Isomeric with anthracene. Occurs in coal tar, q.v., and in products of incomplete combustion. Isoln: Ostermayer, Fittig, Ber. 5, 933 (1872); Glaser, ibid. 982. Purification (from contaminating carbazole and anthracene): Clar, Ber. 65, 852 (1932). Formation from toluene, bibenzil, 9-methylfluorene or stilbene by passage through red-hot tube: Graebe, Ber. 7, 48 (1874); Ann. 167, 161 (1879); Ber. 37, 4145 (1904). Also from coumarone and benzene: Kraemer, Spilker, Ber. 23, 85 (1890). Pschorr synthesis from o-nitrobenzaldehyde and phenylacetic acid: Ber. 29, 500 (1896). From diphenylethylene: Cook, Hewett, J. Chem. Soc. 1933, 1098. Diene synthesis from 1-vinylnaphthalene and maleic anhydride: Cohen, Warren, ibid. 1937, 1315. From o-phenylbenzoic acid: Sch"onberg, Warren, Chem. Ind. (London) 58, 199 (1939). By irradiation of stilbene: Mallory et al., J. Am. Chem. Soc. 84, 4361 (1962). Synthesis by double succinoylation of benzene: Rahman et al., J. Org. Chem. 28, 3571 (1963). Structure: Trotter, Acta Crystallogr. 16, 605 (1963). Review of carcinogenic risk: IARC Monographs 32, 419-430 (1983); of toxicology and human exposure: Toxicological Profile for Polycyclic Aromatic Hydrocarbons (PB95-264370, 1995) 487 pp.