Fluorene , or 9''H''-fluorene is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the formula (C₆H₄)₂CH₂. It forms white crystals that exhibit a characteristic, aromatic odor similar to that of

naphthalene Naphthalene is an organic compound with formula . It is the simplest polycyclic aromatic hydrocarbon, and is a white Crystal, crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 Parts-per notation ...

. Despite its name, it does not contain the element

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

, but rather it comes from the violet

fluorescence Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...

it exhibits. For commercial purposes it is obtained from

coal tar Coal tar is a thick dark liquid which is a by-product of the production of coke and coal gas from coal. It is a type of creosote. It has both medical and industrial uses. Medicinally it is a topical medication applied to skin to treat psoria ...

, where it was discovered and named by

Marcellin Berthelot Pierre Eugène Marcellin Berthelot (; 25 October 1827 – 18 March 1907) was a French chemist and Republican politician noted for the ThomsenBerthelot principle of thermochemistry. He synthesized many organic compounds from inorganic substance ...

in 1867. It is insoluble in water and soluble in many organic solvents. Although sometimes classified as a

polycyclic aromatic hydrocarbon A Polycyclic aromatic hydrocarbon (PAH) is any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter— by engine exhaust fumes, tobacco, incine ...

, the five-membered ring has no aromatic properties. Fluorene is mildly acidic.

Synthesis, structure, and reactivity

Although fluorene is obtained from coal tar, it can also be prepared by dehydrogenation of

diphenylmethane Diphenylmethane is an organic compound with the formula (C6H5)2CH2 (often abbreviated ). The compound consists of methane wherein two hydrogen atoms are replaced by two phenyl groups. It is a white solid. Diphenylmethane is a common skeleton in or ...

. Alternatively, it can be prepared by the reduction of fluorenone with

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

hypophosphorous acid Hypophosphorous acid (HPA), or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula H3PO2. It is a colorless low-melting compound, which is soluble in water, dioxane and alcohols. The formula for th ...

–

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

. The fluorene molecule is nearly planar,D. M. Burns, John Iball (1954), ''Molecular Structure of Fluorene'' Nature volume 173, p. 635. although each of the two benzene rings is coplanar with the central carbon 9. Fluorene can be found after the

incomplete combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

plastics Plastics are a wide range of synthetic or semisynthetic materials composed primarily of polymers. Their defining characteristic, plasticity, allows them to be molded, extruded, or pressed into a diverse range of solid forms. This adaptab ...

such as PS, PE and

PVC Polyvinyl chloride (alternatively: poly(vinyl chloride), colloquial: vinyl or polyvinyl; abbreviated: PVC) is the world's third-most widely produced synthetic polymer of plastic (after polyethylene and polypropylene). About 40 million tons o ...

Acidity

The C9-H sites of the fluorene ring are weakly acidic ( pK_a = 22.6 in

DMSO Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula . This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is ...

.) Deprotonation gives the stable fluorenyl anion, nominally C₁₃H₉⁻, which is

aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

and has an intense orange colour. The anion is a

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

Electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively Electric charge, charged, have an ...

s 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 Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

metal in boiling

dioxane Dioxane may refer to the following chemical compounds: * 1,2-dioxane * 1,3-dioxane * 1,4-dioxane {{Authority control ...

Ligand properties

Fluorene and its derivatives can be deprotonated to give

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s akin to

cyclopentadienide Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pin ...

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 Fluorenylmethyloxycarbonyl chloride (Fmoc-Cl) is a chloroformate ester. It is used to introduce the fluorenylmethyloxycarbonyl protecting group as the Fmoc carbamate. Preparation This compound may be prepared by reacting 9-fluorenylmethanol with ...

(Fmoc chloride) is used to introduce the 9-fluorenylmethyl carbamate (Fmoc)

protecting group A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis. In man ...

on amines in

peptide synthesis In organic chemistry, peptide synthesis is the production of peptides, compounds where multiple amino acids are linked via amide bonds, also known as peptide bonds. Peptides are chemically synthesized by the condensation reaction of the carboxyl ...

. Polyfluorene polymers (where carbon 7 of one unit is linked to carbon 2 of the next one, displacing two hydrogens) are

electrically conductive Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...

and

electroluminescent Electroluminescence (EL) is an optical and electrical phenomenon, in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black body light emission resulting from ...

, and have been much investigated as a

luminophore In chemistry, a luminophore (sometimes shortened to lumophore) is an atom or functional group in a chemical compound that is responsible for its luminescent properties. Luminophores can be either organic or inorganic. Luminophores can be further ...

organic light-emitting diode An organic light-emitting diode (OLED), also known as organic electroluminescent (organic EL) diode, is a type of light-emitting diode (LED) in which the emissive electroluminescent layer is an organic compound film that emits light in respon ...

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.

References

External links

Fluorene
in the National Institute of Standards and Technology database. {{PAHs Molecular electronics Polycyclic aromatic hydrocarbons