Eine logische Rechtschreibung: Carben-Carbenium

Beitrag zum Problem der Formulierung von Carben

Abstract

a) The author quotes the previously discussed arguments in favour of the term carbenium ion and also mentions that the IUPAC now prefers carbenium to carbonium.
b) Among other evidences the nomenclature rule C 82.4 is reported, according to which an organic cation formed by addition of a proton to an unsaturated hydrocarbon is named by adding “-ium" to the name of the hydrocarbon, with elision of terminal “e” if present.
Following examples are given by the IUPAC :

ethene + H⁺ → ethenium CH₃–CH₂⁺
benzene + H⁺ → benzenium C₆H₇⁺
As a logical addition the author gives two further examples:

carbene + H⁺ → carbenium CH₃⁺
benzyne + H⁺→ benzynium C₆H₅⁺

1) According to rule C 83 the name carbenium may be used for CH₃⁺. Only the nucleophilic carbenes however can be regarded as bases conjugate to the carbenium ions
:C<  conjugate to H–C⁺<

The formal relationship of carbenes to carbanions and to carbenium ions is represented in fig. 1, in classical (1a) and in segment notation (1b).
In its singlet state the genuine carbene is a 1.1-dipole and has an empty orbital and a pair of unshared electrons, corresponding to a virtual (electrophilic) and a real (nucleophilic) segment, respectively. In segment notation positive charges can be neutralised but are never annihilated. Therefore the author formulates e.g. the dihalocarbenes as X₂Cl±.
The above carbenes add to olefins to form cyclopropanes. Both segments can react simultaneously leading to stereospecific addition.
Whether the electrophilic or nucleophilic character of a singlet carbene will predominate, depends on the ability of the segments of adjacent atoms to overlap with the real or virtual segment of the divalent carbon. Fig. 4 illustrates the exclusion of overlap with the other segment as a result of orthogonality.
The nucleophilic character of a carbene is mostly exemplified in the aminocarbenes of Wanzlick, which fail to react with olefins (no longer 1.1-dipole.)
2) The carbon atom –C⁺= is designated as a carbynium atom. It is encountered in the benzynium ion, hetarynium ion, dipole form of acetylenes, acyl cations and in diphenylcarbene, which therefore has electrophilic character.
c) In the final chapter the author deals with the azenes R—N± (nitrenes), which are isoelectronic with the carbenes. The aminoazenes can be protonated to azenium ions. Also included are the electron-deficient sulfenium and oxenium ions having a sextet of electrons, as well as the sulfinium salts. An oxinium oxygen atom is also considered.