O-Chem in Real Life: Chlorinated Aromatics
Polychlorinated aromatic hydrocarbons are non-natural organic chemicals that have been synthesized industrially for about 80 years or so. They tended to have many useful material properties, for example, inflammability, high electrical resistivity and high stability to heat etc. They quickly found many uses in electrical applications, as flame retardants etc, at one point being required by fire codes for certain applications.
Their most useful property, however, turns out to be related to their biggest problem. They are very unreactive, which means that they are difficult to get rid of! Being non-natural, they are not easily biodegradable. Consequently they remain in the environment, become transferred into the food chain, and accumulate in higher order organisms towards the top of the food chain. Because some polychlorinated aromatics are also toxic, the consequences are obvious.
Polychlorinated biphenyls (PCB's) are among the best known toxic examples. A large number of isomeric compounds were manufactured (usually as a mixture) by chlorination of biphenyl. The more toxic examples have chlorines in the para- and meta-positions. The more chlorines the more toxic. Toxicity is also associated with flatness (perhaps due to intercalation with DNA). Ortho-chlorinated biphenyls would be less flat due to steric hinderance, which may account for their lower toxicity.
The use of polychlorinated biphenyl has been essentially banned since 1978, nevertheless, they are still present in the environment to this day.
Dioxin, or TCDD, is the term used to describe a series of isomeric polychlorinated aromatics of the general structure shown above. Unlike PCB's, dioxins were never deliberately manufactured. However, they form as by-products in other reactions. One of the more infamous examples is 2,3,7,8-tetrachlorodibenzo-p-dioxin (or 2,3,7,8-TCDD), below. It is formed in small quantities in the synthesis of 2,4,5-trichlorophenoxyacetic acid from 2,4,5-trichlorophenol. 2,4,5-Trichlorophenoxyacetic acid is better known as Agent Orange, the defoliant that was widely used in the Vietnam war.
Dioxins tend to be extremely toxic, and it is thought that health problems related to the use of Agent Orange are related to the presence of dioxin as an impurity.