Carbonium ion: Waxing and waning of a name

Abstract

Carbonium ion" has a familiar ring to most chemists, who are likely to think of a species, usually short-lived, with a positive charge on carbon. The 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 may emerge, become popular in a different context, lose its precision, and fall from popularity. It illuminates the care required in the development 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 (SiH () is the parent compound that has a substituent, ethyl. Carbonium ion has been used in both radicofunctional and substitutive names, not always with care about spacing. In German, the spacing does not appear anyway.The term carboniumion first appeared in the chemical literature in early 1902 in an article on color theory by Adolf Baeyer and Victor Villiger (1). Baeyer and Villiger attempted to explain the color that developed when triphenylmethanol 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 convey that metal-like behavior: carbon-ium. (The strong favor toward Latin words at that time in Germany may have led to that choice instead of Kohlenstoffium for the new term.) Baeyer and Villiger did not use a formula representation for the carboniumsalze, and the term was used only in the generic sense. That is, it 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 Carboniumund Oxonium-Salze.A couple of months later, extensive studies of <t3CX compounds were separately reported in the same journal by Paul Walden (2) and Moses Gomberg (3). The rapidity of chemical 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 designation (2). In the specific names, it appeared as triphenylcarbonium. Walden drew an analogy among Ammonium und Sulfonium-Basen, Iodoniumbasen, and Carboniumbasen and Carboniumsalze. After using these terms, Walden wrote that most of the research had been completed when the Baeyer-Villiger paper appeared (obviously correct with so short a time between the papers), but credit for the ion names is not given. Walden's int...

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