Most are produced synthetically in the laboratory or as a result of industrial processes. Very few halogenated compounds occur naturally. Almost any class of organic compounds (e.g., alcohols, ketones, carboxylic acids) can contain halogen atoms. Organic halogen compounds are derivatives of organic compounds in which one or more hydrogen atoms have been replaced by an equal number of halogen atoms (F, Cl, Br, or I). Marianna Anderson Busch, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 IX.A General Survey Consequently, there is no low energy pathway for reactions to occur, and the compound is inert even though many reactions are thermodynamically favored.
This inertness is a result of the molecule having no vacant bonding site or unshared electron pairs on sulfur to initiate a reaction and the fact that six fluorine atoms shield the sulfur atom from attack. The compound is so inert that it is used as a gaseous insulator, and rats allowed to breathe a mixture of SF 6 and oxygen show no ill effects after several hours of exposure. The molecule has the expected octahedral structure and a dipole moment of zero. Under a pressure of 2 atm, the melting point is −51 ☌. Sulfur hexafluoride sublimes at −64 ☌ to produce a dense gas (6.14 g L −1). The evidence for halogen bridging in electrophilic chlorinations and iodinations of alkynes is much weaker 〈B-78MI11900〉.
Brominations of alkynes to give anti stereospecific addition products provide evidence for unsaturated cyclic bromonium ion intermediates. Electrophilic addition of halogens to alkenes has been reviewed by Schmid and Garratt 〈B-77MI11900〉. Chlorination and iodination of appropriately structured alkenes also provide evidence for bridged halonium intermediates. The concept of a bromonium ion was subsequently used to rationalize observations in studies of many dissociative nucleophilic displacement reactions at centers with a neighboring bromine atom 〈39JA1576, 63MI11900, 64QR45〉. The intermediacy of a three-membered ring cyclic bromonium ion was first suggested by Roberts and Kimball in 1937 to explain the stereochemistry of the electrophilic addition of bromine to alkenes 〈37JA947〉. This chapter will emphasize the heterocyclic species which include three-, four- and five-coordinate halogen in the cyclic system, since they have not been recently or thoroughly reviewed. Reviews concerned with polyvalent iodine species have been published by Sandin 〈43CR(32)249〉, Beringer and Gindler 〈56MI11900〉 and Banks 〈66CR243〉. The chemistry of halonium ions has been the subject of an extensive and complete review by Olah 〈B-75MI11900〉. Heterocyclic halogen compounds include both the cyclic halonium salts and their covalent analogues, compounds with formal expansion of the valence octet of the halogen to 10 or 12 electrons. Martin, in Comprehensive Heterocyclic Chemistry, 1984 1.19.1.1 Historical