Fluorene
Fluorene /ˈflʊəriːn/, or 9H-fluorene, is a polycyclic aromatic hydrocarbon. It forms white crystals that exhibit a characteristic, aromatic odor similar to that of naphthalene. It is combustible. It has a violet fluorescence, hence its name. For commercial purposes it is obtained from coal tar. It is insoluble in water and soluble in many organic solvents.
Synthesis, structure, and reactivity
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. The fluorene molecule is nearly planar, although each of the two benzene rings is coplanar with the central carbon 9.
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, and most 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.
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