The properties of the halogenated alkanes are similar to their nonhalogenated counterparts. CNS depressant action and liver and the potential for kidney damage both increase with degree of chlorination. Unsaturated compounds are more toxic than saturated. Replacing chlorine with bromine increases toxicity, whereas replacing it with fluorine decreases it. Many halogenated compounds induce liver cancer in laboratory tests with rodents.
Methylene chloride may be the least toxic of the chlorinated methanes. It has the lowest irritancy and potential for CNS depression. It is commonly used as a laboratory solvent. Chloroform and carbon tetrachloride are potent CNS depressants and liver and kidney toxins. A summary of the inhalation effects of chloroform in humans is:
200-300 ppmv 1000 ppmv 1500 ppmv 4000 ppmv 14,000-16,000 ppmv
Fatigue and headache
Dizziness and salivation within minutes
Fainting and nausea
Vinyl chloride, a class A carcinogen, is one of the most dangerous compounds in this group. CNS depression and death have been reported upon acute exposure. Chronic exposure results in arthroosteolysis (destruction of bone at the joints), Raynaud's phenomenon (a vascular disease that can lead to gangrene), and sclerodermatous skin changes (hardening and thickening of the skin). It is mutagenic and is a well-established cause of liver cancer at levels formerly thought to be acceptable for occupational exposure.
Polychlorinated biphenyls (PCBs) have the general structure shown below, with chlorines substituted at various positions, forming a great variety of possible congeners. Their empirical formula is C12H12_„Cl„, where n is the number of chlorine substitutions. Thus, a congener with five chlorines will have a molar mass of 328.5 and consists of 54% chlorine by weight. PCBs were produced as mixtures of various congeners plus impurities and sold by the Monsanto Company under the trade name Aroclor 12xx, where xx indicates the average percent chlorine. Aroclor i254, one of the common mixtures sold, thus has an average of five chlorines per molecule. Their vapor pressure and aqueous solubility is very low, and they adsorb strongly to soil and sediment. The solubility of Aroclor i248 ranges from 0.034 to 0.175 mg/L; that of Aroclor 1268 is less than 0.007 mg/L. They both bioconcentrate and biomagnify in aquatic systems. They have the chemical property of being highly stable, even at high temperatures, thus are very persistent in the environment. They are resistant to biochemical degradation. However, some breakdown has been observed with PCBs having fewer than four chlorine atoms.
Cl biphenyl 2,2',3,4'- tetrachlorobiphenyl, a polychlorinated biphenyl (PCB)
Their use has been discontinued because of the environmental problems they cause. However, because of their persistance and of previous releases, they remain a problem in a number of aquatic and marine environments. For example, an advisory has been issued recommending that consumption of striped bass taken from the Hudson River estuary be limited to not more than one per month because of PCB contamination of that river.
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