Archibald Scott Couper in Paris in 1857 or 1858.
Courtesy Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library.
Archibald Scott Couper's bond lines in a French version of his 1858 paper. On the left is his representation of tartaric acid and the product obtained after the loss of water by heating. On the right is the first depiction of a ring system—for cyanuric acid (Az =N). Here Couper used continuous lines and brackets to represent bonds. In other publications, bonds are straight dotted lines—possibly the typesetter's preference. From Annales de chemie et de physique, Série 3, 53 (1858), 488–489.
Chemical Heritage Foundation Collections.
In 1858 Archibald Scott Couper (1831–1892) and August Kekulé von Stradonitz (1829–1896), two young men from different backgrounds—and, as it turned out, entering upon even more disparate career paths—independently recognized that carbon atoms can link directly to one another to form carbon chains. This finding explained the very multiplicity of carbon compounds that had been puzzling chemists. The discovery by these two scientists depended on Kekulé's theory, proposed in 1857, that carbon is tetravalent—valence being defined at the time as the combining capacity of the elements. Couper, in his paper—and in another paper on salicylic acid that appeared earlier in 1858—indicated valence bonds as straight lines linking the symbols for the elements, which is still the practice in most modern structural diagrams.
August Kekulé von Stradonitz in 1862. From Richard Anschütz, August Kekulé, vol. I (Berlin: Verlag Chemie 1929).
By permission of VCH Verlagsgesellschaft and courtesy Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library.
Couper, who was a Scot educated in Glasgow, Edinburgh, Berlin, and Paris, came to chemistry from the study of philosophy and classical languages. This background probably helped him make an analogy between letters in words and carbon atoms in molecules, and focus on how carbon atoms combine with other atoms. His invention of an appropriate symbolic language to indicate the order in which the various atoms are joined in molecules may also stem from his philosophical and linguistic training. Sadly, his paper describing carbon linkage was read before the French Academy a few weeks after Kekulé's similar paper was published in Liebig's Annalen der Chemie und Pharmacie. Couper had entrusted his paper to Charles Adolphe Wurtz, in whose laboratory he worked in Paris, and Wurtz had procrastinated in giving it to an Academy member for presentation. It is not known how much Couper's bitterness over his loss of priority and his subsequent fight with Wurtz contributed to his emotional collapse. He soon retreated to his Scottish home and never published another scientific paper for the remaining 30 years of his life.
A representation of the benzene ring from Kekulé's Lehrbuch der organischen Chemie (1861–1867), which closely resembles the structure that became known as Kekulé's, although it was proposed by others and he continued to have reservations about it.
Courtesy Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library.
Kekulé, a German of Czech descent, was intended by his family to become an architect, but at the University of Giessen he was lured to chemistry by Liebig's lectures. During his studies he also worked in Paris with Charles Gerhardt, another leading chemist in the effort to understand the constitution of organic compounds. Kekulé, after receiving his doctorate from Giessen, served as a research assistant, first in Switzerland, then in England. On a bus ride in London on the way home from visiting a chemist friend, Kekulé envisioned his earliest notion of carbon chains. In a daydream he "saw" carbon atoms joining in a "giddy dance."
Heinrich von Angeli's portrait of August Kekulé von Stradonitz, commissioned by German dye companies for his 60th birthday.
Courtesy Edgar Fahs Smith Memorial Collection, Department of Special Collections, University of Pennsylvania Library.
Kekulé returned to Germany from England to begin his career as a university teacher, which took him from the University of Heidelberg, to the University of Ghent in Belgium, and finally to the University of Bonn, where he oversaw the establishment of a chemical institute. He was a good lecturer, well liked by his many students. As with several other scientists in chemical history, writing a textbook—Lehrbuch der organischen Chemie—proved to be a stimulus for new chemical theories. One afternoon in 1865 in Ghent, while he was working on his textbook, he became sleepy and turned his chair to the fire. Again he saw dancing strings of carbon atoms, but this time he saw one that snake-like took its tail into its mouth, which gave him the idea for the ring form of the benzene molecule. Here then was a structure for the many molecules that would not fit into the existent structural theory.