Indene

Indene

Skeletal formula
Names
Preferred IUPAC name 1H-Indene [1]
Other names Benzocyclopentadiene Indonaphthene Bicyclo[4.3.0]nona-1,3,5,7-tetraene
Identifiers
CAS Number	95-13-6  Y
3D model (JSmol)	Interactive image
Beilstein Reference	635873
ChEBI	CHEBI:41921  Y
ChEMBL	ChEMBL192812  Y
ChemSpider	6949  Y
DrugBank	DB02815  Y
ECHA InfoCard	100.002.176
EC Number	202-393-6
Gmelin Reference	27265
KEGG	C11565  Y
PubChem CID	7219
UNII	67H8Y6LB8A  Y
CompTox Dashboard (EPA)	DTXSID8042052
InChI InChI=1S/C9H8/c1-2-5-9-7-3-6-8(9)4-1/h1-6H,7H2 Y Key: YBYIRNPNPLQARY-UHFFFAOYSA-N Y InChI=1/C9H8/c1-2-5-9-7-3-6-8(9)4-1/h1-6H,7H2 Key: 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 (pKa)	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). Y  verify  (what is  YN ?) Infobox references

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]

Isolation

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]

Reactivity

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]

See also

• Indane
• Indole
• Isoindene
• Indenyl effect
• Indenone

References

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. 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.

External links

• W. v. Miller, Rohde (1890). "Zur Synthese von Indenderivaten". Berichte der Deutschen Chemischen Gesellschaft. 23 (2): 1881–1886. doi:10.1002/cber.18900230227.
• W. v. Miller, Rohde (1890). "Zur Synthese von Indenderivaten". Berichte der Deutschen Chemischen Gesellschaft. 23 (2): 1887–1902. doi:10.1002/cber.18900230228.
• Finar, I. L. (1985). Organic Chemistry. Longman Scientific & Technical. ISBN   0-582-44257-5.