Fluorene

Fluorene

Fluorene

Chemical compound

Not to be confused with Fluorane, Fluorine, Fluorone, or Fluorenone.

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. It has a violet fluorescence, hence its name. For commercial purposes it is obtained from coal tar.[3] It is insoluble in water and soluble in many organic solvents. Although sometimes classified as a polycyclic aromatic hydrocarbon, the five-membered ring has no aromatic properties.[citation needed] Fluorene is mildly acidic.

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Synthesis, structure, and reactivity

Although fluorene is obtained from coal tar, it can also be prepared by dehydrogenation of diphenylmethane.[3] Alternatively, it can be prepared by the reduction of fluorenone with zinc[4] or hypophosphorous acid–iodine.[5] The fluorene molecule is nearly planar,[6] although each of the two benzene rings is coplanar with the central carbon 9.[7]

Fluorene can be found after the incomplete combustion of plastics such as PS, PE and PVC.[8]

Acidity

The C9-H sites of the fluorene ring are weakly acidic (pKa = 22.6 in DMSO.[9]) 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.[10]

Ligand properties

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

A fluorenyl-derived Kaminsky precatalyst for producing syndiotactic polypropylene.[11]

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.[3]

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 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.[12]

See also

References

  1. [1]
    Merck Index, 11th Edition, 4081
  2. [2]
    International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. p. 207. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
  3. [3]
    Griesbaum, Karl; Behr, Arno; Biedenkapp, Dieter; Voges, Heinz-Werner; Garbe, Dorothea; Paetz, Christian; Collin, Gerd; Mayer, Dieter; Höke (2000). "Hydrocarbons". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a13_227. ISBN 978-3527306732.
  4. [4]
    Fittig, Rud. (1873), "Ueber einen neuen Kohlenwasserstoff aus dem Diphenylenketon" Ber. Dtsch. Chem. Ges. volume 6, p. 187.doi:10.1002/cber.18730060169
  5. [5]
    Hicks, Latorya D.; Han, Ja Kyung; Fry, Albert J. (2000). "Hypophosphorous acid–iodine: a novel reducing system". Tetrahedron Letters. 41 (41). Elsevier BV: 7817–7820. doi:10.1016/s0040-4039(00)01359-9. ISSN 0040-4039.
  6. [6]
    D. M. Burns, John Iball (1954), Molecular Structure of Fluorene Nature volume 173, p. 635. doi:10.1038/173635a0
  7. [7]
    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.
  8. [8]
    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.
  9. [9]
    F. G. Bordwell (1988). "Equilibrium acidities in dimethyl sulfoxide solution". Acc. Chem. Res. 21 (12): 456–463. doi:10.1021/ar00156a004.
  10. [10]
    G. W. Scherf; R. K. Brown (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..
  11. [11]
    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.
  12. [12]
    Kurdyukova, I. V.; Ishchenko, A. A. (2012). "Organic dyes based on fluorene and its derivatives". Russian Chemical Reviews. 81 (3): 258–290. Bibcode:2012RuCRv..81..258K. doi:10.1070/RC2012v081n03ABEH004211. S2CID 95312830.
  • Fluorene in the National Institute of Standards and Technology database.
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