Indigo is the dye that makes the “blue” in blue jeans. It has been used for thousands of years to color textiles. Some civilizations have also used indigo in paint and cosmetics. Until the late 1800s, indigo was produced from plant sources on tropical plantations, mainly in India. In fact, the name indigo is derived from India. After harvesting the Indigofera plants, workers on the indigo plantations spent many days preparing just one batch of the dye. The plants were soaked in water to extract the colorless compound, indecan. This mixture was fermented for up to 15 hours, during which the indecan was converted into indoxyl. This yellow liquid was agitated while the color changed to green and then blue. Dark flakes were formed, and the mixture was boiled to remove impurities. The flakes were filtered and pressed to remove moisture, then cut into cubes and air-dried. This dried indigo was ready for market. Because of the lengthy processing required to produce the dye, indigo was very expensive. During the last half of the 19th century, organic chemists discovered how to synthesize different dyes in the laboratory. Although some of the chemical steps involved in the synthesis reactions were complicated, these synthetic dyes were still much cheaper than the natural dyes isolated from plants or shellfish. The chemical synthesis of indigo was first published in 1882. This chemical reaction started with o-nitrobenzaldehyde, a component of coal tar. Acetone was added under basic conditions (dilute NaOH), and the resulting compound formed a dimer, indigo. This initial synthesis reaction was modified in the late 1890s for large-scale commercial production. It required petroleum-based starting reagents and generated toxic byproducts. However, synthetic indigo was easier to produce than natural indigo, and therefore cheaper. The synthetic dye became increasingly popular. By World War I, nearly all of the indigo sold on international markets was synthesized in laboratories. The effects of synthetic indigo on society were more widespread than one might think. Synthetic indigo was not merely the cheaper source of dye. It also contributed to the eventual independence of India from the British Empire. Because the process of generating natural indigo was so labor-intensive, thousands of workers were affected when the plantations became too expensive to efficiently operate during the early 1900s. The plantation workers had to work longer hours in order to try and produce more natural indigo. Since most of the plantations were in India, this contributed to the social and political unrest in that country. Mahatma Gandhi was one of the leaders who used the terrible conditions on the plantations as a means to organize the Indian population to protest British rule. Thus, the economic turmoil caused by the chemical synthesis of indigo contributed to the efforts of the Indian people to become independent of Great Britain. While synthetic indigo has enjoyed a virtual monopoly for nearly a century, another method for generating an environmentally friendly indigo is under development. At the end of the 20th century, the enzymes required for cellular indigo synthesis were cloned into bacteria. When these genetically modified bacteria are fed tryptophan, they synthesize indigo and secrete it into the growth medium. This bio-indigo is not yet economical because the bacteria produce it so slowly. However, scientists continue to enhance the growth conditions of these biological indigo factories. Perhaps one day, the “blue” in blue jeans will be primarily produced by genetically engineered bacteria. Besides indigo, many other natural products have been produced in the laboratory. One of these is quinine, a compound used for hundreds of years to treat malaria. Quinine is naturally derived from the bark of the tropical Cinchona trees found in Amazonia. Chemists had been trying to synthesize this important drug since the mid-1800s, and finally succeeded in the 1940s. The availability of synthetic quinine helped the Allied troops combat malaria in the Pacific during World War II, and thus it may indirectly be partly responsible for the outcome of the war. Another compound derived from bark, this time from the Pacific yew tree, is paclitaxol, better known as Taxol. This drug has potent anti-tumor activity. However, each yew tree makes so little of the compound that the bark of several old trees is required to treat just one cancer patient. Since harvesting the bark kills the tree, there is a tremendous drive to generate a high-yield synthesis reaction for Taxol. While the compound was first synthesized in the laboratory in the 1990s, the yields are too small to be practical. Chemists are continuing to work on a better synthesis pathway for this drug. In addition, derivatives of Taxol are being created that may have even more potent anticancer activity. Other natural products that have synthetic or semi-synthetic versions include antibiotics, antifungals, and anesthetics. Creating synthetic versions of useful natural products certainly benefits society by producing cheaper compounds. However, synthetic chemistry can also be environmentally friendly. Improved synthetic pathways can reduce the amount of toxic by-products formed during some chemical reactions. In addition, the availability of synthetic compounds eliminates the need to continually harvest large quantities of rare plants or other organisms in order to isolate the natural product.
—Theresa Beaty, Ph.D.,is an associate professor in the department of chemistry & physics at LeMoyne College in Syracuse, New York.