BIPHENYL AND TERPHENYLS

Biphenyl (diphenyl, phenylben2ene) and terphenyl are the lowest members of a family of polyphenyls in which benzene rings are attached one to another in a chainlike manner, Many higher polyphenyls are known (1), but only biphenyl and the terphenyls are of commercial significance. Some 

Pure biphenyl is a white crystalline soHd that separates from solvents as plates or monoclinic prismatic crystals. Commercial samples are often slightly yellow or tan in color. Similady, pure terphenyls are white crystalline soHds whereas commercial grades are somewhat yellow or tan. Physical and chemical constants for biphenyl and the three isomeric terphenyls are given in Tables 2 and 3, respectively. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Biphenyl has been produced commercially in the United States since 1926, mainly by The Dow Chemical Co., Monsanto Co., and Sun Oil Co. Currently, Dow, Monsanto, and Koch Chemical Co. are the principal biphenyl producers, with lesser amounts coming from Sybron Corp. and Chemol, Inc. With the exception of Monsanto, the above suppHers recover biphenyl from high boiler fractions that accompany the hydrodealkylation of toluene [108-88-3] to benzene (6). Hydrodealkylation of alkylbenzenes, usually toluene, C Hg, is an important source of benzene, C H, in the United States. Numerous hydrodealkylation (HDA) processes have been developed. Most have the common feature that toluene or other alkylbenzene plus hydrogen is passed under pressure through a tubular reactor at high temperature (34). Methane and benzene are the principal products formed. Dealkylation conditions are sufficiently severe to cause some dehydrocondensation of benzene and toluene molecules. 

---

Liquid crystalline compounds are not very dangerous and only basic precautions should be used in handling them. They are not poisonous or carcinogenic, and do not cause problems when in contact with skin (see also Biphenyls and terphenyls Cinnamic acid Stilbene dyes). 

Pyrolysis. Benzene undergoes thermal dehydrocondensation at high temperatures to produce small amounts of biphenyls and terphenyls (see Biphenyl AND terphenyls). Before the 1970s most commercial biphenyl was produced from benzene pyrolysis. In a typical procedure benzene vapors are passed through a reactor, usually at temperatures above 650°C. The decomposition of benzene iato carbon and hydrogen is a competing reaction at temperatures of about 750°C. Biphenyls are also formed when benzene and ethylene are heated to 130—160°C ia the presence of alkaH metals on activated AI2O3 (33). 

Pure (9-terphenyl can be obtained by fractional distillation. To obtain high purity m- or -terphenyl, the appropriate distillation fraction has to be further purified by recrysta11i2ing, 2one refining, or other refining techniques. Currently, litde demand exists for pure isomers, and only a mixture is routinely produced. Small amounts of acetone, ethanol, or methanol are used to promote dehydrocondensation, and as a result, minor amounts of methyl- or methylene-substituted polyphenyls accompany the biphenyl and terphenyls produced. For most purposes, the level of such products (<1%) is so small that their presence can be ignored. For appHcations requiring removal of these alkyl-polyphenyl impurities, an efficient process for their oxidative destmction has been described (38). 

Table 7. Properties of Biphenyl and Terphenyls of Environmental Importance...

Environmental Health Criteria 140 (1993). Polychlorinated biphenyls and terphenyls. Geneva WHO. 

Polychlorinated Biphenyls and Terphenyls. Hygienic Criteria of Environmental State (1980), Geneva, WHO, Vol. 2, 98 pp. 

Surnina, N. N., Tarasov, V. V. (1992). Some aspects of contamination of environmental objects by polychlorinated biphenyls and terphenyls. Journal of Ecological Chemistry, 2, 5-20. 

Polybrominated biphenyls (No. 152,1994) Polybrominated dibenzo-p-dioxins and dibenzofurans (No. 205,1998) Polychlorinated biphenyls and terphenyls (No. 2,1976,1st edition) ... 

---