Carbonium Ion: Waxing and Waning of a Name James G. Traynham Louisiana State University, Baton Rouge, LA 70803 "Carbonium ion" has a familiar ring to most chemists, who are likely to think of a species, usuallv short-lived, with a positivecharge on carbin. c he concept of a cation with charge on carbon has been central to organic reaction mechanisms, and its development is linked to some of the most prominent chemists who have worked in this area. Usage of the name has changed several times, however, since it was introduced in 1902. A review of the history of this name reveals how a chemical term mav emeree. r .. . become o. o.~ u l ain a dirlrrent runtext, lose its precision, and fall from populari1s. I1 illuminates the care renuired in the drvelonment and use of chemical nomenclature. Comprehension of some of the points to be made will be aided by recall of the basic IUPAC rule about spacing in names of organic compounds. Ethyl alcohol (radicofunctional nomenclature) has a space between parts of the name, because alcohol is a functional class name, not the name of any compound. Ethylsilane (substitutive nomenclature) has no space, because silane (SiH4)is the parent compound that has a suhstituent. etbvl. Carbonium ion has been used in both radicofuncti'onal*and substitutive names, not always with care about spacine. In German. the soacine does not appear anyway. The term carboniumion first appeared in the chemical literature in early 1902 in an article on color theory by Adolf Baever and Victor Villiaer ( I ) . Baever and Villiaer attemoted to explain the color that developed when triphenylmethan01 was mixed with sulfuric acid or triphenylmethyl chloride with aluminum chloride. They likened these reactions to salt formation from metal hydroxides and likened the carbon center to the metal: natrium-carbonium. The designation carbonium was apparently intended merely to convev that metal-like behavior: carbon-ium. (The strong favor roward Latin wurds at thut time in Germany may have led to thut choi~.einsrrad of Kohlenstoffiuni for the new trrm., Baeyer and Villiger did not use a formula representation for the carboniumsalze, and the term was used onlv in the generic sense. That is, i t was not used as part of the name of any specific substance. The focus was clearly on the metal analogy. The originating authors used the terms carboniumsalze, carboniumnitrate, and Carbonium- und OxoniumSalze. A couple of months later, extensive studies of *$X com~ o u n dwere s seoaratelv r e ~ o r t e din the same iournal bv Paul ~ a l d e (2) n and ~ o s e s ~ o k b e(3). r g The rapidity of lhemical publishing in 1902 is indicated by their references to the publication of Baeyer and Villiger. Walden used the term carbonium in specific names as well as a class desienation (2). In the specific names, i t appeared as triphenylcarbonium. Walden drew an analogy among Ammonium und Sulfonium-Basen, Iodoniumbasen, and Carboniumhasen and Carhoniumsalze. After using these terms, Walden wrote that most of the research had been completed when the BaeyerVilliger paper appeared (obviously correct with so short a time between the papers), but credit for the ion names is not given. Walden's intent, like Baeyer's, seems to have been to identify carbon-ium compounds rather than carb-onium ones. Unlike Baeyer and Walden, Gomberg actually used the formula %C+ in his paper (3). While he used Carboniumsalze in the title of his paper, he asserted, in the last paragraph, that carbonium should be reserved for those salts in 930
Journal of Chemical Education
which an increase in valency of carbon occurs, as with other onium salts. He proposed the name carbyl salts for "the salts of alkyl". In spite of the reasonableness of his proposal, apparently it had no impact on usage, perhaps, in part, because of the relative status of Baeyer and Gomberg among organic chemists in 1902. Within a decade, several other articles in German journals discussed carboniumsalze (4). Carbonium was used both for a class name and for the positive carbon in specific names (e.g., Triphenylcarhoniumsalze), but Triphenylmethyl-Ion was also used in the context of reference to ~ r i o h e n v l methyl-Radical. There was no indication that carbonium salts were e x ~ e c t e doutside the triarvlmethvl . svstem. A shift in t h e intended meaning bf carbonium seems to have emerged in an article on "The Basic Properties of Oxygen" in JACS in 1912 (5). The US. authors referred to the systems toluene HBr and toluene HCl as carbonium complexes, hut no picture, either in words or formula, was given. They did refer to oxonium complexes, formulated them as, for example,
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