Dioxin Numbering System
The nomenclature and numbering system used for ring systems like the dioxins is a little unusual. Since the central dioxin ring is connected on either side to benzene rings, the three-ring unit is properly known as dibenzo-f)-dioxin.
The chlorine substitution on the outer rings also should be recognized, so the dioxin shown below is a tetrachlorodibenzo-p-dioxin, or TCDDt
The numbering scheme for the ring carbons in dioxins takes into account the fact that the carbons shared between two rings carry no hydrogen atoms and so need not be numbered. Thus C-l is the carbon next to one joining the rings, and the numbering follows a direct path from there. By convention, the oxygen atoms are also part of the numbered sequence in this scheme, although their locations are not used in naming any of this family of compounds. The initial (C-l) position for the numbering system is chosen to give the lowest possible value to the first substituent; if there is a choice after this criterion has been applied, then that which gives the lowest number to the second substituent is used, etc. Applying these rules, the dioxin shown above is named 2,3,7,8-TCDD, or to give it its full title 2,3,7,8-tetrachlorodibenro-f>-dioxin. No wonder it is simply called "dioxin" in the press!
There are actually 75 different chlorinated dibenzo-p-dioxin compounds, when one includes all the possibilities having between one and eight chlorines, given that a number of isomers exist for most of these eight types. Different members of a chemical family that differ only in the number and position of the same substituent are called congeners.
All dioxin congeners are planar; All carbon, oxygen, hydrogen, and chlorine atoms lie in the same plane. For convenience, we refer to the carbon atoms closest to the central dioxin ring as alpha carbons, and the outer ones as beta carbons:
The unsubstituted dibenzo-p-dioxin molecule has two types of symmetry that are useful when considering substitution patterns. First there is lateral, or left-right, symmetry: The carbon atom labeled ¡3 at the top of the left-side ring is equivalent to the ¡3 carbon at the top of the ring at the right side, and similarly for the two f3 carbons at the bottom. The dioxin ring also has up-down symmetry: The carbon atom labeled /3 at the top of the left-side ring is equivalent to the /3 carbon at the bottom of that ring, and similarly for the two fi carbons of the right-side ring. Thus all the four /3 carbons are equivalent in the unsubstituted dioxin. Similarly, the four a carbons all denote equivalent positions. Consequently, for example, there are only two unique monochlorodibenzo-p-dioxins; due to the equivalence of the four a positions, those that would otherwise be numbered 4-, 6-, and 9-chlorodibenzo-p-dioxin are all equivalent to the 1- molecule. Similarly, 3-, 7-, and 8-chlorodibenzo-p-dioxins are all equivalent to the 2- molecule due to the equivalence of the ¡3 positions. Some or all of the equivalences can be lost when multiple substitution occurs.
By drawing the structures and comparing them, decide whether 1,3-, 2,4-, 6,8-, and 7,9-dichlorodibenzo-p-dioxins are all unique compounds or whether they are all really the same compound. Are 1,2- and 1,8-dichlorodibenzo-f)-dioxins unique compounds? Using a systematic procedure, deduce the structures of all unique dichlorodibenzo-p-dioxins, keeping in mind that before substitution the two rings are equivalent and that the molecule has up-down symmetry.
Chlorophenols as Pesticides
In addition to their use as starting materials in the production of herbicides, chlorophenols find use as wood preservatives (fungicides) and as slimicides. The most common preservative, in use since 1936, is pentachlorophenol (PCP, though not the "angel dust" compound known by the same initials); all the benzene's six hydrogens have been substituted in this compound:
Commercial PCP is not pure pentachlorophenol but is significantly contaminated with 2,3,4,6'tetrachhrophenol. This mixture has many pesticidal applications: It is used as a herbicide (e.g., as a preharvest defoliant), an insecticide (termite control), a fungicide (wood preservation and seed treatment), and a molluscicide (snail control). Some trichlorophenol isomers and some tetra-chlorophenol isomers are also sold as wood preservatives.
Unfortunately, if wood treated with such preservatives is eventually burned, a fraction of the chlorophenols can react to eliminate HC1, thereby producing members of the chlorinated dioxin family. Thus octachlorodibenzo-|)-dioxin, OCDD, is produced as an unwanted by-product in the incomplete combustion of pentachlorophenol products:
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