Retene, methyl isopropyl phenanthrene or 1-methyl-7-isopropyl phenanthrene, C18H18, is a polycyclic aromatic hydrocarbon present in the coal tar fraction, boiling above 360 °C. It occurs naturally in the tars obtained by the distillation of resinous woods. It crystallizes in large plates, which melt at 98.5 °C and boil at 390 °C. It is readily soluble in warm ether and in hot glacial acetic acid. Sodium and boiling amyl alcohol reduce it to a tetrahydroretene, whilst if it be heated with phosphorus and hydriodic acid to 260 °C, a dodecahydride is formed. Chromic acid oxidizes it to retene quinone, phthalic acid and acetic acid. It forms a picrate which melts at 123-124 °C.
The presence of traces of retene in the air is an indicator of forest fires; it is a major product of pyrolysis of conifer trees. It is also present in effluents from wood pulp and paper mills.
Retene, together with cadalene, simonellite and ip-iHMN, is a biomarker of higher plants, which makes it useful for paleobotanic analysis of rock sediments. Ratio of retene/cadalene in sediments can reveal the ratio of the genus Pinaceae in the biosphere.