The Copula Theory
Kolbe must have been an avid observer of the disputes raging in organic chemistry during his student years in Göttingen. Laurent's nucleus theory, Dumas's type theory, Liebig's hydracid theory, Berzelius's defenses of dualism, the angry self-justificatory letters by the last three of these chemists published almost biweekly for a time in the Comptes rendus , and the Schwindler parody, not to mention the lively private correspondence that Wöhler conducted with his feuding intimates Berzelius and Liebig, all occurred while Kolbe was studying with Wöhler.
This was also the time when Berzelius was fashioning the theory of copulated or conjugated compounds, his chief line of defense in response to the arguments of the French substitutionists.[1] Since 1837 Berzelius had been noting the discovery of a number of organic derivatives of inorganic acids and bases (especially alkyl sulfates and sulfonic acids, and ammines) that retained typical properties of the original acid or base, especially an unchanged saturation capacity. In 1839 Gerhardt coined the term copulé to refer to such combinations, the organic adduct being the copule . Berzelius then expanded this concept to explain the near identity in properties of acetic and chloroacetic acid. In the latter substance, the "sesquichloride of carbon," or C2 Cl6 —the copula—was combined with "oxalic acid," or C2 O3 ·H2 O, without affecting the fundamental (acidic) properties of the latter moiety. When Dumas' assistant Louis Melsens found a method to reduce
the chlorinated derivative back to the starting material, Berzelius responded that acetic acid itself must be a "copulated" compound:
Berzelius then developed a general theory of copulated or conjugated compounds. He supposed conjugation to be a nonelectrochemical form of chemical combination, which was why the saturation capacity of the acid remained unaltered. The hydrogen of the organic copula could be substituted by chlorine or other atoms, Berzelius conceded, but since this substitution occurred only in the passive, chemically unimportant copula, it was not surprising that the fundamental chemical properties of the compound did not change. For every substance substituted by chlorine, Berzelius now hypothesized a copula structure, always placing the chlorine in the copula. The theory provided a way for Berzelius to concede the increasingly unavoidable fact of chlorine substitution while maintaining the basis of dualistic organic chemistry. But what seemed to Berzelius to be a flexible modification showing the strength of dualism, seemed instead in the eyes of his opponents to be an unacceptably ad hoc retreat for a dying theory. In his posthumous chemical manifesto, the embittered and dying Laurent blasted the theory:
A word let fall from the pen of Gerhardt, was thus transformed into a luminous idea for dualism. From this time everything was copulated. Acetic, formic, butyric, margaric, &c., acids,—alcohols, ethers, amides, anilides, all became copulated bodies. So that to make acetanilide, for example, they no longer employed acetic acid and aniline, but they re-copulated a copulated oxalic acid with a copulated ammonia. I am inventing nothing—altering nothing. Is it my fault if, while writing history, I appear to be composing a romance? What then is a copula? A copula is an imaginary body, the presence of which disguises all the chemical properties of the compounds with which it is united. . . . The dishonesty is flagrant.[2]
But the copula theory proved to have a scientifically fruitful life. Just before Kolbe's arrival in Marburg, Bunsen provided a compelling argument in favor of Berzelius' theory by isolating the cacodyl radical and a large number of its derivatives, all of which also fit into the emerging copula theory. The acidity of oxygenated arsenic ("arsenious acid," formulated as AsO) did not seem at all affected by the addition of methyl radicals (producing "cacodyl oxide," formulated as
C2 H6 AsO). Cacodyl oxide could be reduced to cacodyl, the first unquestionably organic radical ever isolated and the first organometallic compound ever synthesized. Berzelius was thrilled at this new support for his ideas. "This is a triumphal chariot, which will overrun and smash [Dumas'] flimsy theoretical barricades," he exulted to Wöhler.[3]
The foundations of the copula theory would have been severely weakened, however, had Gerhardt's and Laurent's arguments been accepted, namely, that all compounds must be related to the same number of gaseous volumes (two), that hydrated acids are not oxides combined with water but rather are radicals with replaceable hydrogen, and that certain acids are polybasic. For instance, in response to Berzelius' suggestion that acetic acid consists of oxalic acid, C2 O3 ·H2 O, paired with the conjugate radical methyl, C2 H6 , Gerhardt and Laurent argued that the true formulas for the two components are half of those indicated, that there is no oxalic in acetic acid, that the true oxalic acid is dibasic rather than monobasic, and that it cannot be separated into an oxide and a molecule of water. They had similar problems with cacodyl oxide and other copula formulas.
Despite these objections, in its initial historical context Berzelius' copula theory was a successful theory that was both empirically supported and heuristically fruitful. Indeed, at the time of its formal introduction in 1839-1841, Gerhardt had not yet even fully formulated the arguments just rehearsed. Moreover, one could imagine a variety of potential compromise positions between the Berzelians and the French reformers if the two sides had been willing to look at the matter from their opponent's point of view. Despite the tendency of modern chemists to view Gerhardt's and Laurent's theories as correct, it might be said that there was as much truth to the copula formulas as there was to the French nucleus and type formulas.
Berzelius was excited by the subject of Kolbe's first research, for this was the same sort of work that had just yielded the wonderful fruit of cacodyl derivatives. This point was not lost on Kolbe, who from autumn 1842 had become a student of the chemist whose cacodyl series was still appearing in Liebig's Annalen . Kolbe's first discovery in Marburg was that moist rather than dry chlorine reacting with carbon disulfide resulted in an oxygenated and rearranged product, namely, one predicted earlier by Berzelius now known as trichloromethylsulfonyl chloride. Alkaline hydrolysis produced the corresponding sulfonic acid:
This last substance Kolbe named Chlorkohlenunterschwefelsäure , or dithionic acid (i.e., hyposulfuric acid anhydride) copulated with Kohlensuperchlorür (i.e., carbon "protochloride," or fully chlorinated ethane).[4]
It will be useful for what follows to understand this compound and its formulation from both a modern and a Berzelian-Kolbean perspective. The first point to note is that dualists always worked schematically with the anhydrides of acids, whether or not they had ever been isolated. For dithionic acid (Unterschwefelsäure ), the schematic transformation from modern (hydrous) to Berzelian anhydride composition is H2 S2 O6 - H2 O = S2 O5 . Second, vapor densities were not regarded as a consistent indication of molecular size, so that formulas could be halved or doubled as required by theory. Most organic formulas were doubled with respect to inorganic formulas—that is, four-volume organic formulas were usually standard. Thus, whenever dithionic acid, including its associated water molecule (S2 O5 +H2 O), occurs in a dualistic organic formula, it is equivalent to the modern sulfonic acid moiety: -SO3 H. Finally, the copula—carbon protochloride—could be considered either a two-volume representation of the fully chlorinated ethane discovered by Faraday (= modern C2 Cl6 ) or a four-volume representation of the fully chlorinated methyl radical (= modern -CCl3 ). The first was initially preferred by Berzelius since the copula could then be represented as a known isolated compound. After methyl was also discovered as a (seemingly) stable isolated substance, however, Kolbe preferred the second interpretation. Hence, what is seen by modern chemistry as the combination of two monovalent moieties, sulfonic acid and methyl, was viewed by copula theorists as the nonelectrochemical "coupling" of two stable, theoretically isolable molecules, dithionic acid and carbon protochloride.
In this same brief notice of 1844, Kolbe revealed that the carbon superchloride (CCl4 ) formed by chlorination of carbon disulfide could be partially transformed by passage through a hot reduction tube into another of Faraday's chlorocarbons, CCl2 , today known as tetrachloroethylene. The latter compound could be further chlorinated, or chlorinated and oxidized to chloroacetic acid, depending on conditions and how many molecules were thought to take part in the reactions. In Kolbe's terms (using unbarred atoms), these reactions are
In 1845 Kolbe published his first extended paper, summarizing his work with these compounds over the previous three years, revealing a number of new derivatives, and interpreting all of them as Berzelian copulas.[5] He showed how one could reduce the trichloro sulfonic acid derivative in a stepwise fashion to dichloromethyl-, monochloromethyl-, and fully reduced methylsulfonic acid using either chemically or electrically produced nascent hydrogen. This represented his first application of Bunsen's powerful new battery, and he sagely predicted that it would doubtless prove an important instrument for the future study of chemical constitutions. The paper was fairly bursting with new compounds, new reactions, and new methods—a bravura performance.
But the paper is at least as notable for its theoretical content as for its experimental novelties. Kolbe argued that his new derivatives of dithionic acid formed a perfect analogy to the chlorinated acetic acids of the French chemists, but that the analogy only made sense when formulated as copula compounds and not as "types." The sulfur acids could be chlorinated without significantly affecting their properties only because the methyl that actually undergoes substitution is the passive copula, in precisely the same way that the hydrocarbon copula of acetic acid can be chlorinated without much affecting the "oxalic acid" to which it was coupled. In an ironic twist of language, he argued that the copulated dithionic acids could stand as "prototypes" for all copulas.[6] Furthermore, by starting with a chlorocarbon and ending with the synthesis of chloroacetic acid, Kolbe thought he had proven the preexistence of the former in the latter, confirming Berzelius' prediction.
This new research, Kolbe averred, thus transformed copulas from "the realm of mere hypothesis to a high degree of probability." Of course, the work also further broadened the field of chlorine substitution and could, like A. W. Hofmann's 1843 papers on chlorinated anilines, be viewed as supporting the "newer substitution theory" of the French chemists. This would be a hasty conclusion, he cautioned. Indeed, one could hardly imagine two more different chemical "types" than chloroform and carbon disulfide, yet Regnault and Kolbe had independently shown that each reacts with chlorine to yield an identical product, carbon superchloride. Contrary to Dumas, chemical types were not conserved. Hofmann's research only showed that aniline, too, was a copulated compound.[7]
Kolbe also followed up on Berzelius' comment to Wöhler about Kolbe's compounds occupying the border region between organic and inorganic compounds. As we have seen, one of the new reactions
Kolbe discovered was the transformation of what is now called tetra-chloroethylene into chloroacetic (i.e., trichloroacetic) acid. Because he had prepared the former substance (like all of his chlorinated carbon compounds) from carbon disulfide and ultimately from inorganic carbon and sulfur, and because the product of the reaction was reducible by electrolytic hydrogen to acetic acid, here was a "nearly direct" synthesis of vinegar from the inorganic elements. These and the other reactions Kolbe had discovered showed a "continuous chain" from inorganic to organic compounds, so much so that any distinct boundary between them had disappeared. Kolbe waxed eloquent: as one can now synthesize vinegar, it may prove possible in the future to reduce acetic acid to alcohol, then to synthesize sugars and starches, in which case an immense new field for our endeavors will open up.[8]
This was the first significant organic synthesis since Wöhler's classic urea paper of 1828,[9] and the first total synthesis in history. The previous paragraph shows that Kolbe was well aware of its importance. It is also said that Kolbe was the first here to use the word Synthese in its modern chemical sense. In many respects, his accomplishment was even more dramatic than Wöhler's, since Wöhler had begun with starting materials derived from living creatures, whereas Kolbe had started with carbon and sulfur, whose ties with life were distant and indirect at best. To be sure, Kolbe's acetic acid synthesis did not destroy vitalism any more than his mentor's urea had, but it was an important benchmark on the route to modern synthetic organic methods.
This, Kolbe's first major paper, established patterns he would follow for the rest of his life. Above all, the article reveals his intense interest in chemical theory and, in particular, in theories concerning the inner constitutions of the molecules of organic compounds (especially organic acids). Kolbe's starting point was Berzelian radical theories. Under attack by the French substitutionists, Berzelius had devised a modification of his theory, copulas, that could countenance the distasteful new phenomenon. However, unlike Berzelius, Kolbe embraced chlorine substitution with enthusiasm and never exhibited any discomfort with the notion of electropositive radicals containing electronegative elements. Like Liebig's, his dualism was moderated by the undeniable fact of chlorine substitution. But in all essential respects, he was from first to last a committed, indeed zealous, Berzelian. For Kolbe, radicals were not mere schematic or conventional entities, but real, preexisting parts of molecules. The distinguishing characteristic of his entire career was the discernment and (wherever possible) physical isolation of these radicals.
As might be expected, Berzelius himself was delighted and highly impressed by Kolbe's work; Kolbe treasured the Swede's letter of con-
gratulations of 3 August 1844 to the end of his life. Berzelius described Kolbe's experiments and interpretations in glowing terms in his Jahresberichte and praised the significance of the work in letters to both Wöhler and Bunsen. He averted publicly that Kolbe had provided "as complete a proof as is possible in chemistry" for the existence of copulas.[10] The following year (1845) he met Kolbe, for the first and only time, during his last visit to Germany.
Berzelius even used Kolbe's disproof of Dumas' types as the final decisive argument in a paper entitled "Views Regarding Organic Composition," which he submitted in February 1846 to the Swedish Academy of Sciences and subsequently published in Poggendorff's Annalen der Physik —the last important article of his life. Berzelius thought that Kolbe's work provided "positive proof" for the existence of copulas; it "completes the refutation of the substitution theories and the imagination-game of chemical types." He regarded as quite novel and striking the stepwise replacement of chlorine by hydrogen and vice versa, with no significant change in the properties of the compound. In short, it was the best work he had seen since Bunsen's cacodyl series.[11]
One can imagine the effect such praise from the venerated master must have had on the young Kolbe. Indeed, much of the substance and rhetoric of Berzelius' last article can be discerned in often little-altered form in many of Kolbe's own later diatribes. Berzelius here repeated (no fewer than three times, and once in italics) that the only sure guide to theories of organic compounds is the dualistic theory of inorganic chemistry. He also rebuked the French chemists, especially Dumas, Laurent, and Gerhardt, for letting their imaginations dictate "Phantasiebilder" that have little or no empirical support or connection to the existing theoretical structure of the science. Determining constitutions of organic compounds is both the most important and the most difficult of all goals in chemistry, he averred; all the more vital that it be done with caution, care, and circumspection.