organoiodine chemistry
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Organoiodine compounds are
organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
that contain one or more
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
iodine
bond Bond or bonds may refer to: Common meanings * Bond (finance), a type of debt security * Bail bond, a commercial third-party guarantor of surety bonds in the United States * Chemical bond, the attraction of atoms, ions or molecules to form chemica ...
s. They occur widely in organic chemistry, but are relatively rare in nature. The
thyroxine File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 25 ...
hormones are organoiodine compounds that are required for health and the reason for government-mandated iodization of salt.


Structure, bonding, general properties

Almost all organoiodine compounds feature iodide connected to one carbon center. These are usually classified as derivatives of I. Some organoiodine compounds feature iodine in higher oxidation states. The C–I bond is the weakest of the carbon– halogen bonds. These bond strengths correlate with the
electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...
of the halogen, decreasing in the order F > Cl > Br > I. This periodic order also follows the
atomic radius The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there ...
of halogens and the length of the carbon-halogen bond. For example, in the molecules represented by CH3X, where X is a halide, the carbon-X bonds have strengths, or bond dissociation energies, of 115, 83.7, 72.1, and 57.6 kcal/mol for X = fluoride, chloride, bromide, and iodide, respectively. Of the halides, iodide usually is the best
leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited ...
. Because of the weakness of the C–I bond, samples of organoiodine compounds are often yellow due to an impurity of I2. A noteworthy aspect of organoiodine compounds is their high density, which arises from the high atomic weight of iodine. For example, one millilitre of methylene iodide weighs 3.325 g.


Industrial applications

Few organoiodine compounds are important industrially, at least in terms of large scale production. Iodide-containing intermediates are common in organic synthesis, because of the easy formation and cleavage of the C–I bond. Industrially significant organoiodine compounds, often used as disinfectants or pesticides, are
iodoform Iodoform (also known as triiodomethane and, inaccurately, as carbon triiodide) is the organoiodine compound with the chemical formula C H I3. A pale yellow, crystalline, volatile substance, it has a penetrating and distinctive odor (in older ch ...
(CHI3), methylene iodide (CH2I2), and methyl iodide (CH3I). Although methyl iodide is not an industrially important product, it is an important intermediate, being a transiently generated intermediate in the industrial production of acetic acid and acetic anhydride. The potential for methyl iodide to replace the ubiquitous dependence on methyl bromide as a soil fumigant has been considered, however limited information is available on environmental behavior of the former. Ioxynil (3,5-diiodo-4-hydroxybenzonitrile), which inhibits photosynthesis at photosystem II, is among the very few organoiodine herbicides. A member of the hydroxybenzonitrile herbicide class, ioxynil is an iodinated analog of the brominated herbicide, bromoxynil (3,5-dibromo-4-hydroxybenzonitrile). Iodinated and brominated organic compounds are of concern as environmental contaminants owing to very limited information available on environment fate behavior. However, recent reports have shown promise in biological detoxification of these classes of contaminants. For example, Iodotyrosine deiodinase is a mammalian enzyme with the unusual function of aerobic reductive dehalogenation of iodine- or bromine-substituted organic substrates. Bromoxynil and ioxynil herbicides have been shown to undergo a variety of environmental transformations, including reductive dehalogenation by
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
bacteria. Polyiodoorganic compounds are sometimes employed as X-ray
contrast agent A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...
s, in
fluoroscopy Fluoroscopy () is an imaging technique that uses X-rays to obtain real-time moving images of the interior of an object. In its primary application of medical imaging, a fluoroscope () allows a physician to see the internal structure and function ...
, a type of medical imaging. This application exploits the X-ray absorbing ability of the heavy iodine nucleus. A variety of agents are available commercially, many are derivatives of 1,3,5-triiodo
benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
and contain about 50% by weight iodine. For most applications, the agent must be highly soluble in water and, of course, non-toxic and readily excreted. A representative reagent is Ioversol (Figure to right), which has water-solubilizing diol substituents. Typical applications include urography and
angiography Angiography or arteriography is a medical imaging technique used to visualize the inside, or lumen, of blood vessels and organs of the body, with particular interest in the arteries, veins, and the heart chambers. Modern angiography is perfor ...
. Organoiodine lubricants can be used with
titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...
, stainless steels, and other metals which tend to seize up with conventional lubricants: organoiodine lubricants can be used in
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating ...
s and
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
, and as a cutting oil in machining. File:Methyl iodide.svg, Methyl iodide, precursor to synthetic acetic acid. File:Erythrosine.svg,
Erythrosine Erythrosine, also known as Red No. 3, is an organoiodine compound, specifically a derivative of fluorone. It is a pink dye which is primarily used for food coloring. It is the disodium salt of 2,4,5,7-tetraiodofluorescein. Its maximum absorba ...
, a common food dye. File:Iodoversol.png, Ioversol, an organoiodine compound used as an X-ray
contrast agent A contrast agent (or contrast medium) is a substance used to increase the contrast of structures or fluids within the body in medical imaging. Contrast agents absorb or alter external electromagnetism or ultrasound, which is different from radiop ...
. File:Thyroxine-2D-skeletal.png,
Thyroxine File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 25 ...
(T4), a
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are require ...
. File:Triiodothyronine.svg,
Triiodothyronine Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. Production of T3 and its prohormone thyroxine ...
(T3), another thyroid hormone.


Biological role

In terms of human health, the most important organoiodine compounds are the two thyroid hormones
thyroxine File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4 rect 376 268 820 433 Thyroid-stimulating hormone rect 411 200 849 266 Thyrotropin-releasing hormone rect 297 168 502 200 Hypothalamus rect 66 216 386 25 ...
("T4") and
triiodothyronine Triiodothyronine, also known as T3, is a thyroid hormone. It affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. Production of T3 and its prohormone thyroxine ...
("T3"). Marine natural products are rich sources of organoiodine compounds, like the recently discovered plakohypaphorines from the sponge '' Plakortis simplex''. The sum of
iodomethane Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one ...
produced by the marine environment, microbial activity in
rice paddies A paddy field is a flooded field of arable land used for growing semiaquatic crops, most notably rice and taro. It originates from the Neolithic rice-farming cultures of the Yangtze River basin in southern China, associated with pre-Au ...
, and the burning of biological material is estimated to be 214
kilotonne TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. The is a unit of energy defined by that convention to be , which is the approximate energy released in the detonation of a m ...
s per year. The volatile iodomethane is broken up by oxidation reactions in the atmosphere and a global iodine cycle is established. More than 3000 organoiodine compounds have been identified.


Methods for preparation of the C–I bond


From I2

Organoiodine compounds are prepared by numerous routes, depending on the degree and regiochemistry of iodination sought as well as the nature of the precursors. The direct iodination with is employed with unsaturated substrates: : This reaction is used to determine the
iodine number Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vio ...
, an indicator of the unsaturation of fats and related samples.


From I sources

The iodide anion is a good nucleophile and will displace chloride, tosylate, bromide and other leaving groups, as in the
Finkelstein reaction The Finkelstein reaction named after the German chemist Hans Finkelstein, is an SN2 reaction (Substitution Nucleophilic Bimolecular reaction) that involves the exchange of one halogen atom for another. It is an equilibrium reaction, but the react ...
. Alcohols can be converted to the corresponding iodides using
phosphorus triiodide Phosphorus triiodide (PI3) is an inorganic compound with the formula PI3. A red solid, it is a common misconception that PI3 is too unstable to be stored; it is, in fact, commercially available. It is widely used in organic chemistry for convertin ...
. Illustrative is the conversion of methanol to
iodomethane Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one ...
: :PI3 + 3 → 3 + "" For bulky alcohol substrates, the methiodide of triphenylphosphite has been used. : H3(C6H5O)3Psup>+I + ROH → RI + CH3(C6H5O)2PO + C6H5OH Aromatic iodides may be prepared via a
diazonium salt Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. General properti ...
by treatment with potassium iodide: :C6H5N2+ + KI -> C6H5I + K+ + N2


From I+ sources

Benzene can be iodinated with a combination of iodide and
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
.
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 an ...
is a reagent that is sometimes used to deliver the equivalent of "".


See also

* Organofluorine compounds * Organochlorine compounds *
Organobromine compound Organobromine compounds, also called organobromides, are organic compounds that contain carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons avai ...
s *
Iodane Iodane generally refers to any organic derivative of iodine. Without modifier, ''iodane'' is the systematic name for the parent hydride of iodine, HI. Thus, any organoiodine compound with general formula RI (e.g., iodomethane , or iodobenzene ) ...
s * Periodinanes


References

{{ChemicalBondsToCarbon