Nucleus-chlorinated aromatic hydrocarbons

Dioxin, PCB, DDT, and the like, i.e., chlorinated aromatic hydrocarbons are believed to bind to a special receptor called AH receptor (AH stands for aryl hydrocarbon). The AH receptor bound with dioxin (or PCB) enters into the nucleus of the cell, and binds to a specific site of a DNA. One of the effects caused by such a binding seems to be an induction of an enzyme dependent on cytochrome P-450. Cytochrome P-450-dependent monooxygenase was talked about in Sect. 6.2.2, and is involved in the metabolism of a wide variety of compounds including steroids and foreign substances (drugs). Probably the P-450 enzymes induced by dioxin, PCB, and others would then metabolize steroid hormones unnecessarily, and thus disrupt the endocrine system. The details are still not very well understood. However, you see that the processes are all chemical reactions at the deepest level. 

If chlorine and bromine are allowed to act upon an aromatic hydrocarbon like toluene, which has a side-chain, substitution may occur in the nucleus or the side-chain, according to the conditions. Generally speaking, in the cold and in presence of a halogen carrier, nuclear substitution occurs, Irut at a high temperatuie the halogen passes into the side-chain (see Piep. 

Polyironphenylsiloxane varnish is used as a catalyst for the chlorination of aromatic hydrocarbons in the nucleus and as a thermostabiliser for antiadhesion silicone coatings. 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

Perchlorination of an alkylaromatic hydrocarbon could a priori be achieved via two routes either exhaustive chlorination of its aromatic nucleus, followed by that of its side-chain, or vice versa, as shown for the synthesis of perchlorotoluene (1). 

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