Indigo and Tyrian purple were manufactured by these labor-intensive methods for centuries. It was not until the end of the nineteenth century that a synthetic form of indigo became available. In 1865 the German chemist Johann Friedrich Wilhelm Adolf von Baeyer began investigating the structure of indigo. By 1880 he had found a way to make it in the laboratory from easily obtainable starting materials. It took another seventeen years, however, before synthetic indigo, prepared by a different route and marketed by the German chemical company Badische Anilin und Soda Fabrik (BASF), became commercially viable.
Baeyer’s first synthesis of indigo required seven separate chemical reactions.
This was the beginning of the decline of the large natural indigo industry, a change that altered the way of life for thousands whose livelihood depended on the cultivation and extraction of natural indigo. Today an annual production of over fourteen thousand tons makes synthetic indigo a major industrial dye. Though synthetic indigo (like the natural compound) notoriously lacks colorfastness, it is most often used to dye blue jeans, where this property is considered a fashion advantage. Millions of pairs of jeans are now made from specially pre-faded indigo-dyed denim. Tyrian purple, the dibromo derivative of indigo, was also produced synthetically through a process similar to indigo synthesis, although other purple dyes have superseded it.
Dyes are colored organic compounds that are incorporated into the fibers of textiles. The molecular structure of these compounds allows the absorption of certain wavelengths of light from the visible spectrum. The actual color of the dye we see depends on the wavelengths of the visible light that is reflected back rather than absorbed. If all the wavelengths are absorbed, no light is reflected, and the color of the dyed cloth we see is black; if no wavelengths are absorbed, all light is reflected and the color we see is white. If only the wavelengths of red light are absorbed, then the reflected light is the complementary color of green. The relationship between the wavelength absorbed and the chemical structure of the molecule is very similar to the absorption of ultraviolet rays by sunscreen products—that is, it depends on the presence of double bonds alternating with single bonds. But for the absorbed wavelength to be in the visible range rather than the ultraviolet, there must be a greater number of these alternating double and single bonds. This is shown in the molecule β-carotene, below, which is responsible for the orange color of carrots, pumpkin, and squash.
β-carotene (orange)
Such alternating double and single bonds, as shown in carotene, are referred to as conjugated. β-carotene has eleven of these conjugated double bonds. Conjugation can be extended and the wavelength of absorbed light changed, when atoms such as oxygen, nitrogen, sulfur, bromine, or chlorine are also part of the alternating system.
The indican molecule, from indigo and woad plants, has some conjugation but not enough to appear colored. The indigo molecule, however, has twice indican’s number of alternating single and double bonds and also has oxygen atoms as part of the conjugation combination. It thus has enough to absorb light from the visible spectrum, which is why indigo is highly colored.
Distinct from organic dyes, finely ground minerals and other inorganic compounds have also been used since antiquity to create color. But while the color of these pigments—found in cave drawings, tomb decorations, paintings, wall murals, and frescoes—is also due to absorption of certain wavelengths of visible light, it has nothing to do with conjugated double bonds.
The two common ancient dyes that were used for red shades have very different sources but surprisingly similar chemical structures. The first of these comes from the root of the madder plant. Madder plants, belonging to the Rubiaceae family, contain a dye called alizarin. Alizarin was probably first used in India, but it was also known in Persia and Egypt long before its use by the ancient Greeks and Romans. It is a mordant dye, that is, a dye where another chemical—a metal ion—is needed to fix color to a fabric. Different colors can be obtained by first treating the fabric with different metal salt mordant solutions. The aluminum ion as a mordant produces a rose-red color; a magnesium mordant gives a violet color; chromium a brownish-violet; and calcium a reddish-purple. The bright red color obtained when both aluminum and calcium ions are in the mordant would have resulted from using clay with dried, crushed, and powdered madder root in the dye process. This is likely the dye/mordant combination used by Alexander the Great in 320 B.C., in a ruse to lure his enemy into an unnecessary battle. Alexander had his soldiers dye their uniforms with large patches of a blood-red dye. The attacking Persian army, assuming they were assaulting wounded survivors and expecting little resistance, were easily defeated by the smaller number of Alexander’s soldiers—and, if the story is true, by the alizarin molecule.
Dyes have long been associated with army uniforms. The blue coats supplied by France to the Americans during the American Revolution were dyed with indigo. The French army used an alizarin dye, known as Turkey red as it had been grown for centuries in the eastern Mediterranean, although it probably originated in India and gradually moved westward through Persia and Syria to Turkey. The madder plant was introduced into France in 1766, and by the end of the eighteenth century it had become one of the country’s most important sources of wealth. Government subsidies to industry may have started with the dye industry: Louis Philippe of France decreed that soldiers in the French army were to wear trousers dyed with Turkey red. Well over a hundred years before, James II of England had banned the exportation of undyed cloth to protect English dyers.
The process of dyeing with natural dyes did not always produce consistent results and was often laborious and time consuming. But Turkey red, when it was obtained, was a beautiful bright red and very colorfast. The chemistry of the process was not understood, and today some of the operations involved seem somewhat bizarre and were probably unnecessary. Of the ten individual steps recorded in dyers’ handbooks of the time, many are repeated more than once. At various stages the fabric or yarn is boiled in potash and in soap solution; mordanted with olive oil, alum, and a little chalk; treated with sheep dung, with tanning material, and with a tin salt; and rinsed overnight in river water, in addition to being dyed with madder.
We now know the structure of the alizarin molecule that is responsible for the color of Turkey red and other shades from the madder plant. Alizarin is a derivative of anthraquinone, the parent compound of a number of naturally occurring coloring materials. More than fifty anthraquinone-based compounds have been found in insects, plants, fungi, and lichens. As with indigo, the parent anthraquinone is not colored. But the two OH groups on the right-hand ring in alizarin, combined with alternating double and single bonds in the rest of the molecule, provide enough conjugation for alizarin to absorb visible light.
OH groups are more important for producing color in these compounds than the number of rings. This is shown as well in compounds derived from naphthoquinone, a molecule with two rings compared to anthraquinone’s three.
The naphthoquinone molecule is colorless; colored derivatives of naphthoquinone include juglone, found in walnuts, and lawsone, the coloring matter in Indian henna (used for centuries as a hair and skin dye). Colored naphthoquinones can have more than one OH group, as shown by echinochrome, a red pigment found in sand dollars and sea urchins.
Echinochrome (red)
Another anthraquinone derivative, chemically similar to alizarin, is carminic acid, the principal dye molecule from cochineal, the other red dye from ancient times. Obtained from the crushed bodies of the female cochineal beetle, Dactylopius coccus, carminic acid contains numerous OH groups.
Carminic acid (scarlet)
Cochineal was a dye of the New World, used by the Aztecs long before the arrival of the Spanish conquistador Hernán Cortés in 1519. Cortés introduced cochineal to Europe, but its source was kept secret until the eighteenth century, in order to protect the Spanish monopoly over this precious scarlet dye. Later, British soldiers became known as “redcoats” from their coch
ineal-dyed jackets. Contracts for English dyers to produce fabric in this distinctive color were still in place at the beginning of the twentieth century. Presumably this was another example of government support of the dye industry, as by then British colonies in the West Indies were major cochineal producers.
Cochineal, also called carmine, was expensive. It took about seventy thousand insect bodies to produce just one pound of the dye. The small dried cochineal beetles looked a little like grain; hence the name “scarlet grain” was often applied to the contents of the bags of raw material that were shipped from cactus plantations in tropical regions of Mexico, and Central and South America for extraction in Spain. Today the major producer of the dye is Peru, which makes about four hundred tons annually, around 85 percent of the world’s production.
The Aztecs were not the only people to use insect extractions as dyes. Ancient Egyptians colored their clothes (and the women their lips) with red juice squeezed from the bodies of the kermes insect (Coccus ilicis). The red pigment from this beetle is mainly kermesic acid, a molecule extraordinarily similar to its New World counterpart of carminic acid from cochineal. But unlike carminic acid, kermesic acid never went into widespread use.
Although kermesic acid, cochineal, and Tyrian purple were derived from animals, plants supplied most of the starting materials for dyers. Blue from indigo and woad, and red from the madder plant were the standards. The third remaining primary color was a bright yellow-orange shade from the saffron crocus, Crocus sativus. Saffron is obtained from the stigmas of flowers, the part that catches pollen for the ovary. This crocus was native to the eastern Mediterranean and was used by the ancient Minoan civilization of Crete as early as 1900 B.C. It was also found extensively throughout the Middle East and was used in Roman times as a spice, a medicine, and a perfume as well as a dye.
Once widespread over Europe, saffron growing declined during the Industrial Revolution for two reasons. First, the three stigmas in each hand-picked blossom had to be individually removed. This was a very labor-intensive process, and laborers at this time had largely moved to the cities to work in factories. The second reason was chemical. Although saffron produced a beautiful brilliant shade, especially when applied to wool, the color was not particularly fast. When man-made dyes were developed, the once-large saffron industry faded away.
Saffron is still grown in Spain, where each flower is still hand-picked in the traditional way and at the traditional time, just after sunrise. The majority of the crop is now used for the flavoring and coloring of food in such traditional dishes as Spanish paella and French bouillabaise. Because of the way it is harvested, saffron is the most expensive spice in the world today; thirteen thousand stigmas are required to produce just one ounce.
The molecule responsible for the characteristic yellow-orange of saffron is known as crocetin, and its structure is reminiscent of the orange color of β-carotene, each having the same chain of seven alternating double bonds indicated, below, by the brackets.
Although the art of dyeing no doubt started as a cottage craft and indeed continues in this mode to some extent today, dyeing has been recorded as a commercial enterprise for thousands of years. An Egyptian papyrus from 236 B.C. has a description of dyers—“stinking of fish, with tired eyes and hands working unceasingly.” Dyers’ guilds were well established in medieval times, and the industry flourished along with the woolen trade of northern Europe and silk production in Italy and France. Indigo, cultivated with slave labor, was an important export crop in parts of the southern United States during the eighteenth century. As cotton became an important commodity in England, so too were dyers’ skills in great demand.
SYNTHETIC DYES
Beginning in the late 1700s synthetic dyes were created that changed the centuries-old practices of these artisans. The first of these man-made dyes was picric acid, the triply nitrated molecule that was used in munitions in World War I.
Picric acid (trinitrophenol)
An example of a phenolic compound, it was first synthesized in 1771 and used as a dye for both wool and silk from about 1788. Although picric acid produced a wonderfully strong yellow hue, its drawback, like that of many nitrated compounds, was its explosive potential, something that dyers did not have to worry about with natural yellow dyes. Two other disadvantages were that picric acid had poor light fastness and that it was not easily obtained.
Synthetic alizarin became available in good quantity and quality in 1868; synthetic indigo became available in 1880. In addition, totally new man-made dyes were prepared; dyes that gave bright, clear shades, were colorfast, and produced consistent results. By 1856 eighteen-year-old William Henry Perkin had synthesized an artificial dye that radically changed the dye industry. Perkin was a student at London’s Royal College of Chemistry; his father was a builder who had little time for the pursuit of chemistry because he felt it was unlikely to lead to a sound financial future. But Perkin proved his father wrong.
Over his Easter holidays of 1856, Perkin decided to try to synthesize the antimalarial drug quinine, using a tiny laboratory that he had set up in his home. His teacher, one August Hofmann, a German chemistry professor at the Royal College, was convinced that quinine could be synthesized from materials found in coal tar, the same oily residue that was, a few years later, to yield phenol for surgeon Joseph Lister. The structure of quinine was not known, but its antimalarial properties were making it in short supply and great demand. The British Empire and other European nations were expanding their colonies into malaria-ridden areas of tropical India, Africa, and Southeast Asia. The only known cure and preventive for malaria was quinine, obtained from the increasingly scarce bark of the South American cinchona tree.
A chemical synthesis of quinine would be a great achievement, but none of Perkin’s experiments were successful. One of his trials did, however, produce a black substance that dissolved in ethanol to give a deep purple solution. When Perkin dropped a few strips of silk into his mixture, the fabric soaked up the color. He tested this dyed silk with hot water and with soap and found that it was colorfast. Perkin exposed the samples to light; the color did not fade—it remained a brilliant lavender purple. Aware that purple was a rare and costly shade in the dye industry and that a purple dye, colorfast on both cotton and silk, could be a commercially viable product, Perkin sent a sample of the dyed cloth to a leading dyeing company in Scotland. Back came a supportive reply: “If your discovery does not make the goods too expensive, it is decidedly one of the most valuable that has come out for a very long time.”
This was all the encouragement Perkin needed. He left the Royal College of Chemistry and, with financial help from his father, patented his discovery, set up a small factory to produce his dye in larger quantities and at a reasonable cost, and investigated the problems associated with dyeing wool and cotton as well as silk. By 1859 mauve, as Perkin’s purple was called, had taken the fashion world by storm. Mauve became the favorite color of Eugénie, empress of France, and the French court. Queen Victoria wore a mauve dress to the wedding of her daughter and to open the London Exhibition of 1862. With royal approval from Britain and France, the popularity of the color soared; the 1860s were often referred to as the mauve decade. Indeed, mauve was used for printing British postal stamps up until the late 1880s.
Perkin’s discovery had far-reaching consequences. As the first true multistep synthesis of an organic compound, it was quickly followed by a number of similar processes leading to many different colored dyes from the coal tar residues of the coal gas industry. These are often known collectively as coal tar dyes or aniline dyes. By the end of the nineteenth century dyers had around two thousand synthetic colors in their repertoire. The chemical dye industry had effectively replaced the millennia-old enterprise of extracting dyes from natural sources.
While Perkin did not make money from the quinine molecule, he did make a vast fortune from mauveine, the name he gave to the molecule that produced the beautiful deep purple shade of mauve, and fr
om his later discoveries of other dye molecules. He was the first person to show that the study of chemistry could be extremely profitable, no doubt necessitating a retraction of his father’s original pessimistic opinion. Perkin’s discovery also emphasized the importance of structural organic chemistry, the branch of chemistry that determines exactly how the various atoms in a molecule are connected. The chemical structures of the new dyes needed to be known, as well as the structures of the older natural dyes like alizarin and indigo.
Perkin’s original experiment was based on incorrect chemical suppositions. At the time it had been determined that quinine had the chemical formula of C20H24N2O2, but little was known about the structure of the substance. Perkin also knew that another compound, allyltoluidine, had the chemical formula C10H13N, and it seemed to him possible that combining two molecules of allyltoluidine, in the presence of an oxidizing agent like potassium dichromate to supply extra oxygen, might just form quinine.
From a chemical formula perspective, Perkin’s idea may not seem unreasonable, but we now know that this reaction would not occur. Without knowledge of the actual structures of allyltoluidine and quinine, it is not possible to devise the series of chemical steps necessary to transform one molecule into another molecule. This is why the molecule Perkin created, mauveine, was chemically very different from quinine, the one he had intended to synthesize.
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