2.4.5- Trichlorophenoxyacetic acid Agent Orange

Another herbicide, 2,4,5-trichlorophenoxyacetic acid, is synthesized by starting with the chlorination of benzene to give 1,2,4,5-tetrachloroben-zene, which reacts with caustic to give 2,4,5-trichlorophenol. Conversion to the sodium salt followed by reaction with sodium chloroacetate and acidification gives 2,4,5-trichlorophenoxyacetic acid. Agent Orange is a 1-to-l mixture of the butyl esters of 2,4,5-trichlorophenoxyacetic acid and 2,4,-dichlorophenoxyacetic acid. 

Finally, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T agent orange) and pentachlorophenol (PCP)—two acidic polychlorinated pesticides—were also investigated. In these cases, the presence of A336 contributed to the solubility of... 

Dioxins are sometimes—but rarely—produced in nature, most commonly during volcanic eruptions and forest fires. Their most common source in the environment are industrial reactions in which they occur as by-products of other chemical changes or during the incineration of certain synthetic organic compounds. For example, trace amounts of 2,3,7,8-TCDD occur as an impurity in the herbicide Agent Orange (a mixture of 2,4,5-trichlorophenoxyacetic acid [2,4,5-T] and 2,4-dichlorophenoxyacetic acid [2,4-D]), which was... 

Chlorophenoxy compounds have been widely used as herbicides. Agent Orange was a mixture of the chlorophenoxy herbicides 2,4-D (dichlorophenoxyacetic acid) and 2,4,5-T (trichlorophenoxyacetic acid) that also contained small amounts of the highly toxic contaminant TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin see p 184) derived from the process of manufacturing 2,4,5-T. Manufacture ot 2,4-D by chlorination of phenol does not produce TCDD. Populations involved in manufacture or handling of 2,4,5-T may show elevated levels of TCDD on serum testing and overall increased rates of cancer compared to the general population. 

The most toxic member of the dioxin family is 2,3,7,8-TCDD. Agent Orange is an approximately 1 1 mixture of two herbicides—2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid. Dioxin TCDD was present as a contaminant in Agent Orange used as a defoliant in the Vietnam War. 

The chlorophenoxy herbicides, including 2,4-dichlorophenoxyacetic acid (2,4-D) shown in Figure 4.11 and the related compounds 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and mecoprop, were manufactured on a large scale for weed and brush control and as military defoliants. At one time, 2,4,5-T (now banned), known as the infamous Agent Orange, was used to defoliate vegetation in the Vietnam War and was of particular concern because of contaminant TCDD (see below) present as a manufacturing by-product. 

The reaction sequence shown below illustrates the synthesis of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), a powerful herbicide. A 1 1 mixture of the butyl esters of 2,4,5-T and its dichlorinated analog 2,4-D was used between 1965 and 1970 as a defoliant during the Vietnam War under the code name Agent Orange. Propose mechanisms for the reactions in the synthesis of this substance, whose effects on the health of those exposed to it remain topics of considerable controversy. 

Herbicides may be selective, as for broad-leaved weeds in cereal crops, or unselective, essentially for land clearance. 2,4-Dichlorophenoxyacetic acid (2,4-D), 24.6, was commercialized in the 1940s it is inexpensive, is easy to make, and kills broad-leaved weeds in cereal crops. Worldwide, it is the most widely used herbicide. It is a synthetic auxin, or plant hormone, acting only on dicots and not monocots. There is a whole family of related compounds, prepared from chloroacetic acid (or other halo acids) and various chlorinated phenols. Agent Orange, used as a defoliant in Vietnam in the 1970s, was a 1 1 mixture of 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The main concerns about its use relate to the possibility that dioxins are formed as contaminants in its manufacture. A few countries ban its use for control of weeds in domestic lawns. 

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