A halonium ion in organic chemistry

Organic chemistry

Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 is any onium compound

Onium compounds

Onium compounds are cations derived by the protonation of mononuclear parent hydrides of elements of the nitrogen group , chalcogens , or halogens , and similar cations derived by the substitution of hydrogen atoms in the former by other groups, such as organic radicals, or halogens, for example...

 (ion) containing a halogen

Halogen

The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...

 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 formed from fluorine

Fluorine

Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

, chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

, bromine

Bromine

Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...

, and iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 are called fluoronium

Fluoronium

The fluoronium cation, H2F+, is a polyatomic ion formed by protonation or self-ionic dissociation of hydrogen fluoride:orUnlike for halonium ions based on chlorine, bromine or iodine , hydrocarbyl derivatives of fluoronium, i.e. R2F+, remain unknown....

, chloronium, bromonium, and iodonium, respectively. The simplest halonium ions are of the structure H-X⁺-H (X = F, Cl, Br, I). Halonium ions often have a three-atom cyclic structure resulting from the formal addition of a halogenium ion X⁺ to a C=C double bond

Double bond

A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...

.

History

Halonium ions were first postulated in 1937 by Roberts and Kimball to account for observed diastereoselectivity in halogen addition reaction

Halogen addition reaction

A halogen addition reaction is a simple organic reaction where a halogen molecule is added to the carbon–carbon double bond of an alkene functional group.The general chemical formula of the halogen addition reaction is:...

s to alkene

Alkene

In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...

s. 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 potential

Ionization potential

The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...

s.

In 1970 George A. Olah succeeded in preparing and isolating halonium salt

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

s by adding a methyl halide such as methyl bromide or methyl chloride in sulfur dioxide

Sulfur dioxide

Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 at −78°C to a complex of antimony pentafluoride

Antimony pentafluoride

Antimony pentafluoride is the inorganic compound with the formula SbF5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, the strongest known acid...

 and tetrafluoromethane

Tetrafluoromethane

Tetrafluoromethane, also known as carbon tetrafluoride, is the simplest fluorocarbon . It has a very high bond strength due to the nature of the carbon–fluorine bond. It can also be classified as a haloalkane or halomethane...

 in sulfur dioxide. After evaporation of sulfur dioxide this procedure left crystals of CH₃–X⁺–CH₃SbF, stable at room temperature

Room temperature

-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...

 but not to moisture.

Cyclic and acyclic chloronium, bromonium and iodonium ions have been structurally characterised by X-ray crystallography

X-ray crystallography

X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

, such as the adamantylideneadamantanebromonium cation, also known as dispiro[adamantane-2,3'-[1λ³]bromirane-3',2-adamantan]-1'-ylium, shown below.

skeletal formula Skeletal formula The skeletal formula of an organic compound is a shorthand representation of its molecular structure, developed by the organic chemist, Friedrich August Kekulé von Stradonitz. Skeletal formulae are ubiquitous in organic chemistry, because they are relatively quick and simple to draw. Carbon and...	ball-and-stick model Ball-and-stick model In chemistry, the ball-and-stick model is a molecular model of a chemical substance which is to display both the three-dimensional position of the atoms and the bonds between them...

Compounds containing trivalent or tetravalent halonium ions do not exist but for some hypothetical compounds stability has been computationally tested