Biphenyl and Terphenyls

Biphenyl (diphenyl, phenylbenzene) and terphenyl are the lowest members of a family of polyphenyls in which benzene rings are attached one to another in a chainlike manner, C6H5(C6H4)mC6H5. Many higher polyphenyls are known, but only biphenyl and the terphenyls are of commercial significance. Pure biphenyl is a white crystalline solid that separates from solvents as plates or monoclinic prismatic crystals. Commercial samples are often slightly yellow or tan in color. Similarly, pure terphenyls are white crystalline solids whereas commercial grades are somewhat yellow or tan. Biphenyl and terphenyls may be regarded as substituted benzenes that undergo acylation, alkylation, halogenation, nitration, sulfonation, and other reactions common to benzene. Terphenyls, like biphenyl, undergo the usual reactions of aromatic hydrocarbons. Biphenyl is recovered from high boiler fractions that accompany the hydrodealkylation of toluene to benzene. High purity biphenyl is produced by Monsanto by direct dehydro-condensation of benzene. Terphenyls are also obtained from the higher boiling polyphenyl by-products that accompany the biphenyl. By-product biphenyl is usually sold as a dye carrier in the molten state in tank truck or tank car lots. Grades of higher purity are also sold in the molten state or as flakes in 22.7-kg bags. Biphenyl is defined as a toxic chemical under, and subject to, reporting requirements of Section 313 of Title III of the Superfund Amendments and Reauthorization Act (SARA) of 1986 and 40 CFR, Part 372, under the name biphenyl. It is identified as a hazardous chemical under criteria of the OSHA Hazard Communication Standard (29 CFR 1910.1200). The small amount of mixed terphenyls that are sold as such are shipped in the form of flaked solids in 22.7-kg multiwall bags. Like biphenyl, mixed terphenyls fall under the hazardous chemical criteria of the OSHA Hazard Communication Standard (29 CFR 1910.1200). About 10% of the biphenyl derived from HDA sources is consumed as 93–95% grade in textile dye carrier applications. The remainder is used for alkylation or upgraded to grades for heat-transfer purposes. Most of the terphenyl produced is converted to a partially hydrogenated form. Although biphenyl and the terphenyls fall under the hazardous chemical criteria of the OSHA Hazard Communications Standard, the products themselves are fairly low in toxicity and do not constitute a serious industrial hazard. Because biphenyl is often transported in the molten state, a moderate fire hazard does exist. The widespread use of biphenyl and methyl-substituted biphenyls as dye carriers in the textile industry has given rise to significant environmental concern because of the amount released to the environment in wastewater effluent. The prospect of their conversion by chlorination to PCBs in the course of wastewater treatment has been a subject of environmental focus. Terphenyls do not constitute an environmental threat. Short-chain alkylated biphenyls are the principal biphenyl derivatives in commercial use. Several functionalized biphenyls either are, or show promise of becoming, commercially significant polymer building blocks. Keywords: Biphenyl; Terphenyl; Isomers; Dye carriers; Eutetic; Polychlorinated biphenyls (PCBs); Polychlorinated terphenyls (PCTs); Textiles