Although fluorene is obtained from coal tar, it can also be prepared by dehydrogenation of
diphenylmethane. Alternatively, it can be prepared by the reduction of
fluorenone with
zinc or
hypophosphorous acid–
iodine. The fluorene molecule is nearly planar, although each of the two benzene rings is coplanar with the central carbon 9. Fluorene can be found after the
incomplete combustion of
plastics such as
PS,
PE and
PVC.
Acidity The C9-H sites of the fluorene ring are weakly acidic (
pKa = 22.6 in
DMSO.) Deprotonation gives the stable
fluorenyl anion, nominally C13H9−, which is
aromatic and has an intense orange colour. The anion is a
nucleophile.
Electrophiles react with it by adding to the 9-position. The purification of fluorene exploits its acidity and the low solubility of its sodium derivative in hydrocarbon solvents. Both protons can be removed from C9. For example, 9,9-fluorenyldipotassium can be obtained by treating fluorene with
potassium metal in boiling
dioxane.
Ligand properties Fluorene and its derivatives can be deprotonated to give
ligands akin to
cyclopentadienide. for producing
syndiotactic polypropylene. ==Uses==