polybrominated biphenyl

polybrominated biphenyl

[pol-ee-broh-muh-ney-tid]
polybrominated biphenyl or PBB, any of a group of organic compounds used as a fire retardant. In 1973 several thousand pounds of PBB were accidentally mixed with livestock feed that was later distributed to farms in W central Michigan. Some 1.5 million chickens, 30,000 cattle, 5,900 swine, and 1,470 sheep that became contaminated with PBB before the mistake was discovered had to be destroyed. Later studies indicated that PBB had spread through the food chain; in one test of a sample of Michigan's residents, 97% of those tested had traces of PBB in fat tissue. Affected cattle suffered loss of appetite and weight loss (often leading to death), decreased milk production, and increased miscarriages. Laboratory studies have linked PBB with liver cancer in rats and with low birth weight, liver damage, and weakened resistance to disease in human beings.
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Biphenyl (or diphenyl or phenyl benzene or 1,1'-biphenyl or lemonene) is a solid organic compound that forms colorless to yellowish crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula C12H10. Biphenyl is notable as a starting material for the production of polychlorinated biphenyls (PCBs), which were once widely used as dielectric fluids and heat transfer agents. Biphenyl is also an intermediate for the production of a host of other organic compounds such as emulsifiers, optical brighteners, crop protection products, and plastics.

Properties

Biphenyl occurs naturally in coal tar, crude oil, and natural gas and can be produced from these sources by distillation. Biphenyl is insoluble in water, but soluble in typical organic solvents. The biphenyl molecule consists of two connected benzene rings, without any additional functionalization, and is therefore not very reactive. The flashpoint is 113 °C and the autoignition temperature is 540 °C.

Stereochemistry

Rotation about the single bond in biphenyl, and especially its ortho-substituted derivatives, is sterically hindered. For this reason, some substituted biphenyls show atropisomerism; that is, the individual C2-symmetric-isomers are optically stable. Some derivatives, as well as related molecules such as BINAP, find application as ligands in asymmetric synthesis. In the case of unsubstituted biphenyl, the equilibrium torsional angle is 44.4° and the torsional barriers are 6.0 kJ/mol at 0° and 6.5 kJ/mol at 90°.

Biological aspects

Biphenyl prevents the growth of molds and fungus, and is therefore used as a preservative (E230, in combination with E231, E232 and E233), particularly in the preservation of citrus fruits during transportation.

It is mildly toxic, but can be degraded biologically by conversion into nontoxic compounds. The ability to do so by adding oxygen molecules to the aromatic rings of biphenyl is found in some bacteria. It has been demonstrated that some bacteria that have degraded biphenyl can then also degrade polychlorinated biphenyls (PCBs).

Biphenyl compounds

Substituted biphenyls can be prepared synthetically by various coupling reactions including the Suzuki reaction and the Ullmann reaction and have many uses. polychlorinated biphenyls were once used as cooling and insulating fluids and polybrominated biphenyls are flame retardants. The biphenyl motif also appears in drugs such as Valsartan and Telmisartan. The abbreviation E7 stands for a liquid crystal mixture consisting of several cyanobiphenyls with long aliphatic tails used commercially in liquid crystal displays.

References

  • Biphenyl (1,1- Biphenyl). Wiley/VCH, Weinh. (1991), ISBN 3-527-28277-7

See also

External links

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