As with most organic halides, aryl halides most often are synthetic intermediates for the production of other useful substances. For example, chlorobenzene is the starting aryl halide for the synthesis of DDT; it also is a source of benzenol which, in turn, has many uses .
Several aromatic chloro compounds are used extensively as insecticides, herbicides, fungicides, and bactericides. They also have acquired much notoriety because in some instances their indiscriminant usage has led to serious problems. For example, hexachlorophene is an external bactericide that until recently was used in cosmetic preparations such as soaps, deodorants, and so on. Its use has been discontinued because of compelling evidence that it can be absorbed through the skin in amounts that are dangerous, if not lethal, for infants and small children.
Other pesticides, notably DDT and the herbicides 2,4-D and 2,4,5-T have been partially banned for different reasons.
Both 2,4-D and 2,4,5-T are herbicides that have been used for weed control and as defoliating agents in jungle warfare. Apart from the arguments for or against the use of chemicals for such purposes, there have been reports of serious dermatitis among the industrial workers who produce these substances.
The cause finally was traced to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which is produced as an impurity in the manufacture of 2,4,5-T.
This substance (TCDD) is very toxic. In addition to the dermatitis it produces, it is a potent teratogen (induces birth abnormalities). The lethal dose is less than 10“6 g for guinea pigs. Its presence in 2,4,5-T can be eliminated, but the conditions by which it is formed are pertinent to our present discussion.
The production of 2,4,5-T involves the substitution of one chlorine of 1,2,4,5-tetrachlorobenzene with hydroxide ion to give 12. This is followed by a second displacement reaction, this time on chloroethanoate by the sodium salt of 12:
If the temperature of the first step exceeds 160°, then two molecules of 12 react in a double nucleophilic displacement to give TCDD.
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