz) of dye. It is most commonly associated with the production of denim cloth and blue jeans, where its properties allow for effects such as stone washing and acid washing to be applied quickly. Most indigo dye produced today is synthetic, constituting several thousand tons each year. Indigo is a natural dye extracted from the leaves of some plants of the Indigofera genus, in particular Indigofera tinctoria dye-bearing Indigofera plants were commonly grown and used throughout the world, in Asia in particular, as an important crop, with the production of indigo dyestuff economically important due to the historical rarity of other blue dyestuffs Indigo dye is an organic compound with a distinctive blue color.
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