Editing Indigo dye (section)

===Chemical synthesis===
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|image1=Indigo.Baeyer-Drewson.Synthesis.svg|caption1=Heumann'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>