Creation of the First Synthetic Vat Dye

Date 1901

After centuries of efforts to create dyes with the brilliant colors of nature as well as permanence and functionality, success marked the dawn of the age of synthesis.

Locale Ludwigshafen, Germany

Key Figures

• René Bohn (1862-1922), German synthetic organic chemist
• Karl Heumann (1850-1894), German chemist
• Roland Scholl (1865-1945), Swiss chemist
• August Wilhelm von Hofmann (1818-1892), German organic chemist
• Sir William Henry Perkin (1838-1907), English chemist and student in Hofmann’s laboratory

Summary of Event

Today, the presence of color in living spaces and daily utensils is largely taken for granted. The precise date when the search began for ways to make human surroundings attractive through the use of color is unknown. By contrast, humankind’s search for the means to obtain and improve on nature’s colors has been recorded and analyzed in great detail. One of many possible critical turning points in this research was the introduction of the first vat dye by a chemist at the German firm Badische Anilin- und Soda-Fabrik (BASF) in 1901.

The term “vat dye” is used to describe a method of applying the dye, but it also serves to characterize the structure of the dye, because all currently useful vat dyes share a common unit. One fundamental problem in dyeing relates to the extent to which the dye is water-soluble. A beautifully colored molecule that is easily soluble in water might seem attractive given the ease with which it binds with fiber; however, this same solubility will lead to the dye’s rapid loss in daily use.

Vat dyes are designed to solve this concern with molecules that can be made water-soluble, but only during the dyeing or vatting process. The structural unit that can be changed chemically and then re-formed involves two groups of a carbon and an oxygen atom connected by two pairs of electrons. Such a carbon-oxygen double bond, or carbonyl group, is easily reduced through the addition of a molecule of hydrogen. After the reduced (or leuco-dye) crystals are safely trapped within the fibers, they are reoxidized, usually with air.

The excellent water-fastness of vat dyes is complemented by the fact that many of them have a very simple chemical structure, which results in great chemical stability and particularly in light-fastness. One modern textbook of dye chemistry asserts that “the fastness properties of vat dyes are surpassed by no other class of dyestuffs.”

From prehistoric times until the mid-nineteenth century, all dyes were derived from natural sources. With a few important exceptions, these coloring materials came from plants. Over thousands of years, only about a dozen dyes proved to be of lasting practical importance. Among the most lasting dyes in nature are the red and blue dyes such as those found in alizarin and indigo. Both of these substances have the carbonyl groups characteristic of vat dyes, and both played central roles in the earliest commercially significant synthesis of a vat dye.

The development of modern chemistry and that of dye chemistry are closely linked. The early years of the nineteenth century found chemists beginning to study the natural world in a scientific way. In France, Antoine-Laurent Lavoisier made chemistry a science by insisting that accurate nomenclature and analysis were essential to progress. In England, John Dalton stated the basic foundation by bringing the Greek atomos, or atoms, into modern form. In Germany, Justus von Liebig promoted intensive laboratory education for analysis and publication of results in scholarly journals. There was great growth, but fundamental problems remained.

Organic chemistry, which deals with the compounds of the element carbon and is associated with living matter, hardly existed. Synthesis of carbon compounds simply was not attempted. Considerable amounts of data had accumulated showing that organic or living matter is basically different from the compounds of the mineral world. In general, organic material was considered to be fragile and difficult to work with. Carbon compounds did not fit easily into the well-established electrical picture of rocks, metals, and salts. Researchers neatly, if unproductively, solved this problem by stating that these molecules contained carbon, hydrogen, nitrogen, and “vital force.” It was widely believed that although one could work with various kinds of organic matter in physical ways and even analyze their composition, such matter could be produced only in a living organism.

In 1828, Friedrich Wöhler found that it was possible to synthesize the clearly organic compound urea from clearly mineral compounds. The concept of “vitalism” began to fade as more chemists reported the successful preparation of compounds previously isolated only from plants or animals.

One especially rich field ripe for exploration was that of coal tar, and August Wilhelm von Hofmann was an active worker in this area. He and his students made careful studies of this complex mixture. The high-quality stills they designed allowed the isolation of pure samples of important compounds for further study. Of greater importance was the collection of able students Hofmann attracted. Among them was Sir William Henry Perkin, who today is regarded as the founder of the dyestuffs industry.

In 1856, Perkin undertook the task of synthesizing quinine from a nitrogen-containing coal tar material called toluidine. Luck played a decisive role in the outcome of his experiment. The sticky compound Perkin obtained contained no quinine, so he turned to investigate the simpler related compound, aniline. A small amount of the impurity toluidine in his aniline gave Perkin the first synthetic dye, Mauveine.

From this beginning, the great dye industries of Europe, particularly Germany, grew. Trial-and-error methods gave way to more systematic searches as the structural theory of organic chemistry was formulated. The academic and the industrial, the theoretical and the synthetic advanced in large measure through mutual stimulation. As the twentieth century began, great progress had been made, and German firms dominated the industry. BASF was incorporated at Ludwigshafen in 1865 and undertook extensive explorations of both alizarin and indigo. René Bohn, a chemist, had made important discoveries in 1888 that helped the company recover lost ground in the alizarin field. Bohn was well trained, having earned his doctorate at the University of Zurich in studies with Karl Heumann, who had developed two successful syntheses of indigo. In 1901, Bohn undertook the synthesis of a dye he hoped would combine the desirable attributes of both alizarin and indigo.

As so often happens in science, nothing like the expected occurred. Bohn realized that the beautiful blue crystals he produced represented a far more important product than he had been seeking. Not only had he created the first synthetic vat dye, Indanthrene, but also, by studying the reaction at higher temperature, he produced a useful yellow dye, Flavanthrone. By 1907, Swiss chemist Roland Scholl had shown unambiguously that the structure proposed by Bohn for Indanthrene was correct, and a major new area of theoretical and practical importance was opened in the field of organic chemistry.

Significance

Bohn’s discovery led to the development of many new and useful dyes. The list of patents issued in his name fills several pages in Chemical Abstracts indexes. The true importance of this work, however, is to be found in a consideration of all synthetic chemistry, which may perhaps be represented by this particular event. By the end of the twentieth century, more than two hundred dyes related to Indanthrene were in commercial use. The colors represented by these substances are a rainbow, making nature’s finest hues available to all. The dozen or so natural dyes have been synthesized into more than seven thousand superior products through the creativity of the chemist.

Despite these desirable outcomes, some observers doubt that there is any real benefit to society from the development of new dyes. This must be considered, given the fact that natural resources are limited. With so many urgent problems to be solved, scientists are not sure whether they should devote their time to a search for greater luxury. A more germane question, however, may be whether scientists can afford not to continue the search. If the field of dye synthesis reveals a single theme, it must be that one should expect the unexpected. Time after time, the effort to reach one goal has led to the achievement of something quite different. No one can say with any certainty where the crucial clue to the solution of a problem will be found. Certainly no one would predict that cancer will be cured through dye research. On the other hand, improvements in dyes used for staining tissue are always needed.

It is perhaps noteworthy in this context that fifty years after Bohn’s discovery, an English team headed by Professor William Bradley began a long series of studies aimed at determining how Indanthrene is formed. Their efforts were directed not at the synthesis of new dyes but at obtaining a fundamental understanding applicable in other fields of research. Several textbooks on dye chemistry have pointed to the importance of knowledge of that field to other sciences—one more assertion that basic research often reveals the unexpected.

Bibliography

Fieser, Louis F., and Mary Fieser. Current Topics in Organic Chemistry. New York: Reinhold, 1963. Excellent historical perspective and a clear explanation of the chemistry and backgrounds of the people involved.

Gordon, P. E., and P. Gregory. Organic Chemistry in Colour. New York: Springer-Verlag, 1983. Contains an excellent historical introduction to the general field, along with a specific description of Bohn’s work on several dyes.

Ihde, Aaron J. The Development of Modern Chemistry. 1964. Reprint. Mineola, N.Y.: Dover, 1984. Through superb writing, provides access to this highly technical world for the nonchemist. Gives authoritative information on the scientific and commercial aspects of the dye industry.

Rys, P., and H. Zollinger. Fundamentals of the Chemistry and Application of Dyes. New York: Wiley-Interscience, 1972. Some historical material and an adequate presentation of Bohn’s synthetic efforts. Brief but effective; pays unusual attention to the central idea and general properties.

Thorpe, Jocelyn Field, and Christopher Kelk Ingold. Synthetic Colouring Matters: Vat Colours. London: Longmans, Green, 1923. An extremely detailed and valuable history of the entire field published only a few years after it became well established. Especially important historically in showing the close relationship between organic and dye chemistry as seen through the eyes of Thorpe’s student Ingold.

Zollinger, Heinrich. Color Chemistry: Syntheses, Properties, and Applications of Organic Dyes and Pigments. 3d ed. Zurich: VHCA, 2003. A brief historical introduction coupled with an extensive and technical discussion of a wide range of modern chemical ideas as illustrated by dye chemistry. Written for the specialist, but contains many insights of interest to anyone attempting to see dye chemistry in the historical perspective of developing modern chemistry.