Indene

Indene is an aromatic, polycyclic hydrocarbon with chemical formula C

8. 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.[1]

## Contents

* 1 Isolation
* 2 Reactivity
* 3 Applications
* 4 See also
* 5 References
* 6 External links

## 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.[2]

## 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:

8 \+ RLi → LiC

7 \+ RH

Indenyl is a ligand in organometallic chemistry, giving rise to many transition metal indenyl complexes.[3]

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.[4]

## Applications

* Indene can also be used directly to synthesize 2-Aminoindan & NM-2-AI.[5]

2-Indanone [615-13-4]

* Indene is used as a starting material to make 2-indanone [615-13-4].[6] 2-indanone in-turn is used to synthesize the following drugs: Aprindine, Delapril, Indanorex, Indantadol, Pip-AI [23928-93-0], Pyr-AI, S-15535 & BChE-IN-7 [2416910-85-3], 2-aminoindane analog of fentanyl: O=C(CC)N(C1=CC=CC=C1)C2CCN(C3CC(C=CC=C4)=C4C3)CC2.

## See also

* Indane
* Indole
* Isoindene
* Indenyl effect
* Indenone

## References

1. ↑ Wu, Jie; Qiu, Guanyinsheng (2014). "Generation of Indene Derivatives by Tandem Reactions". Synlett 25 (19): 2703–2713. doi:10.1055/s-0034-1379318.
2. ↑ Collin, Gerd; Mildenberg, Rolf; Zander, Mechthild; Höke, Hartmut; McKillip, William; Freitag, Werner; Imöhl, Wolfgang. "Ullmann's Encyclopedia of Industrial Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH.
3. ↑ 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.
4. ↑ Haid, S.; Gugeler, K.; Kästner, J.; Campisi, D. (2025). "Superhydrogenation of indene at low temperatures". Astronomy & Astrophysics 701: A34. doi:10.1051/0004-6361/202451572.
5. ↑ "Redox-Photosensitized Aminations of 1,2-Benzo-1,3-cycloalkadienes, Arylcyclopropanes, and Quadricyclane with Ammonia". The Journal of Organic Chemistry 68 (20): 7618–7624. 1 October 2003. doi:10.1021/jo030053+. https://pubs.acs.org/doi/10.1021/jo030053+.
6. ↑ "2-INDANONE". Organic Syntheses 41: 53. 1961. doi:10.15227/orgsyn.041.0053. http://orgsyn.org/demo.aspx?prep=CV5P0647.

## 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. https://zenodo.org/record/1425600.
* W. v. Miller, Rohde (1890). "Zur Synthese von Indenderivaten". Berichte der Deutschen Chemischen Gesellschaft 23 (2): 1887–1902. doi:10.1002/cber.18900230228. https://zenodo.org/record/1425559.
* Finar, I. L. (1985). Organic Chemistry. Longman Scientific & Technical. ISBN 0-582-44257-5.

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