King of Dyes: Indigo

King of Dyes: Indigo
Introduction
In 1905, the Nobel Prize in Chemistry was awarded to Adolf von Baeyer, a German chemist, for his outstanding contributions to the synthesis of indigo and the hydrogenation of aromatic hydrocarbons.
Ancient Dye: Indigo
Blue jeans made of twill cotton dyed with indigo became popular in the late 1970s around the world, and their popularity can be inferred from the sharp rise in indigo production. For example, the annual production of indigo in the United States was 15,000 tons in the 1950s. However, by the mid-1960s production had almost completely ceased, but in the late 1970s production had risen again, with the United Chemical Company alone producing 3,000t annually, and since then production has increased by more than 100 percent around the world.
The use of indigo dye has a history of 40 to 50 years, some of the clothes worn by the mummies in ancient Egypt, the blue linen fabrics excavated in Mawangdui in China, etc. are all dyed with indigo, and more than 200 years ago, the flags of the French Revolution and the American War of Independence were also dyed with the color of indigo. This dye can be long-term, widely loved by people, is because it is in the dyeing firmness and light fastness are other dyes can not be compared, it is often referred to as “the king of dyes”.
Indigo is obtained from indigo plants (Indigofera group) such as: woad, polygonum, mullein, etc. They have been widely cultivated in China and India, and their roots are called Panax quinquefolius, the leaves are called Dazhongye, and the processed sediment is called qingdai, which are all used for medicine.

When dyeing with indigo plant, it is first chopped up and put in an urn with water to let it ferment, the fermentation liquid contains cryptochrome with indole phenol as the main component, when the fabric is soaked through it and taken out to dry, the cryptochrome is oxidized by the air, forming water-insoluble indigo and dyeing on the fabric. Therefore, indigo is a kind of reducing dye, more because the dyeing is carried out in the urn with small air contact area, also known as urn dyeing dyes.
In the middle of the 19th century, the industrial revolution led by the textile revolution also promoted the demand for dyestuffs to increase dramatically. Although hundreds of thousands of acres of good land were opened up in East India, especially in Bengal, for the cultivation of indigo plants, the needs of the dyeing and printing industry still could not be met, and the subject of synthesizing indigo dyestuffs by chemical means was put in front of the chemists.
Synthesis of Indigo
In 1905, the Nobel Prize in Chemistry was awarded to Adolf von Baeyer, a German chemist, for his outstanding contributions to the synthesis of indigo and the hydrogenation of aromatic hydrocarbons. However, before Baeyer’s research on indigo, its experimental formula: C 8 H5NO and molecular formula: C16 H10N2O2 had already been determined. It was also found that indigo could produce o-aminobenzoic acid when it was melted with caustic potash at low temperature, aniline when it was melted at high temperature, and indigo red when it was oxidized with nitric acid or chromium trioxide. However, because the understanding of molecular structure was in its infancy at that time, the structure of indigo could not be deduced.
In 1865, Kekuler proposed the ring structure of benzene, and in the same year, Bayer carried out the study of indigo. He firstly thought that indigo was an oxygenated compound, and thus might be the oxidized product of some kind of “parent” substance. This parent substance is similar to aniline to some extent, Bayer named it as indole, and its oxidation product should be indole phenol; a higher level of oxidation product is indole quinone, that is, indigo with red crystallization.
Although there are many differences between indole phenol and phenol, but are the parent of a hydrogen atom is replaced by hydroxyl products, so Bayer also hoped to follow the phenol can be reduced to benzene as from indole phenol indole, but due to the indole phenol is very easy to resin, so that after more than half a year of work, but still can not get the desired results.
When Bayer said to his colleague Stahlschmidt, who was teaching a course on chemical processes, about the difficulties encountered during the experiments, he got an important piece of information from him, which was that zinc powder, which had been used as a filler for paints in the past, had already been used as a reducing agent in industry.

So Bayer immediately used it in the reduction of indole phenol, although again after many times to feel, but still can not get the result, in a desperate situation, try to put the two together in the burning tube heating, unexpectedly in the burning tube was heated to red heat, really obtained indole (1866).
Bayer’s long and systematic work on the structure and synthesis of indigo was due to his serendipitous discovery of the important method of zinc reduction, which led to the parent material of indigo. When his student C. Graebe applied this method to the study of the red vegetable dye, alizarin, he not only determined its chemical skeleton, but also quickly extended the laboratory production of alizarin to industrial scale (1871). This success not only stimulated the dye makers, but also boosted Bayer’s determination to study indigo.
In 1870, Bayer and his students together treated indigo with phosphorus trichloride and then obtained indigo by reducing it with zinc powder and 2 hydrochloric acid, which enabled Bayer to see the dawn of man-made indigo for the first time. At this time, the indigo used, though still from indigo plants, could be obtained from phenylacetic acid by Bayer in 1878.
In 1879, Bayer further discovered that if the bromide of o-nitrocinnamic acid was co-cooked with alkali, a small amount of indigo could be obtained. Later on, he discovered the method of producing indigo from o-nitrophenylpropionic acid and applied for the first patent on the synthesis of indigo for the invention on March 19th, 1880; in December of the same year, he published the first scientific paper on the synthesis of indigo, and put forward the structural formula of indigo in 1883 , which was deduced through chemical reaction. This formula, deduced by chemical reactions, was only slightly different from the one obtained 45 years later (1928) by X-ray diffraction (2) in the cis and trans forms:

With the structure of indigo, it was possible to find a more convenient method of synthesis and to produce new dyes by chemical modification of the structure. The industrial production of indigo did not ultimately follow Bayer’s special procedure in the laboratory, but rather K. Heumann’s route (1890) using aniline and acetic acid as the initial raw materials, but the impact of production on industry and agriculture was global. By the end of the 19th century, chemical plants producing synthetic indigo had replaced farms growing indigo plants everywhere. It was also surprising to discover that the ancient Tyrian purple was indigo with two bromine atoms, a valuable violet dye that had long been produced in great secrecy from the bones of snail snails; the isomer of indigo, indirubin, however, has magical properties. Indirubin) has a miraculous effect on the treatment of blood cancer.
Altdorf-Bayer
Altdorf Bayer, born on October 31, 1835 in Berlin, his father was a general in the Prussian army, retired as the director of the Prussian Geodetic Institute, his mother was the daughter of a famous jurist and historian. Bayer became interested in chemistry when he was a young boy, as he later said in his lecture on the synthesis of indigo “: I have been aware of those marvelous and peculiar-smelling dyes from East India since I was a child”.
In 1853, Bayer left Berlin, where there was no chemistry laboratory at that time, to study chemistry in Heidelberg, where he studied under R. Bunsen and met A. Kekule, and was awarded his doctoral degree in 1858 for his thesis on organic arsenides.
After graduation, Bayer first worked in the laboratory of A. W. Hoffmann, and then returned to Berlin in 1860 to work as a teacher at a technical school, while conducting scientific research. In the course of his research on uric acid he discovered an interesting acid which he called barbituric acid, named after his best friend Barbara, and which was later developed into a large class of sleeping pills and narcotics.
In 1865, Bayer began to turn his attention to the study of indigo, in the process of research, not only discovered phenolphthalein, but also produced a phenol and formaldehyde condensate, but unfortunately for this viscous substance, was considered a non-crystalline and dye has nothing to do with the in-depth study, and this discovery was made in the depth of the research of the Beakeland (L. Beakeland) produced phenolic plastics, leading to the dawn of the age of plastics. Plastics era.
In 1872 Bayer to the newly formed University of Strasbourg as Professor of Chemistry, three years later, due to the University of Munich’s famous chemist Liebig (J. F. Liebig) died. J. F. Liebig, a famous chemist at the University of Munich, died and was appointed professor of chemistry at the university, becoming Liebig’s successor, and then continued to teach and conduct scientific research here until he was 80 years old.
After publishing the structural formula of indigo, Bayer shifted his research direction to terpenes and highly unsaturated polyynes, and successively determined the molecular structures of α2-pinene, carene, terpineol, and other hydrogenated aromatic hydrocarbons, and in 1885 proposed the “tensile theory”, which became the earliest application of the “Tensile theory” by J. H. Van’t Hoff, the first professor of chemistry at the university. H. Van’t Hoff and J. A. Le Bel. J. H. Van’t Hoff and J. A. Le Bel) of the tetrahedral configuration theory of carbon atom, expounded the stability of carbocyclic compounds, Bayer tensile theory later on, although there is a more accurate expression, but it is still in the history of chemistry is still not lost its significance.
With the industrialization of the production of indigo, alizarin and other dyes, also led to the development of the German organic chemical industry. In the factories of these new industries, most of the personnel in important positions were trained by Bayer, an experimentalist and theorist, who was also a famous chemical educator and tutor at that time. Among the many talents under Bayer, especially the German chemist E. Fischer, who was famous for his research on purine chemistry, glycochemistry, and protein chemistry, etc., was a shining star, which really fulfills the old saying in our country that “the young are better than the blue”.