Dioxin

Not to be confused with dioxane or digoxin.

Dioxin is a heterocyclic, organic, antiaromatic compound with the chemical formula C4H4O2. There are two isomers, 1,2-dioxin (or o-dioxin) and 1,4-dioxin (or p-dioxin). Their chemical structures are shown at right. The ortho isomer 1,2-dioxin is very unstable due to its peroxide-like characteristics. The known properties of 1,4-dioxin are listed in the infobox to the right.

1,4-dioxin can be prepared by cycloaddition, namely by the Diels-Alder reaction.

Dioxin is used as a blanket term for a family of chemical compounds that are formed through combustion, chlorine bleaching and manufacturing processes. The combination of heat and chlorine creates dioxin. Since chlorine is often a part of the earth's environment, natural ecological activity such as volcanic activity and forest fires can lead to the formation of dioxin.Nevertheless, dioxin, a highly carcinogenic and toxic compound, is mostly created by human activity.

Other meanings

Skeletal formulas and substituent numbering schemes of Dioxin isomers
The skeletal formula and substituent numbering scheme of dibenzo-1,4-dioxin, the parent compound of PCDDs

The word dioxin can also in a general way refer to compounds whose molecules have a dioxin core skeletal structure with substituent molecular groups attached to it. For example, dibenzo-1,4-dioxin is a compound whose structure consists of two benzo- groups fused onto a 1,4-dioxin ring as shown below (see also Dibenzodioxin).

Because of their extreme importance as environmental pollutants, current scientific literature uses the name dioxins commonly for simplification to denote the chlorinated derivatives of dibenzo-1,4-dioxin, more precisely the polychlorinated dibenzodioxins (PCDDs), among which TCDD, a tetrachlorinated derivative, is the best known. The polychlorinated dibenzodioxins, which can also be classified in the family of halogenated organic compounds, have been shown to bioaccumulate in humans and wildlife due to their lipophilic properties, and are known teratogens, mutagens, and carcinogens. References under the main article on polychlorinated dibenzodioxins.

Additionally, sometimes with dioxins a similar, but unrelated compound type the polychlorinated dibenzofurans of like importance are also implied.

Toxicity

Some dioxin derivatives are carcinogenic, and directly correllated with an increase in the likelihood of developing cancer (see above). Scientists are working to establish their exact toxicity. The job is made difficult because dioxins are not a single compound, but a mixture. Toxicity depends on the particular molecular arrangement of the compound. The compound of highest known toxicity is 2,3,7,8-tetrachlorodibenzo-1,4-dioxin (pictured here as well).

Polychlorinated dibenzodioxins

The word dioxins sometimes refers to polychlorinated dibenzodioxins (PCDDs), a class of chlorinated dioxin derivatives. PCDD exposures are suspected in famous cases including Vietnam War veteran illnesses, the Seveso disaster, and the poisoning of Viktor Yushchenko.

Body response

Small amounts of dioxin at the scale of picograms, particularly dibenzo-dioxins, induces CYP 1A1 strongly. Dioxin does so by binding to cytosolic Aryl Hydrocarbon Receptor (AhR). The AhR is then translocated to the nucleus where it binds to the Xenobiotic Response Element (XRE) and promotes the transcription of the CYP 1A1 gene.