Editing Indigo dye (section)

==Sources==

===Natural sources===
{{see also|#Indigo derivatives}}
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, notably the Indian Subcontinent. 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 {{lang|ja-Latn|Ryukyu|italic=no}} Islands and Taiwan is ''[[Strobilanthes cusia]]''.

Until the introduction of ''Indigofera'' species from the south, ''[[Polygonum tinctorum|Persicaria tinctoria]]'' (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. In [[Central America|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. 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.

Several plants contain indigo, which, when exposed to an oxidizing 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.

The precursor to indigo is [[indican]], a colorless, water-soluble derivative of the amino acid [[tryptophan]], and ''Indigofera'' leaves contain as much as 0.2–0.8% of this compound. Pressing cut leaves into a vat and soaking hydrolyzes the indican, releasing β-{{sc|D}}-[[glucose]] and [[indoxyl]]. The indoxyl dimerizes in the mixture, and after 12–15 hours of [[fermentation]] yields the yellow, water-soluble [[Leuco dye|leucoindigo]]. Subsequent exposure to air forms the blue, water-insoluble indigo dye.<ref>{{cite book |last=Schorlemmer |first=Carl |author-link=Carl Schorlemmer |title=A Manual of the Chemistry of the Carbon compounds; or, Organic Chemistry |url=https://archive.org/details/amanualchemistr02schogoog |location=London |publisher=Macmillan and co. |year=1874}} Quoted in the [[Oxford English Dictionary]], second edition, 1989</ref><ref>{{cite book |last1=Freeman |first1=H. S. |title=Colorants for non-textile applications |last2=Peters |first2=A. T. |publisher=Elsevier |year=2000 |isbn=978-0-444-82888-0 |location=Amsterdam [Netherlands] New York |pages=382–455 |chapter=9 - Natural Dyes}}</ref> The dye precipitates from the fermented leaf solution upon oxidation, but may also be precipitated when mixed with a strong base<ref>{{cite web |url=https://www.coyuchi.com/the-naturalista/indigo_dyeing/ |title=Indigo Dyeing |website=Coyuchi Inc. |language=en |access-date=2019-05-24 |archive-date=2019-05-24 |archive-url=https://web.archive.org/web/20190524001631/https://www.coyuchi.com/the-naturalista/indigo_dyeing/ |url-status=dead}}</ref> such as [[sodium hydroxide|lye]]. The solids are filtered, pressed into cakes, dried, and powdered. The powder is then mixed with various other substances to produce different shades of blue and purple.

Natural sources of indigo also include mollusks: the ''[[Murex]]'' genus of sea snails produces a mixture of indigo and [[6,6'-Dibromoindigo|6,6'-dibromoindigo]] (red), which together produce a range of purple hues known as [[Tyrian purple]]. 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]].

===Chemical synthesis===
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|image1=Indigo.Baeyer-Drewson.Synthesis.svg|caption1=Heumann's synthesis of indigo
|image2=Indigo Synthesis V.1.svg|caption2=Pfleger's synthesis of indigo}}
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Given its economic importance, indigo has been prepared by many methods. The [[Baeyer–Drewsen 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 was highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but proved impractical for industrial-scale synthesis. Johannes Pfleger<ref name="history.evonik.com"/> and {{ill|Karl Heumann|de}} eventually came up with industrial mass production synthesis from [[aniline]] by using [[mercury (element)|mercury]] as a catalyst. The method was discovered by an accident by Karl Heumann in Zurich which involved a broken thermometer.<ref name="ingenious.org.uk"/>

The first commercially practical route of producing indigo is credited to Pfleger in 1901. In this process, [[N-Phenylglycine|''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 {{convert|200|°C}} 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.<ref name=Ullmann/> The preparation of indigo dye is practised in college laboratory classes according to the original Baeyer-Drewsen route.<ref>{{cite journal |last1=McKee |first1=James R. |last2=Zanger |first2=Murray |year=1991 |title=A microscale synthesis of indigo: Vat dyeing |journal=Journal of Chemical Education |volume=68 |issue=10 |page=A242 |doi=10.1021/ed068pA242 |bibcode=1991JChEd..68..242M}}</ref>