Halonium ion

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A halonium ion in organic chemistry is any onium ion containing a bridged halogen atom carrying a positive charge. This cation has the general structure R-X⁺-R where X is any halogen and R any organic residue and this structure can be cyclic or an open chain molecular structure.

Halonium ions were first postulated in 1937 by Roberts and Kimball ^[1] to account for observed diastereoselectivity in halogenation of alkenes. They correctly argued that if the initial reaction intermediate in bromination is the open-chain X^--C-C⁺, rotation around the C-C single bond would be possible leading to a mixture of equal amounts of dihalogen cis isomer and trans isomer which is not the case. They also asserted that a positively charged halogen atom is isoelectronic with oxygen and that carbon and bromine have comparable ionization potentials.

In 1970 George A. Olah succeeded in preparing and isolating halonium salts ^[2] by adding a methyl halide such as methyl bromide or methyl chloride in sulfur dioxide at -78°C to a complex of antimony pentafluoride and tetrafluoromethane in sulfur dioxide. After evaporation of sulfur dioxide this procedure left crystals of CH₃-X⁺-CH₃SbF₆^-, stable at room temperature but not to moisture.

[edit] References

1. ↑  The Halogenation of Ethylenes Irving Roberts, George E. Kimball J. Am. Chem. Soc.; 1937; 59(5); 947-948. Abstract
2. ↑  Friedel-Crafts chemistry. V. Isolation, carbon-13 nuclear magnetic resonance, and laser Raman spectroscopic study of dimethylhalonium fluoroantimonates George A. Olah, John R. DeMember J. Am. Chem. Soc.; 1970; 92(3); 718-720. Abstract