What is Dioxin?

This pictorial shorthand describes the spatial relationship of the atoms that make up the molecule. How does the name relate to the structure? The dibenzo part comes from the two hexagons with inscribed circles. This is shorthand for benzene sub-structures. There is a carbon atom at the vertex of each hexagon.

The two benzene rings are joined by two oxygen atoms (5 and 10). By joining the benzene rings in this way the center of the molecule, containing the oxygens, is also a six-membered ring. A six-membered ring like this with two oxygens in it is called dioxin. When the oxygens are at opposite sides of the ring, as in the figure, they are called para to each other, hence the -p- in the name. Voila! Two benzene rings joined with two oxygens in this fashion is dibenzo-p-dioxin, the so-called "parent" compound or structure or back-bone.

The four chlorine atoms are joined to the carbon atoms labeled 2,3,7, and 8. If the four chlorines were attached to different carbons (1, 2, 3, and 4 for instance), then the substance would be an isomer of 2,3,7,8-tetrachlorodibenzo-p-dioxin. If nothing is indicated as attached to a carbon that has room for something to be attached, it has hydrogen atom(s) attached there. Therefore, there are hydrogens attached to carbons 1, 4, 6, and 9. (I won't go into how to tell if there is room, just trust me.) Counting up atoms leads to the empirical formula: C12H4O2Cl4.

You have just had a short-course in chemical nomenclature. The concept is that a chemical name refers to a specific molecular geometry and chemical formula. The name is a complete description of the molecule, and the rules for naming them are by international convention (IUPAC - International Union of Pure and Applied Chemists). As you can see, however, the names tend to be long and complicated.

Abbreviations are quite common, especially when a substance with a long name impacts the public domain in some fashion. 2,3,7,8-tetrachlorodibenzo-p-dioxin has gone under the pseudonyms dioxin, tetrachloro dioxin, 2,3,7,8-tetrachloro dioxin, 2,3,7,8-TCDD, and TCDD (often with the prefix "deadly"). The term chlorinated dioxins refers generically to chlorinated dibenzo-p-dioxins.

TCDD was originally discovered as a by-product in the manufacture of trichlrophenol, an intermediate chemical in the manufacturing process for some pesticides. It does not occur in a pure form in nature. Public interest was initially aroused when employees were exposed to 2,3,7,8-TCDD as a low concentration (part per million) constituent in industrial chemical exposure incidents involving trichlorophenol. Since TCDD displays high toxicity to some experimental animals, there was concern over the possible health effects on these people.

Toxicology studies of the effects of chlorinated dioxins on test animals indicate that the toxicity is not the same for each possible isomer (placement of chlorine around the molecule). Those isomers which contain chlorines in the 2,3,7, and/or 8 positions are more toxic than when hydrogen is attached to those positions. This led to using the term dioxin to describe all isomers of chlorinated dioxins containing from 1 to 8 (the maximum number) chlorines, and 2,3,7,8-dioxins to refer to the "toxic" isomers.

Pure 2,3,7,8-TCDD was synthesized in 1968. It is a white, micro-crystalline solid (looks like table salt) which is insoluble in water and sparingly soluble in some organic solvents.

In summary, the term dioxin does not have a single, simple definition. It has referred to a group of related substances and to individual members of the group. The arrangement of the atoms in these substances is known, and they all have the same carbon-oxygen basic structure and geometry with chlorine atoms attached to various carbon atoms.

This article is Copyright 1994 by Lewis A. Shadoff. It may be freely redistributed in its entirety provided that this copyright notice is not removed. It may not be sold for profit or incorporated in commercial documents without the written permission of the copyright holder. While all information in this article is believed to be correct at the time of writing, this article is for educational purposes only and does not purport to provide legal advice.