Dioxin Production in the Preparation of 245T
Traditionally, the industrial synthesis of the herbicide called 2,4,5-T (discussed in Chapter 10) started with 2,4,5-trichlorophenol, which itself was produced by reacting NaOH with the appropriate tetrachlorobenzem. The OH group replaces one chlorine atom in the process. Unfortunately, during this synthesis there occurs an additional reaction that converts a very small portion of the trichlorophenol product into "dioxin." In this side reaction, two trichbrophenoxy anions react with each other, resulting in the elimination of two chloride ions:
In this process a new six-membered ring is formed which links the two chlorinated benzene rings. This central ring has two oxygen atoms located para (i.e., opposite) to each other, as is found in the simple molecule 1,4-dioxin or para-dioxin (p-dioxin).
Although the molecule labeled "dioxin" above is correctly known as a tetrachlorodibenzo-p-dioxin, it has become popularly known simply as "dioxin," with the understanding that it is the most toxic of a class of related compounds.
The side reaction that produces dioxin as a by-product is kinetically second-order in chlorophenoxide. In other words, the rate of the reaction depends on the square (or second power) of the ion's concentration. Consequently, the rate of dioxin production increases dramatically as the initial chlorophenoxide ion concentration increases. Also, the rate of this side reaction increases more rapidly with increasing reaction temperature than does the main reaction. Therefore, the extent to which the trichlorophenol and consequently the commercial herbicide become contaminated with the dioxin by-product can be minimized by controlling concentration and temperature in the preparation of the original trichlorophenol. Today, the contamination of commercial 2,4,5-T by this dioxin can be kept to less than 0.1 ppm by keeping both the phenoxide concentration and the temperature low. Nevertheless, its manufacture and use in North America were phased out in the mid-1980s because of concerns about its dioxin content, however small.
A 1:1 mixture of the herbicides 2,4-D and 2,4,5-T called Agent Orange was used extensively as a defoliant during the Vietnam War. Since the mixture contained dioxin levels of about 10 ppm, it is clear that the reaction used to produce the trichlorophenol used for 2,4,5-T preparation was not carefully controlled so as to minimize contamination. As a result, the soil in southern Vietnam is contaminated by dioxins. The consequences of this contamination for the residents, and for the American troops who were exposed while spraying was underway, are still controversial. There is some evidence that the rate of melanoma has increased in Air Force personnel who were involved in the spraying. It was realized recently that the defoliation potential of Agent Orange was originally tested in the 1960s by the U.S. armed forces near Gagetown, New Brunswick, Canada, and that the soil in the area is polluted by the substance.
Environmental contamination by dioxin also occurred as the result of an explosion in a chemical factory in Seveso, Italy, in 1976. The factory produced 2,4,5-trichlorophenol from tetrachlorobenzene, as described above. On one occasion the reaction was not brought to a complete halt before the workers left for the weekend. The reaction continued unmonitored, and the heat subsequently released by the reaction eventually resulted in an explosion. Since the trichlorophenol had been heated to a high temperature, a considerable amount of dioxin—probably several kilograms—was produced. The explosion distributed the toxin into the environment, and many wildlife deaths resulted from the contamination. Although a large number of humans, both adults and children, were also exposed to the chemical as a result of this explosion, no serious health effects to humans were found for many years. Recent studies, however, have established that the rates of several types of cancers are elevated in people who lived in the zones most exposed to dioxin from the explosion. Specifically, the risk of contracting breast cancer increased in proportion to the dioxin exposure, as measured by the level of the substance in women's blood samples taken soon after the explosion.
Continue reading here: Dioxin Numbering System
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