Indene is a flammable polycyclic hydrocarbon with chemical formula C9H8. It is composed of a benzene ring fused with a cyclopentene ring and occurs as a colorless to pale yellow liquid. A principal industrial use of indene is in the production of indene/cumarone thermoplastic resins.
It is also known as benzocyclopentadiene, and occurs naturally in coal-tar fractions boiling around 175-185 °C. It can be obtained from this fraction by heating with sodium (indene is acidic), separating the solid sodio-indene, and recovering by steam distilling it.
It condenses with ethyl oxalate in the presence of sodium ethoxide to form indene-oxalic ester, and with aldehydes or ketones in the presence of alkali to form benzofulvenes. The latter are highly coloured.