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Unlike its lighter congeners, the
halogen The halogens () are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would ...
iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...
forms a number of stable
organic compounds Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
, in which iodine exhibits higher
formal Formal, formality, informal or informality imply the complying with, or not complying with, some set of requirements ( forms, in Ancient Greek). They may refer to: Dress code and events * Formal wear, attire for formal events * Semi-formal atti ...
oxidation states In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to other atoms are fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. Concep ...
than −1 or
coordination number In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion ...
exceeding 1. These are the hypervalent organoiodines, often called iodanes after the
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
rule used to name them. These iodine compounds are hypervalent because the iodine atom formally contains in its
valence shell In chemistry and physics, valence electrons are electrons in the outermost shell of an atom, and that can participate in the formation of a chemical bond if the outermost shell is not closed. In a single covalent bond, a shared pair forms with b ...
more than the 8 electrons required for the
octet rule The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The ru ...
. Hypervalent iodine oxyanions are known for oxidation states +1, +3, +5, and +7; organic analogues of these moieties are known for each oxidation state except +7. In terms of chemical behavior, λ3 and λ5iodanes are generally oxidizing and/or electrophilic species. They have been widely applied towards those ends in
organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
.


Nomenclature

Several different naming conventions are in use for the hypervalent organoiodines. All begin with nonstandard formal charge assignments. In iodane chemistry, carbon is considered ''more'' electronegative than iodine, despite the Pauling electronegativities of those respective atoms. Thus iodobenzene () is an iodine(I) compound, (dichloroiodo)benzene () and iodosobenzene () iodine(III) compounds, and iodoxybenzene () an iodine(V) compound. With that convention in place, IUPAC names assume complete electron transfer. Thus when iodine is ligated to an organic residue and two
Lewis acids A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
, it is in the +3
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
and the corresponding compound is a λ3iodane. A compound with iodine(V) would be a λ5iodane, and a hypothetical iodine(VII)containing compound would be a λ7iodane. Organyl-iodine ethers, a kind of λ3iodane, are sometimes called organic hypoiodites. Alternatively, the hypervalent iodines can be classified using neutral electron counting. Iodine itself contains 7 valence electrons, and, in a monovalent iodane such as iodobenzene (), the
phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ...
ligand donates one additional electron to give a completed octet. In a λ3iodane, each X-type ligand donates an additional electron, for 10 in total; the result is a decet structure. Similarly, many λ5iodanes are dodecet molecules, and hypothetical λ7iodanes are tetradecet molecules. As with other hypervalent compounds, NXL notation can be used to describe the formal electron count of iodanes, in which N stands for the number of electrons around the central atom X (in this case iodine), and L is the total number of ligand bonds with X. Thus, λ3iodanes can be described as 10I3 compounds, λ5iodanes as 12I5 compounds, and hypothetical λ7iodanes as 14I7 compounds.


Electron structure

As with other hypervalent compounds, iodanes bonding was formerly described using ''d''-orbital participation. 3-center-4-electron bonding is now believed to be the primary bonding mode. This paradigm was developed by J.J. Musher in 1969. One such bond exists in iodine(III) compounds, two such bonds reside in iodine(V) compounds and three such bonds would reside in the hypothetical iodine(VII) compounds.


Examples

Hypervalent organoiodine compounds are prepared by the oxidation of an organyl iodide. In 1886, German chemist
Conrad Willgerodt Conrad Heinrich Christoph Willgerodt (2 November 1841 – 19 December 1930) was a German chemist who first described the Willgerodt rearrangement, Willgerodt reaction. Alongside the Willgerodt reaction, he had also discovered Iodosobenzene and chlo ...
prepared the first hypervalent iodine compound, iodobenzene dichloride (), by passing
chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
gas through iodobenzene in a cooled solution of
chloroform Chloroform, or trichloromethane (often abbreviated as TCM), is an organochloride with the formula and a common solvent. It is a volatile, colorless, sweet-smelling, dense liquid produced on a large scale as a precursor to refrigerants and po ...
:
This preparation can be varied to produce iodobenzene pseudohalides. Cleaner preparations begin with
solutions Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solutio ...
of
peracetic acid Peracetic acid (also known as peroxyacetic acid, or Percidine) is an organic compound with the formula CH3CO3H. This peroxy acid is a colorless liquid with a characteristic acrid odor reminiscent of acetic acid. It can be highly corrosive. Perac ...
in glacial acetic acid, also due to Willgerodt:
The iodobenzene diacetate product hydrolyzes to the polymeric iodosobenzene (PhIO), which is stable in cool alkaline solution. In hot water (or, in Willgerodt's original preparation, steam distillation), iodosobenzene instead disproportionates to iodoxybenzene and iodobenzene: : 2-Iodobenzoic acid reacts with
oxone Potassium peroxymonosulfate is widely used as an oxidizing agent, for example, in pools and spas (usually referred to as monopersulfate or "MPS"). It is the potassium salt of peroxymonosulfuric acid. Potassium peroxymonosulfate per se is rarely e ...
or a combination of potassium bromate and
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
to produce the insoluble λ5iodane 2-iodoxybenzoic (IBX) acid. IBX acid is unstable and explosive, but
acetylation : In chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply ''acetates''. Deacetylation is the opposite react ...
tempers it to the stabler Dess-Martin periodinane.
Aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated (in which all ...
hypoiodites can be synthesized through a variant on the Williamson ether synthesis: an
alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organyl substituent. Alkoxides are strong bases and, whe ...
reacts with
iodine monochloride Iodine monochloride is an interhalogen compound with the formula . It is a red-brown chemical compound that melts near room temperature. Because of the difference in the electronegativity of iodine and chlorine, this molecule is highly polar ...
, releasing the alkyl hypoiodite and
chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
. Alternatively, the Meyer-Hartmann reaction applies: a silver alkoxide reacts with elemental iodine to give the hypoiodite and
silver iodide Silver iodide is an inorganic compound with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a grey colouration. The silver contamination arises because some samp ...
. They are unstable to visible light, cleaving into alkoxyl and
iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...
radicals. The synthesis of organyl periodyl derivatives (λ7-iodanes) has been attempted since the early 20th century. Efforts so far have met with failure, although aryl λ7chloranes are known. Organic diesters of iodine(VII) are presumed intermediates in the periodate cleavage of diols ( Malaprade reaction), although no carbon-iodine(VII) bond is present in this process.


Diaryliodonium salts

Diaryliodonium salts are compounds of the type . They are formally composed of a diaryliodonium cation paired with a counteranion, but crystal structures show a long, weak, partially-
covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
bond between the iodine and the counterion. Some authors have described this interaction as an example of halogen bonding, but the interaction exists even with traditionally noncoordinating ions, such as
perchlorate A perchlorate is a chemical compound containing the perchlorate ion, , the conjugate base of perchloric acid (ionic perchlorate). As counterions, there can be metal cations, quaternary ammonium cations or other ions, for example, nitronium cat ...
,
triflate In organic chemistry, triflate (Preferred IUPAC name, systematic name: trifluoromethanesulfonate), is a functional group with the Chemical formula, formula and Chemical structure, structure . The triflate group is often represented by , as opp ...
, or tetrafluoroborate. As a result, other authors regard the diaryliodonia as λ3-iodanes. The salts are generally T-shaped, with the counteranion occupying an apical position. The overall geometry at the iodine atom is pseudotrigonal bipyramidal. The placement of ligands exhibits apicophilicity: the
phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ...
group and chlorine group attain apical positions, while the other phenyl group and a
lone pair In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC ''Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. Lone ...
of electrons hold equatorial ones. Salts with a halide counterion are poorly soluble in many organic solvents, possibly because the halides bridge dimers. Solubility improves with
triflate In organic chemistry, triflate (Preferred IUPAC name, systematic name: trifluoromethanesulfonate), is a functional group with the Chemical formula, formula and Chemical structure, structure . The triflate group is often represented by , as opp ...
and tetrafluoroborate counterions. In general, the salts can be prepared from preformed hypervalent iodines such as iodic acid, iodosyl sulfate or iodosyl triflate. The first such compound was synthesised in 1894, via the silver hydroxide-catalyzed coupling of two aryl iodides (the Meyer–Hartmann reaction): Alternatively, the iodane may be formed ''in situ'': an aryl iodide is oxidized to an aryliodine(III) compound (such as ArIO), followed by a ligand exchange. The latter can occur with organometallized arenes such as an arylstannane or -silane (a nucleophilic aromatic substitution reaction) or unfunctionalized arenes in the presence of a Brønsted or Lewis acid (an
electrophilic aromatic substitution Electrophilic aromatic substitution (SEAr) is an organic reaction in which an atom that is attached to an aromatic ring, aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitut ...
reaction). Diaryliodonium salts react with
nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
s at iodine, replacing one ligand to form the substituted arene ArNu and iodobenzene ArI. Diaryliodonium salts also react with metals M through ArMX intermediates in
cross-coupling reaction In organic chemistry, a cross-coupling reaction is a reaction where two different fragments are joined. Cross-couplings are a subset of the more general coupling reactions. Often cross-coupling reactions require metal catalysts. One important re ...
s.


Uses

Hypervalent iodine compounds are predominantly used as oxidizing reagents, although they are specialized and expensive. In some cases they replace more toxic oxidants. Iodobenzene diacetate (PhIAc2) and iodobenzene di(trifluoroacetate) are both strong oxidizing agents used in organic oxidations, as well as precursors for further organoiodine compounds. A hypervalent iodine (III) reagent was used as oxidant, together with ammonium acetate as nitrogen source, to provide 2-Furonitrile, a pharmaceutical intermediate and potential artificial sweetener. Current research focuses on the use of iodanes in carbon-carbon and carbon-heteroatom bond-forming reactions. In one study, an intramolecular C-N coupling of an alkoxyhydroxylamine to its
anisole Anisole, or methoxybenzene, is an organic compound with the formula . It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances. The compound is mainly ...
group is accomplished with a catalytic amount of aryliodide in trifluoroethanol.


See also

* Carbonyl oxidation with hypervalent iodine reagents * Phenol oxidation with hypervalent iodine reagents


References

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External links


Hypervalent Iodine Chemistry
Oxidizing agents