The Decline of Dualistic Organic Chemistry

Behind Kolbe's two mentors, Wöhler and Bunsen, lay the dominating spirit of Berzelius, whose reign as virtual king of chemical theoreticians was only beginning to be challenged while Kolbe was studying at Göttingen. Wöhler was Berzelius' official (and extraordinarily industrious) representative in Germany. The correspondence of the two chemists over two dozen years occupies nearly 1500 pages in Wallach's massive edition; the warmth of their scientific and personal relationships emerges clearly from virtually every page. Wöhler's own theoretical convictions, such as they were, nearly always came directly from the Jahresberichte and the Lehrbuch of Berzelius.

We have seen how Berzelius offered advice to Wöhler on both the

experimental and interpretational aspects of Kolbe's first extended investigation; Wöhler certainly shared these suggestions with his student. Although no letters between Berzelius and Kolbe have survived, it is known that they had begun to correspond on an occasional basis at least by the summer of 1843.^[44] It is clear from both correspondence and his reports in the Jahresberichte that Berzelius became ever more excited about Kolbe's research.^[45] For his part, Kolbe revered Berzelius; he kept one highly complimentary letter from the Swede (dated 3 August 1844) for the rest of his life as a "talisman" against the seductiveness of fraudulent hypotheses.^[46]

The bedrock of Berzelian chemistry, dating from Berzelius' first important paper (1803), was the conviction that chemical affinity was reducible to polar forces between electrically dissimilar molecular components. The system built up over the years by Berzelius and his school was thus called electrochemical dualism. Applied to inorganic combinations, this system became widely accepted in the 1810s and 1820s.

Applied to the emerging field of organic chemistry in the 1830s, the theory led to the formulation of various organic radicals that were thought to function integrally and electropositively, adding to negative components as metallic elements do in inorganic compounds. Whether electronegative oxygen could enter into an organic hydrocarbon radical was a disputed point right from the beginning of the older radical theory. Berzelius expressed strong doubts about the reasonableness of oxygenated radicals, preferring conceptually clean, theoretically consistent, purely electropositive hydrocarbon radicals. Liebig took the more empirical viewpoint that at least some of the oxygen associated with many organic radicals seemed to appear in entire series of compounds of the radical and so ought to be considered as an integral part of the radical.^[47]

Upsetting the coherence of dualistic organic chemistry was the phenomenon of chlorine substitution, wherein highly negative chlorine appeared to replace highly positive hydrogen in organic compounds, without major alteration of the properties of the compound. Since a cardinal thesis of dualistic organic theory was the direct dependence of chemical properties on electrical characteristics of molecular components, this appeared distressingly anomalous. Adumbrated by Liebig's mentor Gay-Lussac, then by Liebig's and Berzelius' archrival Dumas, organic chlorination reactions were first thoroughly explored by Dumas' student Auguste Laurent beginning in 1830. Laurent developed a nonelectrochemical theory of organic reactions based on chlorine substitutions which he initially called the theory of fundamental and derived radicals and later the nucleus theory. According

to this theory, fundamental radicals could be transformed into derived radicals either by substitution within the radical or by addition or elimination of atoms outside the radical. The most important factor for Laurent was not the identity of an atom but its position. The electrochemically opposite substances hydrogen and chlorine could play the same chemical role inside a radical; conversely, a chlorine atom inside or outside a radical would exhibit different chemical properties. Similarly, oxygen could replace hydrogen inside the radical with no great alteration of properties, but oxygen introduced outside the radical would make a neutral substance acidic.

Dumas at first attempted to salvage dualism in the face of Laurent's discoveries. Briefly allied with Liebig at the end of 1837, they published a joint manifesto that advocated a complete analogy between inorganic elemental radicals and organic compound radicals. Attacked by Berzelius as having been corrupted by Laurentian theory, Dumas indignantly refuted the charge. But in August 1838 Dumas discovered chloroacetic acid, and he noted to his surprise that the replacement of three-fourths of the electropositive hydrogen content of acetic acid by highly electronegative chlorine had little real effect on its properties.

Dumas promptly abandoned dualistic organic chemistry, formulating a new theory loosely based on Laurent's work. According to Dumas' "type theory," there are series of organic compounds each of which must be considered to be based on a single "type" formed from the same number of chemical equivalents combined in the same way. Contrary to dualistic organic theory, substitution reactions—even between electrochemical extremes, such as chlorine for hydrogen in chloroacetic acid—cannot alter the chemical type, hence cannot alter the fundamental chemical properties. In a modification of this theory (1840), Dumas conceded that substitution sometimes does alter the fundamental chemical properties without changing the total number or apparent mode of arrangement of the chemical equivalents. In such instances, a new chemical type is created, but without altering the "molecular or mechanical" type. Only addition or elimination of atoms could create a new mechanical type.

Despite his joint declaration with Dumas, Liebig had also been traveling away from the Berzelian orthodoxy. One of his greatest experimental and theoretical masterpieces, the joint work with Wöhler on benzoyl derivatives (1832), had suggested to him that oxygen must be considered as an integral part of certain organic radicals and that the hydrogen atom of benzaldehyde (benzoyl hydride) is replaceable by oxygen, chlorine, or other electrochemically dissimilar substances. Thus the door to apostasy was opened for Liebig.

For Berzelius, all acid-base reactions resulted from electrochemical

addition of a negative anhydrous (usually oxy-) acid to a positive base. Just as potassium sulfate, for example, was regarded as the result of the combination of sulfuric acid (anhydride) and potash:

so potassium acetate was regarded as the combination of acetic acid (anhydride) and potash:

It must be noted that Berzelius did not regard vapor densities of compounds as indicative of molecular size, and so there was no problem with formulating acetate as a "four-volume" molecule. Indeed, only a four-volume (and not a modern two-volume) formula succeeds in representing acetic acid anhydride without fractional oxygen atoms. He also used an atomic weight for potassium that was twice what later chemists adopted, hence he required only half the number of potassium symbols in the formula as compared to what we have become used to. Looking at the issue in the reverse sense, we note that the modern two-volume formula for acetic acid is C₂ H₄ O₂ ; doubling the formula and then subtracting a molecule of water (H₂ O) yields the Berzelian formula, which can be depicted as coupled to a (modern) potassium oxide component, K₂ O. Although Berzelius was led to these ideas by apparently consistent application of his basic assumptions, and although they were widely accepted for many years, the result of these manipulations was that inorganic and organic chemistry became based on two different fiducial standards: two- and four-volume formulas, respectively. This conflict was to create serious difficulties for Berzelian chemistry during the 1840s and led to its destruction in the 1850s.

Initially, Berzelius preferred to consider the acid, as in the equation cited above, as the trioxide of the acetyl radical, C₄ H₆ . We will see in the next chapter how he shifted his position after 1838 to view the oxygen content as combined with half the carbon of the molecule, the other half being a hydrocarbon moiety: C₂ H₆ ·C₂ O₃ . Since C₂ O₃ is the (two-volume) formula for oxalic acid (anhydride), all carboxylic acids could subsequently be formulated as homologous hydrocarbons coupled with oxalic acid.

From earlier work by Davy and Dulong and his own benzoyl radical paper, Liebig developed a new theory of acid-base reactions that depended not on dualistic addition of bases to acids but rather on

substitution by bases for a replaceable hydrogen atom of acids. The nonelectrochemical and substitutionist implications of this theory appeared to ally Liebig with Laurent, a point stressed by Berzelius in correspondence with his recalcitrant German friend. In fact, Liebig's organic hydracid theory led directly to a tragic polemic between them, at first waged quietly and privately, then increasingly publicly. The differences separating Berzelius and Liebig were quite real: Liebig had essentially abandoned dualistic precepts in organic chemistry, though he also had no patience for the French type theorists. The kindly Wöhler, who dearly loved both men, was caught in the middle. Unable to understand such passion over matters of theory, Wöhler tried for years without success to effect a reconciliation.

The wear on Liebig's nerves and emotions was also substantial, although not sufficient to overcome his firm (Berzelius would say obstinate) convictions. Worn down to the point of physical and psychical illness both by his arguments with Berzelius and by an increasingly vitriolic series of priority disputes with French substitutionists such as Dumas, Laurent, Malaguti, and Persoz, Liebig ostentatiously declared to Wöhler in early 1840 his abandonment of all chemical theory and his resolution henceforth to devote himself to practical pursuits such as agricultural and physiological chemsitry. Liebig was not alone in feeling an uncomfortable sense of disorientation and dismay at the rapid evolution of theory. J. F. W. Johnston described the science in 1840 as being "unhinged . . . tottering and disjointed." He called for a return to the comfortable Berzelian orthodoxy, condemned the "rage for the new " sweeping the Continent, and labeled Liebig as well as Dumas "chemical chartists."^[48]

Liebig's abandonment of theory was not mere rhetoric. After 1839, Liebig abandoned his own hydracid theory and really ceased to participate in the organic-chemical theoretical dialectic. One indication of his disgust was his unauthorized decision in February 1840 (which was during Kolbe's Göttingen years) to publish a whimsical and hilarious French-language lampoon of the type theory, which had been written on a lark by Wöhler.^[49] The putative author, S. C. H. Windler (Schwindler = swindler), claimed to have succeeded in gradually replacing all the atoms in copper acetate by chlorine, producing a material composed entirely of chlorine but retaining the properties of copper acetate. The paper portrayed with some justice, but also with malice, the extent to which Dumas, Laurent, and others were inclined to exaggeration. Whether due to the strain of the disputes, or to the Schwindler critique, or to the attainment of a level of professional success that reduced the hunger for acclaim, Dumas also retreated after

1840 from a leading theoretical role. Since many of the remaining leaders of German chemistry, including Wöhler, Bunsen, Gmelin, Rose, Mitscherlich, Will, and Kopp, were not inclined toward theory, and the great Berzelius increasingly appeared to the chemical world to be old-fashioned, something of a vacuum in German organic theory emerged in the 1840s—a void into which Kolbe eagerly stepped.