Halonium ion

File:Cyclopentene-bromonium-3D-balls.png

A ball-and-stick model of the bromonium ion formed from cyclopentene

A halonium ion in organic chemistry is any onium compound (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 to account for observed diastereoselectivity in halogen addition reactions to 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 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.

A halonium ion also seems to refer generically to the simpler onium compounds (ions) based on halogens: fluoronium, H₂F⁺; chloronium, H₂Cl⁺; bromonium, H₂Br⁺; iodonium, H₂I⁺.