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Indene

Bicyclic hydrocarbon compound with formula C9H8

Summary

Bicyclic hydrocarbon compound with formula C9H8

Indonaphthene Bicyclo[4.3.0]nona-1,3,5,7-tetraene Bordwell pKa Table in DMSO

Indene is an aromatic, polycyclic hydrocarbon with chemical formula . 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.

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.

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

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.

References

References

  1. [[International Union of Pure and Applied Chemistry]]. (2014). "Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013". [[Royal Society of Chemistry.
  2. {{PGCH. 0340
  3. (2014). "Generation of Indene Derivatives by Tandem Reactions". Synlett.
  4. (2000). "Resins, Synthetic".
  5. (1987). "Ring-Slippage Chemistry of Transition Metal Cyclopentadienyl and Indenyl Complexes". Chemical Reviews.
  6. (2025). "Superhydrogenation of indene at low temperatures". Astronomy & Astrophysics.
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