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The Defection of Hofmann and Frankland

Some additional problems in this paper concern Kolbe's orientation toward Hofmann's and Frankland's research on substitution reactions. As already noted, Kolbe and Hofmann were exact contemporaries, arrived in London the same month, and became very close friends during the next two years. Hofmann later reported the results of some joint work that they performed in this period.[65] At this time, Hofmann and Kolbe were of very similar minds regarding constitutional theories. Hofmann's work on aniline appeared fully to support Berzelius' copula theory, the aniline derivatives all being formulated as ammonias with the structurally unspecified hydrocarbon C12 H4 as a copula. Consistent with the theory, all the substituted anilines retained the characteristic basic properties of ordinary aniline, analogous to free ammonia. In June 1848, two months before Berzelius' death, Hofmann published a paper extolling Berzelius and his theory and adducing all


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the recent powerful experimental support for copulas, including Kolbe's work on acids and his own on bases. Still, he plaintively noted, "vainly have I hoped" to split aniline directly into NH3 and C12 H4 .[66]

But the following year he found compelling reasons to abandon the copula theory, turning to embrace a thoroughgoing substitution theory and Liebig's amidogen theory (NH2 plus a radical). Hofmann's conversion appears to have turned on one notable failed analogy between ammonia and aniline: whereas ammonia and benzoic acid can react together losing two water molecules (four "water equivalents") to form benzonitrile, there is no corresponding reaction between aniline and benzoic acid. The only way theoretically to rationalize this fact was to consider aniline not as a compound containing proximate ammonia but rather as one containing proximate amidogen (NH2 ) in its structure; then there would be insufficient hydrogen in aniline to yield two water molecules unless some additional hydrogen were abstracted from the hydrocarbon moiety. Thus, aniline is not NH3 ,C12 H4 , but rather NH2 ,C12 H5 .[67]

As trivial an adjustment as this may seem, the implications were large. Copula theorists insisted wherever possible on using isolable compounds as their proximate components and explaining their combination as nonelectrochemical addition. This joining of proximate components, they averted, had nothing to do with substitution of hydrogen. By contrast, Hofmann's new constitution for aniline did indeed suggest substitution of Laurent's phenyl radical for a hydrogen atom of ammonia, and it denied the preexistence of ammonia in aniline. In short, Hofmann was saying that aniline could not be a copulated ammonia; it was instead a substituted ammonia.

In this paper, Hofmann placed all the weight motivating his conversion on the nitrile-forming reaction just discussed. Nonetheless, he did mention Wurtz's discovery earlier that year of methylamine, ethylamine, and amylamine (what Gerhardt called primary amines), which appeared to be ammonia with one hydrogen substituted by organic radicals. This new development was surely a motivating factor as well. At the very end of the paper, he mentioned one more new kind of reaction that, he thought, put the matter beyond all question: his own discovery, just made, of secondary and tertiary amines, representing further substitutions of the second or third hydrogens of ammonia or of the second hydrogen of aniline, by various hydrocarbon radicals.

This paper was read to the Chemical Society on 5 November 1849. By 26 December, Hofmann had submitted to the Royal Society a detailed memoir on the subject just broached, detailing one of the most classic pieces of organic chemical research in the nineteenth century. Using alkyl iodides as reagents, Hofmann described the preparation,


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properties, analysis, and theoretical classification of dozens of new substituted ammonias, including mono-, di-, and tri-methyl, ethyl, and amyl amines and anilines.[68] In a letter to Kolbe written while the paper was in press he described the work and related how much he had enjoyed it: "This investigation was a great deal of fun for me, for in 6 weeks the whole matter was settled. These reactions are so precise, that not a single experiment failed. The number of bases has now become virtually unlimited."[69]

In a letter that has apparently not survived, Kolbe must have communicated to Hofmann in 1850 an outline of his ideas for the review article on organic radicals, for in the letter just cited (undated, but ca. March 1850 by context), Hofmann reacted to some of Kolbe's ideas. Kolbe must have been attempting to maintain a strict interpretation of copulas, for Hofmann, now a true substitutionist, responded:

I cannot yet accommodate myself to your point of view, in particular it seems more complicated than mine, and moreover it does not explain why ethyl can be inserted twice into aniline and three times into ammonia. This is a fact which I, as you can see, have established by these experiments. But I will suspend judgment until I have heard all your reasons.[70]

Kolbe's other good friend from his London years was simultaneously undergoing a similar conceptual evolution. It was Frankland who in 1848 had introduced the use of alkyl iodides as reagents, and he used them with phenomenal success in the preparation of new organometallic compounds. These reactions appeared to proceed by substitution mechanisms. The radicals seemed to substitute indifferently for hydrogen, not only in the (mostly hypothetical) metal hydrides but also in Paul Thenard's alkyl phosphines and in Hofmann's and Wurtz' nitrogen bases.

At the same meeting of. the Chemical Society where Hofmann first mentioned his preparation of secondary and tertiary amines from alkyl iodides and signaled his conversion from copulas to substitution, Frankland's first paper on organometallic compounds, in which he drew similar implications, was read in absentia. An alkyl iodide, Frankland wrote, was analogous to hydriodic acid or any other hydracid, and the alkyl radical can substitute for hydrogen in a variety of organic and inorganic substances. He drew a specific analogy to Wurtz', Hofmann's, and Thenard's compounds, all the result of substitution reactions. Frankland was quite serious about this matter: the same comment was repeated in a paper read to the Chemical Society on 18 February 1850.[71]

Thus, in addition to his strong statements and recently revealed evi-


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dence in favor of the original radical theory of 1832-1834, from the beginning of his second German period (October 1848 to January 1850) Frankland was also paying close attention to the newest research on substitution and fitting his results into that theoretical program as well. In a classic paper written shortly after his return to England, Frank-land was explicit. One could depict metal oxides as "true molecular types" for the organometallic compounds, he stated, citing Laurent and Dumas as authorities for the concept. For the fourth time in three years, he emphasized the analogy between his new compounds and those of the type theorists: "It is obvious that the establishment of this view of the constitution of the organometallic bodies will remove them from the class of organic radicals, and place them in the most intimate relation with ammonia and the bases of Wurtz, Hofmann, and Paul Thenard." Frankland was clearly offering the olive branch to the French type theorists: "The formation and examination of the organometallic bodies promise to assist in effecting a fusion of the two theories which have so long divided the opinions of chemists, and which have too hastily been considered irreconcilable."[72]

This apparent defection from the copula theory (and implicitly at least toward the French chemists) of his two loyal friends must have given Kolbe pause, and may account for the long delay between draft and publication of his 1850 article. There is evidence in Kolbe's paper of a serious effort toward accommodation of the uncomfortable new research, but also of the construction of a seawall against the incoming tide of typist ideas. Hofmann and Wurtz were "certainly right," he said, regarding the amidogen constitution of aniline and primary amines. Furthermore, Hofmann's work did demonstrate to a certain extent the truth of the type theorists' dictum that substitution of electrochemically foreign elements (such as chlorine or bromine in aniline) does not fundamentally alter the substance's properties. But that same research also showed that the extreme interpretation of type theory is wrong: a steady decrease of basicity occurs as aniline becomes more halogenated, until finally it becomes virtually neutral in character. Hofmann's and Frankland's research also placed beyond any doubt the actual existence of organic radicals as components of compounds, a point that some French chemists such as Dumas and Gerhardt had at times denied.[73]

Kolbe then asked whether it was the "radical hydrogen" of ammonia (i.e., that hydrogen atom whose removal creates amidogen) which is substituted by methyl or ethyl to create methylamine or ethylamine, and he answered his query in the affirmative.[74] Thus, Kolbe assumed the chemical nonequivalence of the three hydrogens of ammonia, simply from the circumstance that in some compounds nitrogen has


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only two hydrogens associated with it. The fact that Hofmann had shown how to substitute all three hydrogens in succession by similar and smooth reactions did not seem to weaken this assumption.

This is an example that once more illustrates Kolbe's habit of assuming theoretical details that are unmotivated, or even subtly contradicted, by empirical information; others have been cited earlier. His mind was so intensely and habitually oriented toward chemical theory in general, and molecular constitutions in particular, that such assumptions seem to have been an integral part of his mentality. This nearly obsessive concern would continue in the future to be both his greatest strength and his greatest weakness as a scientist.


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