Ahalonium ion is anyonium ion containing ahalogen atom carrying a positive charge. Thiscation has the general structureR−+X−R′ where X is any halogen and no restrictions on R,[1] this structure can be cyclic or an open chain molecular structure. Halonium ions formed fromfluorine,chlorine,bromine, andiodine are calledfluoronium,chloronium,bromonium, andiodonium, respectively.[1] The 3-membered cyclic variety commonly proposed as intermediates in electrophilic halogenation may be called haliranium ions, using theHantzsch-Widman nomenclature system.
The simplest halonium ions are of the structureH−+X−H (X = F, Cl, Br, I). Many halonium ions have a three-atom cyclic structure, similar to that of anepoxide, resulting from the formal addition of a halogenium ionX+ to a C=Cdouble bond, as when a halogen is added to analkene.[1] The formation of 5-membered halonium ions (e.g., chlorolanium, bromolanium ions) vianeighboring group participation is also well studied.[2]
Diaryliodonium ions ([Ar2I]+X−) are generally stable, isolable salts which exhibit a T-shaped geometry with the aryl groups at ~90 degrees apart;[3] for more details, seehypervalent iodine.
The tendency to form bridging halonium ions is in the order I > Br > Cl > F. Whereas iodine and bromine readily form bridged iodonium and bromonium ions, fluoronium ions have only recently been characterized in designed systems that force close encounter of the fluorine lone pair and a carbocationic center. In practice, structurally, there is a continuum between a symmetrically bridged halonium, to an unsymmetrical halonium with a long weak bond to one of the carbon centers, to a true β-halocarbocation with no halonium character. The equilibrium structure depends on the ability of the carbon atoms and the halogen to accommodate positive charge. Thus, a bromonium ion that bridges a primary and tertiary carbon will often exhibit a skewed structure, with a weak bond to the tertiary center (with significant carbocation character) and stronger bond to the primary carbon. This is due to the increased stability of tertiary carbons to stabilize positive charge. In the more extreme case, if the tertiary center is doubly benzylic for instance, then the open form may be favored. Similarly, switching from bromine to chlorine also weakens bridging character, due to the higher electronegativity of chlorine and lower propensity to share electron density compared to bromine.
These ions are usually only short-livedreaction intermediates; they are very reactive, owing to highring strain in the three-membered ring and the positive charge on the halogen; this positive charge makes them greatelectrophiles. In almost all cases, the halonium ion is attacked by anucleophile within a very short time. Even a weak nucleophile, such aswater will attack the halonium ion; this is howhalohydrins can be made.
On occasion, a halonium atom will rearrange to acarbocation. This usually occurs only when that carbocation is an allylic or a benzylic carbocation.[4]
Halonium ions were first postulated in 1937 by Roberts and Kimball[5] to account for observedantidiastereoselectivity inhalogen addition reactions toalkenes. They correctly argued that if the initial reaction intermediate in bromination is the open-chain X–C–C+ species, rotation around the C–Csingle bond would be possible leading to a mixture of equal amounts of dihalogensyn isomer andanti isomer, which is not the case. They also asserted that a positively charged halogen atom isisoelectronic with oxygen and that carbon and bromine have comparableionization potentials. For certain aryl substituted alkenes, theanti stereospecificity is diminished or lost, as a result of weakened or absent halonium character in the cationic intermediate.
In 1970George A. Olah succeeded in preparing and isolating haloniumsalts[6] by adding amethyl halide such asmethyl bromide ormethyl chloride insulfur dioxide at −78 °C to a complex ofantimony pentafluoride andtetrafluoromethane in sulfur dioxide. After evaporation ofsulfur dioxide this procedure left crystals of[H3C–+X–CH3][SbF6]–, stable atroom temperature but not to moisture. A fluoronium ion was recently characterized in solution phase (dissolved in sulfur dioxide orsulfuryl chloride fluoride) at low temperature.[7]
Cyclic and acyclic chloronium,[8] bromonium, and iodonium ions have been structurally characterised byX-ray crystallography, such as the bi(adamantylidene)-derived bromonium cation shown below.[9]
 |  |
| skeletal formula | ball-and-stick model |
Compounds containing trivalent or tetravalent halonium ions do not exist but for some hypothetical compounds stability has been computationally tested.[10]
