Organoiodine chemistry

Organoiodine compounds are organic compounds that contain one or more carbon–iodine bonds. They occur widely in organic chemistry, but are relatively rare in nature. The thyroxine 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 of the halogen, decreasing in the order F > Cl > Br > I. This periodic order also follows the atomic radius 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. 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 (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.

Polyiodoorganic compounds are sometimes employed as X-ray contrast agents, in fluoroscopy, 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-triiodobenzene 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.

Structure of Ioversol, an organoiodine compound used as an X-ray contrast agent.

Biological role

In terms of human health, the most important organoiodine compounds are the two thyroid hormones thyroxine ("T₄") and triiodothyronine ("T₃"). Marine natural products are rich sources of organoiodine compounds, like the recently discovered plakohypaphorines from the sponge Plakortis simplex.

thyroxine (T₄)

triiodothyronine (T₃)

The sum of iodomethane produced by the marine environment, microbial activitiy in rice paddies, and the burning of biological material is estimated to be 214 kilotonnes per year. The volatile iodomethane is broken up by oxidation reactions in the atmosphere and a global iodine cycle is established.

Moreover, iodine can add to double bonds of docosahexaenoic acid and arachidonic acid of cellular membranes, making iodolipids which seem to have an apoptic action and are less reactive to free oxygen radicals.

Methods for preparation of the C–I bond

Organoiodine compounds are prepared by numerous routes, depending on the degree and regiochemistry of iodination sought and the nature of the precursors. The direct iodination with I₂ is employed with unsaturated substrates:

RHC=CH₂ + I₂ → RHIC-CIH₂

The iodide anion is a good nucleophile and will displace chloride, tosylate, bromide and other leaving groups, as in the Finkelstein reaction. Aromatic iodides may be prepared via a diazonium salt in the Sandmeyer reaction.

Because of its low ionization energy, iodine is readily converted to reagents that deliver the equivalent of "I⁺". A representative electrophilic iodination reagent is iodine monochloride.

See also

• Organofluorine compounds
• Organochlorine compounds
• Organobromine compounds
• Periodinanes