The German Genius

hydrocarbon—a substance containing carbon and hydrogen only. 11
    At first, Faraday called the waste “bicarburet of hydrogen,” later known as benzene. At that stage, there was not the slightest inkling that benzene is the stable backbone underlying a vast series of substances that would come to be known as “the aromatic compounds.” * 12
    Seven years after Faraday’s identification of benzene in 1832, von Liebig and Wöhler began their second important collaboration, this time working on the aromatic compounds. In the first phase they isolated from the oil of bitter almonds a substance they named hydrobenzoyl (today called benzaldehyde), which they found to contain carbon, hydrogen, and oxygen only. But it was their next move that was to prove crucial: they performed a series of transformations—and found that treating benzaldehyde with chlorine gave benzoyl chloride, a substance that could be further transformed into benzoyl iodide by potassium iodide and so forth. This was the first demonstration of a series of systematic chemical transformations that could be carried out with related organic substances. What von Liebig and Wöhler were the first to realize was that throughout this series of transformations a sizable backbone of the molecular structure, which they calculated as C 14 H 10 O 2 (as then written), remained unchanged . This backbone, which they labeled “benzoyl,” they called a “compound radical.” The idea of a “radical,” in this context, meant a collection of elements “mimicking the behaviour of a single element.” Lavoisier had considered the idea, but in a much simpler form, and as it related only to inorganic examples. A new era in chemistry was opening up. This was confirmed when the Berlin chemist Eilhardt Mitscherlich performed a transformation that von Liebig and Wöhler had missed. In 1834, by heating benzoic acid with lime, he obtained “nothing other than Faraday’s bicarburet of hydrogen”—benzene itself. This substance, benzene, C 6 H 6 , in time became revered as the truly irreducible nucleus or “radical” of the aromatic compounds. 13
    As more discoveries about the properties of the many organic compounds accumulated, it became ever clearer how exceptional benzene was. All other substances with a low ratio of hydrogen to carbon were unstable. Benzene had the same ratio of the two elements (1:1) as the highly explosive gas acetylene, but the benzene nucleus, as von Liebig and Wöhler repeatedly demonstrated, “could pass unchanged through a whole series of substitution reactions that could lead back again, given the right manipulations, to benzene itself,” behavior that set it “quite apart” from inorganic compounds. “The eventual solution to the nature of benzene would be one of the great attainments of the human mind.” 14
    Berzelius and other older chemists never really came to grips with organic substitution reactions, and the next moves were made mainly by French, Alsatian, and German chemists who gravitated to von Liebig’s Giessen laboratory. In this environment the structures and the properties of the aromatic compounds were gradually isolated, until the fundamental reality was grasped: organic chemistry is very largely the chemistry of “functional groups”—a term not yet invented—attached to a relatively inert hydrocarbon skeleton. Charles Frédéric Gerhardt, a Swiss brought up in Alsace, who studied under von Liebig at Giessen, was the first to understand how structure and function were related. 15 For example:

     

     

     

     
    Brilliant as this insight was, this picture concealed a more complex—but more fundamental—truth. What was behind this concept was something not discovered until the 1860s: valency. 16 Valency, in everyday language, is “the combining power of one atom for another”—in a way, the number of “hooks” an atom has available to join it to its neighbors. By the 1850s, water was identified as H 2 O (hydrogen monovalent, oxygen bivalent), but the behavior of carbon was still perplexing, since methane was CH 4 , ethane was C 2 H 6 , ethylene was C 2 H 4, and acetylene C 2 H 2 . Is the valency of carbon, 4, 3, 2, or 1? The answer, eventually, was found to be four, and what accounts for the difficulty nineteenth-century chemists faced is that carbon atoms form chains and rings with each other .

     

     
    Once this phenomenon had been discovered, organic chemistry gave up more of its structural secrets.