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Indene
Indene
Indene
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Indene
Skeletal formula
Skeletal formula
Ball-and-stick model of the indene molecule
Names
Preferred IUPAC name
1H-Indene[1]
Other names
Benzocyclopentadiene
Indonaphthene
Bicyclo[4.3.0]nona-1,3,5,7-tetraene
Identifiers
3D model (JSmol)
635873
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.002.176 Edit this at Wikidata
EC Number
  • 202-393-6
27265
KEGG
UNII
  • InChI=1S/C9H8/c1-2-5-9-7-3-6-8(9)4-1/h1-6H,7H2 checkY
    Key: YBYIRNPNPLQARY-UHFFFAOYSA-N checkY
  • InChI=1/C9H8/c1-2-5-9-7-3-6-8(9)4-1/h1-6H,7H2
    Key: YBYIRNPNPLQARY-UHFFFAOYAJ
  • c1ccc2c(c1)CC=C2
Properties
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)
Insoluble
Acidity (pKa) 20.1 (in DMSO)[3]
−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 ?)

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

[edit]

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

[edit]

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]

Applications

[edit]
2-Indanone [615-13-4]

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Indene

View on Grokipedia
from Grokipedia
Indene is a bicyclic aromatic with the molecular formula C₉H₈ and a of 116.16 g/mol, consisting of a ring fused to a ring, rendering it a colorless to pale yellow liquid with an aromatic odor. It has a of 182 °C, a of -1.5 °C, and a of 0.997 g/cm³ at 20 °C, and is insoluble in but highly soluble in organic solvents such as and . Indene is primarily obtained as a fraction from distillation, where it constitutes about 1-2% of the crude material, and can also be synthesized through methods like the cyclization of diaryl-1,3-dienes under or via palladium-catalyzed reactions. Its principal industrial application involves the production of indene-coumarone thermoplastic resins through with coumarone (), which yield corrosion-resistant materials used in paints, varnishes, adhesives, and water-resistant coatings for products and . Beyond resins, indene serves as a versatile building block in , including the preparation of substituted indenes for pharmaceutical derivatives, such as indene compounds evaluated for bioactivity against enzymes like , and in the development of adducts like indene-C₆₀ for photovoltaic and electronic applications. Its reactivity, stemming from the strained five-membered ring and , facilitates Diels-Alder reactions and metal complex formations, contributing to its role in and research.

Structure and properties

Molecular structure

Indene possesses the molecular formula C₉H₈ and a molar mass of 116.16 g/mol. This bicyclic compound features a benzene ring fused to a five-membered cyclopentene ring, with the rings sharing two adjacent carbon atoms in an ortho-fused arrangement. In the standard IUPAC numbering, the cyclopentene ring encompasses positions 1 through 3 and 3a to 7a, while the benzene ring occupies positions 4 through 7 and 7a; the five-membered ring exhibits bond alternation, including a localized double bond between carbons 1 and 2, and a methylene group (-CH₂-) at position 3. The fused system supports delocalization of 10 π electrons across the structure, contributing to its aromatic character primarily through the moiety, while classifying indene as a non-alternant due to the uneven distribution of π centers. Indene exists predominantly as the stable 1H-indene , whereas the isoindene , featuring a rearranged position, is significantly less stable. The representation typically depicts the ring as a with alternating s fused to a five-membered ring containing one and a CH₂ group, emphasizing the conjugated π system.

Physical and chemical properties

Indene is a colorless to pale with an aromatic . It has a of 0.997 g/mL at 20°C. The is -1.8°C, and the is 181.6°C at 760 mmHg. Indene has a of 78.3°C and is classified as a Class IB flammable liquid. Indene is insoluble in water (<0.1 g/100 mL) but miscible with organic solvents such as ethanol, ether, and benzene. Regarding acid-base properties, indene exhibits a pKa of 20.1 in DMSO, reflecting the weakly acidic nature of the C-H bond at position 1. Indene is air-stable under normal conditions but can polymerize upon heating or exposure to acids or bases. This stability is partly attributed to its fused ring structure, which confers aromatic character.

Production

Isolation from natural sources

Indene is primarily isolated from coal-tar fractions, a byproduct of coal carbonization processes, where it occurs in the light oil distillate boiling between 175–185°C and comprises approximately 1-2% of this fraction. This natural occurrence made coal tar the historical primary source for indene recovery. Indene was first identified in coal tar during the 19th century, with commercial-scale isolation emerging in the early 20th century to supply raw materials for resin production. The classical isolation process begins with fractional distillation of crude coal tar to separate the C9 hydrocarbon-rich fraction, typically containing indene alongside compounds like coumarone and indane. This fraction is then selectively extracted by treatment with sodium metal, exploiting the acidic C-H bond at the 1-position of indene to form the insoluble sodio-indene salt, which precipitates and allows removal of non-acidic impurities. The sodio-indene is subsequently decomposed via steam distillation or acidification to yield purified indene. In modern practice, indene is also extracted from petroleum pyrolysis oils and coke oven byproducts, which arise from high-temperature cracking processes in refining and steelmaking. These sources are processed using solvent extraction or adsorption methods, such as liquid-liquid extraction with polar solvents or selective adsorption on activated carbon, to concentrate and isolate indene from aromatic mixtures. For instance, solvent-based extraction from pyrolysis oils has demonstrated effective recovery of indene for further applications. Post-distillation fractions typically achieve 70-80% indene purity, which is further refined to greater than 95% through additional vacuum or precision distillation to meet industrial specifications.

Laboratory and industrial synthesis

Indene can be synthesized in the laboratory through classical acid-catalyzed cyclization methods, such as the dehydration and ring closure of o-alkylstyrenes or phenyl-substituted allylic alcohols using polyphosphoric acid (PPA) as a dehydrating agent. A variant of the Haworth synthesis employs PPA on derivatives of 3-phenylpropionic acid to form indene precursors, followed by dehydration to yield the fused ring system under heating conditions around 100–120°C. These approaches provide moderate yields (typically 50–70%) and are suitable for small-scale preparation, though they often require subsequent purification to remove oligomeric byproducts. Modern laboratory syntheses have advanced to more efficient Brønsted acid-catalyzed cyclizations of 1,3-dienes, particularly diaryl- or alkyl aryl-1,3-dienes, using triflic acid (TfOH) as the catalyst. Treatment of these dienes with 5–10 mol% TfOH in dichloromethane at room temperature affords substituted indenes in yields ranging from 70% to 95%, with high regioselectivity due to the electrophilic addition mechanism. Additionally, palladium-catalyzed annulation reactions between internal alkynes and aryl halides enable the construction of the indene core via carboannulation, often employing Pd(OAc)₂ with phosphine ligands in the presence of a base like K₂CO₃, achieving yields up to 85% for polysubstituted variants. These methods are versatile for introducing substituents at the 1- or 2-positions, facilitating applications in materials science. An illustrative example of acid-catalyzed cyclization is the conversion of allylbenzene to indene: \ceC6H5−CH2−CH=CH2−>[H2SO4][100∘C]indene+H2O\ce{C6H5-CH2-CH=CH2 ->[H2SO4][100^\circ C] indene + H2O} This reaction proceeds via of the , followed by and dehydration, though yields are typically low (20–40%) without optimized conditions. For substituted indenes relevant to pharmaceuticals, directed ortho-metalation (DoM) of or carbamate-protected arenes with sec-BuLi, followed by electrophilic quenching or , allows regioselective installation of alkyl or alkenyl groups at the ortho position, enabling subsequent cyclization to indene scaffolds used in nonsteroidal drugs. Cross-coupling strategies, such as –Miyaura reactions between indenyl boronic acids and aryl halides, further functionalize the 2- or 3-positions, providing access to bioactive derivatives like those in indene-based inhibitors with overall efficiencies exceeding 70% over two steps. Current production favors isolation from reforming fractions as a cost-effective alternative, supplemented by synthetic enhancement through of 1-indanol, obtained via catalytic of indanone byproducts from cracking. Over catalysts like HZSM-5 at 90–100°C, 1-indanol dehydrates to indene with selectivities above 90%, boosting yields in integrated processes.

Chemical reactivity

Polymerization reactions

Indene readily undergoes due to its structural features, including the strained five-membered ring and conjugated π-system, which enable reaction with acids, bases, or heat. The primary industrial approach is , employing catalysts such as or BF₃ at temperatures of 20–35°C, yielding polyindene with molecular weights typically in the range of 500–5000. In this process, initiation occurs via at the C3 position of the five-membered ring, generating a resonance-stabilized that propagates through to additional indene monomers, with chain termination via proton loss or recombination; the overall reaction is represented as n\ceC9H8→(\ceC9H8)nn \ce{C9H8} \rightarrow (\ce{C9H8})_n. Indene also participates in copolymerization with coumarone (), forming indene-coumarone resins via similar cationic mechanisms, resulting in materials with a temperature around 50°C. These resins find application as tackifiers in adhesives due to their compatibility and cohesive strength.

Substitution and addition reactions

Indene undergoes predominantly at the C2 or C3 positions of the five-membered ring, where the electron density is higher due to the with the ring. For example, with adds across the C1=C2 double bond to yield 1,2-dibromoindane. Deprotonation of indene occurs selectively at the C1 position (the vinylic proton) using strong bases like , forming the indenyl anion (lithium indenylide), which is widely used as a ligand precursor in . The reaction proceeds as follows: \ceC9H8+n−BuLi−>LiC9H7+C4H10\ce{C9H8 + n-BuLi -> LiC9H7 + C4H10} This anion exhibits enhanced nucleophilicity and stability due to delocalization across both rings. Addition reactions of indene are facilitated by its conjugated diene system in the five-membered ring. In the Diels-Alder cycloaddition, indene acts as a , reacting with electron-poor dienophiles like to form the endo , benzonorbornene-5,6-dicarboxylic anhydride, via a concerted [4+2] mechanism involving the C1=C2 bond. of indene typically employs (Pd/C) as a catalyst under , leading to full saturation of the C1=C2 and formation of . Oxidation of indene with selectively cleaves the five-membered ring, yielding homophthalic acid (2-(carboxymethyl)) as the primary product, with the reaction involving oxidative scission at the C1-C2 bond and subsequent formation on the ring. The simplified equation is: \ceC9H8+3[O]−>C6H4(COOH)(CH2COOH)\ce{C9H8 + 3[O] -> C6H4(COOH)(CH2COOH)} where [O] represents the oxidant. This method provides a purer product compared to oxidation, which also generates as a . Indene participates in condensation reactions with aldehydes or ketones under basic conditions to form benzofulvene derivatives, involving at C1 followed by and . For instance, reaction with yields 1-benzylideneindene, a fulvene-like structure with extended conjugation. Cesium serves as an efficient for this transformation, enabling high yields with various aromatic aldehydes.

Applications and uses

In resins and polymers

Indene is primarily utilized as a feedstock for the production of indene-coumarone resins, which are formed through copolymerization processes dating back to the early 1900s. These resins result from the acid-catalyzed of indene derived from fractions, often alongside coumarone (), yielding materials with versatile industrial applications. The resins exhibit characteristics, including solubility in hydrocarbons, and are commonly employed in varnishes, adhesives, and inks due to their balanced mechanical properties. Polyindene segments within the structure provide enhanced and tackiness, improving and durability in end-use formulations. Global output of indene from sources is approximately 20,000 tons per year as of the 2020s, supporting the scale of manufacturing. To produce modified variants, indene undergoes acid-catalyzed copolymerization with styrene or , which adjusts softening points and compatibility with other polymers. These resins offer advantages such as low production costs, superior electrical insulation, and resistance to , making them suitable for demanding applications in coatings and composites. The inherent reactivity of indene enables efficient resin formation under mild catalytic conditions.

In pharmaceuticals and other compounds

Indene serves as a key in several pharmaceutical compounds, particularly as a building block for non-steroidal drugs (NSAIDs). , an indene derivative chemically designated as (Z)-5-fluoro-2-methyl-1-[[p-(methylsulfinyl)benzylidene]]indene-3-acetic acid, is widely used for its , , and properties, acting as a that is metabolized to its active sulfide form to inhibit enzymes. Derivatives of , such as novel indene-based compounds, have been synthesized to enhance anti-proliferative effects while reducing gastrointestinal toxicity associated with traditional NSAIDs. In the realm of anticancer agents, indene scaffolds feature prominently in due to their ability to interact with biological targets like . For instance, dihydro-1H-indene derivatives designed as site inhibitors exhibit potent antiproliferative activity against lines by disrupting . Indeno[1,2-b] derivatives have also shown promising cytotoxic effects against various tumor cells, with structure-activity relationships highlighting the indene core's role in DNA intercalation and inhibition. Although indene itself is not abundant in nature, motifs resembling indene—such as (the saturated analog)—appear in bioactive natural products, including lignan-like molecules from and indene glycosides isolated from marine actinomycetes like Salinispora pacifica, which display and cytotoxic activities. Chiral indene derivatives, often accessed via asymmetric synthesis, are incorporated into bioactive molecules to confer stereospecific interactions in enzymatic . Beyond pharmaceuticals, indene is a precursor for indenyl ligands in metallocene catalysts, such as indenylzirconocene dichloride, which facilitate stereoselective olefin polymerization in the production of polyolefins for industrial applications. Indene derivatives contribute to dyes, exemplified by triphenylamine-cinnamaldehyde-indane-1,3-dione hybrids used in photoinitiators for and photocomposites due to their high extinction coefficients in the visible range. In fragrances, hydrogenated indane derivatives like pentamethylindane impart woody, amber notes, enhancing compositions with long-lasting olfactory profiles. The synthetic versatility of indene, particularly through substitution reactions enabling facile derivatization, supports its use in cross-coupling methodologies for constructing complex drug scaffolds in medicinal chemistry. Over 100 patents since 2000 describe indene-based active pharmaceutical ingredients (APIs), underscoring its prevalence in therapeutic development for conditions like inflammation and cancer. Indene's role continues to expand in and . For example, dihydro-indenoindene derivatives serve as blue fluorescent emitters for organic light-emitting diodes (OLEDs), offering low oxidation potentials and high quantum yields for efficient . In anticancer research, indene-fullerene adducts serve as electron acceptors in organic photovoltaics, while novel indene-dione hybrids exhibit against tumor cells. As of 2024, the global indene market is valued at approximately USD 1.2 billion, projected to reach USD 1.8 billion by 2033 at a CAGR of 5.1%, driven by demand in resins, , and sustainable materials.

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