Indene (original) (raw)

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Bicyclic hydrocarbon compound with formula C9H8

Indene

Skeletal formulaSkeletal formula
Ball-and-stick model of the indene molecule
Names
Preferred IUPAC name 1_H_-Indene[1]
Other namesBenzocyclopentadieneIndonaphtheneBicyclo[4.3.0]nona-1,3,5,7-tetraene
Identifiers
CAS Number 95-13-6 checkY
3D model (JSmol) Interactive image
Beilstein Reference 635873
ChEBI CHEBI:41921 checkY
ChEMBL ChEMBL192812 checkY
ChemSpider 6949 checkY
DrugBank DB02815 checkY
ECHA InfoCard 100.002.176 Edit this at Wikidata
EC Number 202-393-6
Gmelin Reference 27265
KEGG C11565 checkY
PubChem CID 7219
UNII 67H8Y6LB8A checkY
CompTox Dashboard (EPA) DTXSID8042052 Edit this at Wikidata
InChI InChI=1S/C9H8/c1-2-5-9-7-3-6-8(9)4-1/h1-6H,7H2 checkYKey: YBYIRNPNPLQARY-UHFFFAOYSA-N checkYInChI=1/C9H8/c1-2-5-9-7-3-6-8(9)4-1/h1-6H,7H2Key: YBYIRNPNPLQARY-UHFFFAOYAJ
SMILES c1ccc2c(c1)CC=C2
Properties
Chemical formula C9H8
Molar mass 116.16
Appearance Colorless liquid[2]
Density 0.997 g/mL
Melting point −1.8 °C (28.8 °F; 271.3 K)
Boiling point 181.6 °C (358.9 °F; 454.8 K)
Solubility in water Insoluble
Acidity (p_K_a) 20.1 (in DMSO)[3]
Magnetic susceptibility (χ) −80.89×10−6 cm3/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards Flammable
Flash point 78.3 °C (172.9 °F; 351.4 K)
NIOSH (US health exposure limits):
PEL (Permissible) none[2]
REL (Recommended) TWA 10 ppm (45 mg/m3)[2]
IDLH (Immediate danger) N.D.[2]
Related compounds
Related compounds Benzofuran, Benzothiophene, Indole
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). checkY verify (what is checkY☒N ?) Infobox references

Chemical compound

Indene is an aromatic, polycyclic hydrocarbon with chemical formula C9H8. It is composed of a benzene ring fused with a cyclopentene ring. This flammable liquid is colorless although samples often are pale yellow. The principal industrial use of indene is in the production of indene/coumarone thermoplastic resins. Substituted indenes and their closely related indane derivatives are important structural motifs found in many natural products and biologically active molecules, such as sulindac.[4]

Indene occurs naturally in coal-tar fractions boiling around 175–185 °C. It can be obtained by heating this fraction with sodium to precipitate solid "sodio-indene". This step exploits indene's weak acidity evidenced by its deprotonation by sodium to give the indenyl derivative. The sodio-indene is converted back to indene by steam distillation.[5]

Indene readily polymerises. Oxidation of indene with acid dichromate yields homophthalic acid (_o_-carboxylphenylacetic acid). It condenses with diethyl oxalate in the presence of sodium ethoxide to form indene–oxalic ester, and with aldehydes or ketones in the presence of alkali to form benzofulvenes, which are highly coloured. Treatment of indene with organolithium reagents gives lithium indenyl compounds:

C9H8 + RLi → LiC9H7 + RH

Indenyl is a ligand in organometallic chemistry, giving rise to many transition metal indenyl complexes.[6]

At temperatures ranging from 30-75 Kelvin (the temperature range of most dark molecular clouds and photodissociation regions), Indene undergoes a process called superhydrogenation. It begins with saturation of carbon atoms in the pentagonal ring which is then followed by the hydrogenation of the benzene unit. Quantum tunneling also plays a role is this.[7]

  1. ^ 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.
  2. ^ a b c d NIOSH Pocket Guide to Chemical Hazards. "#0340". National Institute for Occupational Safety and Health (NIOSH).
  3. ^ Bordwell FG (1988). "Equilibrium acidities in dimethyl sulfoxide solution". Accounts of Chemical Research. 21 (12): 456–463. doi:10.1021/ar00156a004. Bordwell pKa Table in DMSO Archived 2008-10-09 at the Wayback Machine
  4. ^ Wu, Jie; Qiu, Guanyinsheng (2014). "Generation of Indene Derivatives by Tandem Reactions". Synlett. 25 (19): 2703–2713. doi:10.1055/s-0034-1379318.
  5. ^ Collin, Gerd; Mildenberg, Rolf; Zander, Mechthild; Höke, Hartmut; McKillip, William; Freitag, Werner; Imöhl, Wolfgang (2000). "Resins, Synthetic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. ISBN 978-3-527-30673-2.
  6. ^ O'Connor, Joseph M.; Casey, Charles P. (1987). "Ring-Slippage Chemistry of Transition Metal Cyclopentadienyl and Indenyl Complexes". Chemical Reviews. 87 (2): 307–318. doi:10.1021/cr00078a002.
  7. ^ Haid, S.; Gugeler, K.; Kästner, J.; Campisi, D. (2025). "Superhydrogenation of indene at low temperatures". Astronomy & Astrophysics. 701: A34. arXiv:2507.22036. doi:10.1051/0004-6361/202451572.