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PCP PCP OCDD

OCDD is the most prevalent dioxin congener found in human fat and in many environmental samples.

Indeed, pentachlorophenols are one of die largest chemical sources of dioxins to the environment; however, the main dioxin they contain, OCDD, is not particularly toxic, as discussed in a later section. Commercial supplies of chlorinated phenols themselves are contaminated with various dioxins.

PROBLEM 11-2

In naming OCDD and pentachlorophenol, no numbers are used to specify the positions of the chlorine substituents. Why is that not necessary here, whereas it is required in, e.g., 2,3,7,8-TCDD?

In general, any two phenol molecules that each have a chlorine on a carbon atom that is next to that having an OH group can combine to produce a dibenzo-p-dioxin molecule. The two phenols that combine need not be identical but simply need to make contact when they have been heated sufficiently to facilitate HC1 elimination and dioxin formation. Similarly, coupling of phenoxide anions can occur with CP elimination, as discussed previously in the case of 2,4,5-T synthesis. The methodology of problem solving that can be used to deduce the chlorophenolic origin of environmental dioxins is discussed in Box 11-1.

PROBLEM 11-3

(a) Deduce the structures and the correct numbering for the two retra-chlorophenol isomers that exist in addition to the 2,3,4,6 isomer mentioned in the text, (b) For each of these two isomers, deduce the structure and names of the dioxin(s) that would result if two molecules of that isomer were to react together.

Deducing the Probable Chlorophenoiic Origins of a Dioxin

The chlorophenoiic source of dioxins found in environmental samples can he deduced by reversing the logic used in the text to deduce which dioxin would be produced by the coupling of two specific chlorophenols.

Consider the congener 1,2,7,8-tetra-chlorodibenzo-p-dioxin; it could have been formed by elimination of two HC1 molecules from two chlorophenol molecules in the following two ways (here T stands for trichlorophenol).

dioxin structure came from the chlorophenol on the right side of the molecule and the bottom oxygen from the chlorophenol on the left, leads to the possibility that the trichlorophenol molecules that combined were the 2,4,5 and the 2,3,6 congeners. Thus a 1,2,7,8-tetrachlorodibenzo-jb-dioxin molecule in the environment could have arisen by combination of a 2,4,5-trichlorophenol molecule with either a 2,3,4' or a 2,3,6-substituted congener.

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