Halonium ion
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[[Image:Cyclopentene-bromonium-3D-balls.png|thumb|right|200px|A [[ball-and-stick model]] of the bromonium ion formed from [[cyclopentene]]]]
A '''halonium ion''' in [[organic chemistry]] is any [[onium compounds|onium compound]] (ion) containing a bridged [[halogen]] atom carrying a positive charge. This [[cation]] has the general structure R-X<sup>+</sup>-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 <ref>The Halogenation of Ethylenes Irving Roberts and George E. Kimball [[J. Am. Chem. Soc.]]; '''1937'''; 59(5) pp 947 - 948; {{DOI|10.1021/ja01284a507}}</ref> to account for observed [[diastereoselectivity]] in [[halogen addition reaction]]s to [[alkene]]s. They correctly argued that if the initial reaction intermediate in bromination is the open-chain X<sup>-</sup>-C-C<sup>+</sup>, 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 potential]]s.
In 1970 [[George A. Olah]] succeeded in preparing and isolating halonium [[salt]]s <ref>''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. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1970/92/i03/f-pdf/f_ja00706a058.pdf Abstract]</ref> 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 CH<sub>3</sub>-X<sup>+</sup>-CH<sub>3</sub>SbF<sub>6</sub><sup>-</sup>, 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]], H<sub>2</sub>F<sup>+</sup>; [[chloronium]], H<sub>2</sub>Cl<sup>+</sup>; [[bromonium]], H<sub>2</sub>Br<sup>+</sup>; [[iodonium]], H<sub>2</sub>I<sup>+</sup>.
==References==
{{reflist}}
{{commonscat|halonium ions}}
[[Category:Cations]]
[[pt:Íon halônio]]