Indene is a flammable polycyclic hydrocarbon

Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 with chemical formula

Chemical formula

A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 C₉H₈. It is composed of a benzene

Benzene

Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....

 ring

Simple aromatic ring

Simple aromatic rings, also known as simple arenes or simple aromatics, are aromatic organic compounds that consist only of a conjugated planar ring system with delocalized pi electron clouds. Many simple aromatic rings have trivial names. They are usually found as substructures of more complex...

 fused with a cyclopentene

Cyclopentene

Cyclopentene is a chemical compound with the formula 58. It is a colorless liquid with a petrol-like odor. It is one of the cycloalkenes.Cyclopentene is produced industrially in large amounts...

 ring. This aromatic liquid is colorless although samples often are pale yellow. The principal industrial use of indene is in the production of indene/coumarone thermoplastic resins.

Isolation

Indene occurs naturally in coal-tar fractions

Fraction (chemistry)

A fraction in chemistry is a quantity collected from a sample or batch of a substance in a fractionating separation process. In such a process, a mixture is separated into fractions, which have compositions that vary according to a gradient. A fraction can be defined as a group of chemicals that...

 boiling around 175–185 °C. It can be obtained by heating this fraction with sodium

Sodium

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 to precipitate solid "sodio-indene." This step exploits indene's weak acidity evidenced by its deprotonation

Deprotonation

Deprotonation is the removal of a proton from a molecule, forming the conjugate base.The relative ability of a molecule to give up a proton is measured by its pKa value. A low pKa value indicates that the compound is acidic and will easily give up its proton to a base...

 by sodium to give the indenyl derivative. The sodio-indene is converted back to indene by steam distillation

Distillation

Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

.

Reactivity

Indene readily polymerises

Polymerization

In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

. Oxidation of indene with acid dichromate yields homophthalic acid (o-carboxylphenylacetic acid). It condenses with ethyl oxalate in the presence of sodium ethoxide

Sodium ethoxide

Sodium ethoxide is an alkoxide salt with the chemical formula C2H5ONa.-Preparation:It is commercially available as a white solid, or as a solution in ethanol. It is easily prepared in the laboratory by reacting sodium metal with ethanol:...

 to form indene-oxalic ester, and with aldehyde

Aldehyde

An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s or ketone

Ketone

In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

s in the presence of alkali

Alkali

In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...

 to form benzofulvenes. The latter are highly coloured. An indene is also a precursor to the indenyl anion, a ligand

Ligand

In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 in organometallic chemistry

Organometallic chemistry

Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...

 with some notability due to the indenyl effect

Indenyl effect

In organometallic chemistry, the indenyl effect refers to the enhanced rates of substitution displayed by η5-indenyl complexes vs the related η5-cyclopentadienyl complexes.-Mechanism:...

.