Fluorene

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.[2] 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. Fluorene is mildly acidic.

Not to be confused with Fluorane, Fluorine, Fluorone, or Fluorenone.
Fluorene[1]
Names
Preferred IUPAC name
9H-Fluorene
Systematic IUPAC name
Tricyclo[7.4.0.02,7]trideca-2,4,6,9,11,13-hexaene
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.001.541
EC Number
  • 201-695-5
KEGG
RTECS number
  • LL5670000
UNII
Properties
C13H10
Molar mass 166.223 g·mol−1
Density 1.202 g/mL
Melting point 116 to 117 °C (241 to 243 °F; 389 to 390 K)
Boiling point 295 °C (563 °F; 568 K)
1.992 mg/L
Solubility organic solvents
log P 4.18
Acidity (pKa) 22.6
-110.5·10−6 cm3/mol
Hazards
Safety data sheet Sigma-Aldrich
NFPA 704 (fire diamond)
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilHealth code 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
1
0
Flash point 152 °C (306 °F; 425 K)
Lethal dose or concentration (LD, LC):
16000 mg/kg (oral, rat)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Synthesis, structure, and reactivity

Although fluorene is obtained from coal tar, it can also be prepared by dehydrogenation of diphenylmethane.[2] Alternatively, it can be prepared by the reduction of fluorenone with zinc.[3] The fluorene molecule is nearly planar,[4] although each of the two benzene rings is coplanar with the central carbon 9.[5]

Acidity

The C9-H sites of the fluorene ring are weakly acidic (pKa = 22.6 in DMSO.[6]) 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.[7]

Ligand properties

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

A fluorenyl-derived Kaminsky precatalyst for producing syndiotactic polypropylene.[8]

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.[2]

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.[9]

See also

References
  1. Merck Index, 11th Edition, 4081
  2. 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.
  3. Fittig, Rud. (1873), "Ueber einen neuen Kohlenwasserstoff aus dem Diphenylenketon" Ber. Dtsch. Chem. Ges. volume 6, p. 187.doi:10.1002/cber.18730060169
  4. D. M. Burns, John Iball (1954), Molecular Structure of Fluorene Nature volume 173, p. 635. doi:10.1038/173635a0
  5. 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.
  6. F. G. Bordwell (1988). "Equilibrium acidities in dimethyl sulfoxide solution". Acc. Chem. Res. 21 (12): 456–463. doi:10.1021/ar00156a004.
  7. 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..
  8. 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.
  9. 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.CS1 maint: uses authors parameter (link)
  • Fluorene in the National Institute of Standards and Technology database.
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