Introduction

The 1905 Nobel Prize in Chemistry was awarded to the German chemist Adolf von Baeyer, in recognition of his Synthetic indigo as well as outstanding contributions in the field of hydrogenated aromatics.

I. Ancient Dye: Indigo

Blue jeans, made from indigo-dyed twill cotton fabric, rapidly gained worldwide popularity in the late 1970s; the surge in demand is evident from the sharp increase in indigo production. For example, annual indigo output in the United States was 15,000 tonnes in the 1950s, but by the mid-1960s production had virtually ceased. By the late 1970s, however, output rebounded, with Union Carbide alone producing 3,000 tonnes per year, and since then production has been expanding at rates exceeding 100% globally.

The use of indigo dye dates back some forty to fifty years; garments worn by ancient Egyptian mummies and blue hemp textiles unearthed at the Mawangdui site in China, for example, were all dyed with indigo. More than two centuries ago, the flags flown during the French Revolution and the American War of Independence were also dyed in indigo. The reason this dye has enjoyed long-standing and widespread popularity is that its dyeing fastness and lightfastness are unmatched by other dyes, which is why it is often referred to as the “King of Dyes.”

Indigo is derived from indigo plants of the genus Indigofera, such as Isatis tinctoria, Polygonum tinctorium, and Indigofera suffruticosa. These plants were once widely cultivated in China, India, and other regions. The roots are known as Banlangen, the leaves as Daqingye, and the processed precipitate as Qingdai; all are used in traditional medicine.
When dyeing with indigo plants, the plant material is first chopped up, placed in a vat, and soaked in water to allow fermentation. The fermentation broth contains leucoindigo, whose main component is indophenol. After the fabric is thoroughly immersed in this solution and then removed to air-dry, the leucoindigo is oxidized by exposure to air, forming water-insoluble indigo that remains on the fabric. Thus, indigo is a reduction dye; moreover, since the dyeing process takes place in a vat with limited contact with air, it is also referred to as vat dye.

In the mid-19th century, the Industrial Revolution, spearheaded by the textile industry, also led to a dramatic surge in demand for dyes. Although hundreds of thousands of acres of fertile land were then brought under cultivation for indigo plants, particularly in East India and especially in Bengal, this still fell short of meeting the needs of the dyeing and printing industry, thereby placing the task of synthesizing dyes by chemical means squarely before chemists.

II. Synthetic Indigo

In 1905, the Nobel Prize in Chemistry was awarded to the German chemist Adolf von Baeyer in recognition of his outstanding contributions to the synthesis of indigo and to the hydrogenation of aromatic hydrocarbons. However, prior to Baeyer’s research on indigo, its empirical formula—C8H5NO—and its molecular formula—C16H10N2O2—had already been determined. It was also discovered that when indigo was melted at low temperatures in caustic potash, it yielded o-aminobenzoic acid; at higher temperatures, it produced aniline; and oxidation with nitric acid or chromium trioxide yielded indigo red. Nevertheless, since understanding of molecular structure was still in its infancy at the time, it was not possible to deduce the structure of indigo.

In 1865, Kekulé proposed the cyclic structure of benzene; in the same year, Bayer initiated research on indigo. He initially regarded indigo as an oxygen-containing compound and thus as the oxidation product of some “parent” substance. This parent substance was, to a certain extent, analogous to aniline; Bayer named it indole, and its oxidation product should be indophenol. A further higher-level oxidation product would be indigoquinone, namely the red crystalline substance known as indigotin.

Although indophenol and phenol differ in many respects, they are both derivatives in which a hydrogen atom on the parent molecule has been replaced by a hydroxyl group; accordingly, Bayer also hoped to synthesize indole from indophenol in the same way that phenol can be reduced to benzene. However, indophenol is prone to resinification, so even after more than six months of work, the desired product still could not be obtained.

When Bayer described the difficult experimental conditions he had encountered to his colleague Stahlschmidt, who was teaching a course on chemical engineering, he received an important piece of information: zinc powder, which had long been used as a pigment filler, was now being employed as an industrial reducing agent.

Bayer immediately applied this method to the reduction of indophenol; however, despite numerous repeated trials, no satisfactory results were obtained. In desperation, he tried heating the two substances together in a combustion tube. To his great surprise, when the tube was heated to red heat, indole was indeed produced (1866).

Bayer’s long-term, systematic pursuit of research on the structure and synthetic methods of indigo can be attributed to his serendipitous discovery of the crucial zinc–powder reduction method, which enabled him to obtain the parent compound of indigo. When his student C. Graebe applied this method to the study of the red plant dye alizarin, not only was its chemical skeleton elucidated, but the laboratory synthesis of alizarin was soon scaled up for industrial production (1871). This success not only spurred dye manufacturers but also reinforced Bayer’s resolve to continue his research on indigo.

In 1870, Bayer, together with his students, treated indigo carmine with phosphorus trichloride and then reduced the resulting product with zinc dust in hydrochloric acid, thereby obtaining indigo. This marked the first glimpse of synthetic indigo for Bayer. Although the indigo carmine used at that time was still derived from the indigo plant, by 1878 Bayer had already succeeded in synthesizing it from phenylacetic acid.

In 1879, Bayer further discovered that a small amount of indigo could also be obtained by co-boiling the bromide of o-nitrocinnamic acid with an alkali. Shortly thereafter, he developed a method for synthesizing indigo from o-nitrophenylpropionic acid and, on March 19, 1880, filed the first patent for synthetic indigo. In December of the same year, he published the first scientific paper on the synthesis of indigo, and in 1883 he proposed the structural formula of indigo. This structure, deduced through chemical reactions, differed only slightly from the structure determined 45 years later (in 1928) by X-ray diffraction (2), specifically in terms of its cis–trans configuration:

With the structural understanding of indigo, it became possible to seek more convenient synthetic methods and, through chemical modification of its structure, to develop new dyes. Although industrial production of indigo ultimately did not follow Bayer’s specialized laboratory procedure but instead adopted K. Heumann’s route using aniline and acetic acid as starting materials (1890), once this process was put into operation it exerted a global impact on both industry and agriculture. By the end of the 19th century, chemical plants producing synthetic indigo had largely replaced the small farms across various regions that had previously cultivated indigo plants. Moreover, it was astonishingly discovered that the ancient Tyrian purple was, in fact, indigo bearing two bromine atoms; this precious purple dye had long been obtained under extremely secretive conditions from the shells of murex snails. Furthermore, an isomer of indigo—indirubin—has been found to possess remarkable therapeutic effects against leukemia.

III. Adolf Bayer

Adolf Bayer was born in Berlin on October 31, 1835. His father had been a general in the Prussian Army and, after retiring, served as the director of the Prussian Academy of Geodesy; his mother was the daughter of a renowned jurist and historian. Even as a boy, Bayer developed a deep interest in chemistry, as he later remarked in a lecture on the synthesis of indigo: “From my earliest childhood, I was fascinated by those marvelous dyes from the East Indies, with their distinctive odors.”

In 1853, Bayer left Berlin—where no chemistry laboratory yet existed—attracted by the opportunity to study chemistry, and went to Heidelberg, where he became a student of Robert Bunsen and also met August Kekulé. In 1858, he was awarded his doctorate for a dissertation on organoarsenic compounds.

After graduating, Bayer first worked in the laboratory of A. W. Hoffmann; in 1860 he returned to Berlin to teach at a technical school while continuing his scientific research. During his studies on uric acid, he discovered a noteworthy acid and named it barbituric acid, after the name of his closest female friend, Barbara. Subsequently, this class of compounds was developed into a major group of sedatives and anesthetics.

In 1865, Bayer turned its attention to the study of indigo. During this research, it not only discovered phenolphthalein but also synthesized the condensation product of phenol and formaldehyde. Unfortunately, this viscous substance was regarded as an amorphous, non-crystalline material unrelated to dyes and thus received little further investigation. Later, however, through L. Baekeland’s in-depth studies, this discovery led to the development of phenolic resins, ushering in the age of plastics.

In 1872, Bayer accepted a professorship in chemistry at the newly established University of Strasbourg. Three years later, following the death of the renowned chemist Justus von Liebig at the University of Munich, Bayer was appointed to the chair of chemistry there, succeeding Liebig. Thereafter, he devoted himself to teaching and research at the university until the age of eighty.

Following the publication of the structural formula for indigo, Bayer shifted his research focus to terpenes and highly unsaturated polyacetylene compounds, and subsequently elucidated the molecular structures of hydrogenated aromatic hydrocarbons such as α-pinene, camphor, and terpineol. In 1885, he also proposed the “strain theory,” becoming one of the earliest scholars to apply J. H. Van’t Hoff’s and J. A. Le Bel’s tetrahedral carbon-atom model to explain the stability of cyclic carbon compounds. Although Bayer’s strain theory was later refined into a more precise formulation, it nonetheless remains of great significance in the history of chemistry.

The industrialization of dye production, including indigo and alizarin, also spurred the development of Germany’s organic chemical industry. In the factories of these emerging industries, the majority of personnel in key positions had received their chemical training from Bayer—himself an experimentalist, a theoretical chemist, and a renowned chemistry educator and mentor of his time. Among the many talented individuals who studied under Bayer, the German chemist Emil Fischer—renowned worldwide for his pioneering research in purine chemistry, carbohydrate chemistry, and protein chemistry—stands out as a truly brilliant star. Indeed, his achievements perfectly embody the Chinese proverb “The student surpasses the teacher.”

(Source: Encyclopedia of Dyeing and Finishing)