Chemistry:Fluorene

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Fluorene /ˈflʊəriːn/, or 9H-fluorene is an organic compound with the formula (C6H4)2CH2. It forms white crystals that exhibit a characteristic, aromatic odor similar to that of naphthalene. Despite its name, it does not contain the element fluorine, but rather it comes from the violet fluorescence it exhibits. For commercial purposes it is obtained from coal tar,[1] where it was discovered and named by Marcellin Berthelot in 1867.[2]

As a polycyclic aromatic hydrocarbon, it is insoluble in water and soluble in many organic solvents. However, thanks to its central ring, fluorene is relatively acidic – powerful bases may remove one of the protons of the CH2 group.

Synthesis, structure, and reactivity

Although fluorene is obtained from coal tar, it can also be prepared by dehydrogenation of diphenylmethane.[1] Alternatively, it can be prepared by the reduction of fluorenone with zinc[3] or hypophosphorous acid–iodine.[4] The fluorene molecule is nearly planar,[5] although each of the two benzene rings is coplanar with the central carbon 9.[6]

Fluorene can be found after the incomplete combustion of plastics such as polystyrene, polyethylene and polyvinyl chloride.[7]

Acidity

The C9–H sites of the fluorene ring are weakly acidic (pKa = 22.6 in DMSO).[8] Deprotonation gives the stable fluorenyl anion, nominally C
13H−
9, 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.[9]

Ligand properties

Fluorene and its derivatives can be deprotonated to give ligands akin to cyclopentadienide.

A fluorenyl-derived Kaminsky precatalyst for producing syndiotactic polypropylene.[10]

Uses

Fluorene is a precursor to other fluorene compounds; the parent species has few applications. Fluorene-9-carboxylic acid is a precursor to pharmaceuticals. Oxidation of fluorene gives fluorenone, which is nitrated to give commercially useful derivatives. 9-Fluorenylmethyl chloroformate (Fmoc chloride) is used to introduce the 9-fluorenylmethyl carbamate (Fmoc) protecting group on amines in peptide synthesis.[1]

Polyfluorene polymers (where carbon 7 of one unit is linked to carbon 2 of the next one, displacing two hydrogens) are electrically conductive and electroluminescent, and have been much investigated as a luminophore in organic light-emitting diodes.

Fluorene is used to make 9-benzylfluorene [1572-46-9].[11] The importance of this is that it is used in the synthesis of UCL 1608 [371172-30-4], a medicine discovered by C. Robin Ganellin & Co.[12]

Fluorene dyes

Fluorene dyes are well developed. Most are prepared by condensation of the active methylene group with carbonyls. 2-Aminofluorene, 3,6-bis(dimethylamino)fluorene, and 2,7-diiodofluorene are precursors to dyes.[13]

See also

References

  1. ↑ 1.0 1.1 1.2 Griesbaum, Karl; Behr, Arno; Biedenkapp, Dieter; Voges, Heinz-Werner; Garbe, Dorothea; Paetz, Christian; Collin, Gerd; Mayer, Dieter et al. (2000). "Ullmann's Encyclopedia of Industrial Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a13_227. 
  2. ↑ (in fr) Annales de chimie et de physique. Masson. 1867. https://books.google.com/books?id=lLZJAQAAMAAJ&pg=PA222. 
  3. ↑ Fittig, Rud (1873). "Ueber einen neuen Kohlenwasserstoff aus dem Diphenylenketon". Berichte der deutschen chemischen Gesellschaft 6: 187. doi:10.1002/cber.18730060169. 
  4. ↑ Hicks, Latorya D.; Han, Ja Kyung; Fry, Albert J. (2000). "Hypophosphorous acid–iodine: a novel reducing system". Tetrahedron Letters (Elsevier) 41 (41): 7817–7820. doi:10.1016/s0040-4039(00)01359-9. ISSN 0040-4039. 
  5. ↑ Burns, D. M.; Iball, John (1954). "Molecular structure of fluorene". Nature 173: 635. doi:10.1038/173635a0. 
  6. ↑ Gerkin, R. E.; Lundstedt, A. P.; Reppart, W. J. (1984). "Structure of fluorene, C13H10, at 159 K". Acta Crystallographica Section C: Crystal Structure Communications 40 (11): 1892–1894. doi:10.1107/S0108270184009963. Bibcode: 1984AcCrC..40.1892G. 
  7. ↑ Wang, Zhenlei; Richter, Henning; Howard, Jack B.; Jordan, Jude; Carlson, Joel; Levendis, Yiannis A. (2004-06-01). "Laboratory Investigation of the Products of the Incomplete Combustion of Waste Plastics and Techniques for Their Minimization". Industrial & Engineering Chemistry Research 43 (12): 2873–2886. doi:10.1021/ie030477u. ISSN 0888-5885. 
  8. ↑ Bordwell, F. G. (1988). "Equilibrium acidities in dimethyl sulfoxide solution". Accounts of Chemical Research 21 (12): 456–463. doi:10.1021/ar00156a004. 
  9. ↑ Scherf, G. W.; Brown, R. K. (1960). "Potassium derivatives of fluorene as intermediates in the preparation of C9-substituted fluorenes. I. The preparation of 9-fluorenyl potassium and the infrared spectra of fluorene and some C9-substituted fluorenes". Canadian Journal of Chemistry 38: 697. doi:10.1139/v60-100. .
  10. ↑ Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. (1988). "Syndiospecific propylene polymerizations with group IVB metallocenes". Journal of the American Chemical Society 110 (18): 6255–6256. doi:10.1021/ja00226a056. PMID 22148816. Bibcode: 1988JAChS.110.6255E. 
  11. ↑ "Ruthenium Supported Ionic Liquid Based PMO as an Efficient Heterogeneous Catalyst for sp 3 C─H Bond Alkylation of 9H ‐Fluorene With Primary Alcohols". Asian Journal of Organic Chemistry 15 (3). March 2026. doi:10.1002/ajoc.70342. https://aces.onlinelibrary.wiley.com/doi/10.1002/ajoc.70342. Retrieved 3 September 2026. 
  12. ↑ "Synthesis and Structure−Activity Relationships of Cetiedil Analogues as Blockers of the Ca 2+ -Activated K + Permeability of Erythrocytes". Journal of Medicinal Chemistry 44 (20): 3244–3253. 1 September 2001. doi:10.1021/jm001113w. https://pubs.acs.org/doi/10.1021/jm001113w. 
  13. ↑ Kurdyukova, I. V.; Ishchenko, A. A. (2012). "Organic dyes based on fluorene and its derivatives". Russian Chemical Reviews 81 (3): 258–290. doi:10.1070/RC2012v081n03ABEH004211. Bibcode: 2012RuCRv..81..258K. 
  • Fluorene in the National Institute of Standards and Technology database.



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