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Cyclopentadiene is an
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
with the
formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...
C5H6.LeRoy H. Scharpen and Victor W. Laurie (1965): "Structure of cyclopentadiene". ''The Journal of Chemical Physics'', volume 43, issue 8, pages 2765-2766. It is often abbreviated CpH because the
cyclopentadienyl anion In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp−. It is formed from the deprotonation of the molecule cyclopentadiene. Properties The cyclopentadienyl anion i ...
is abbreviated Cp. This colorless liquid has a strong and
unpleasant odor An odor (American English) or odour (Commonwealth English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive via their sense ...
. At room temperature, this cyclic
diene In organic chemistry a diene ( ) (diolefin ( ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nomenclature. ...
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
izes over the course of hours to give
dicyclopentadiene Dicyclopentadiene, abbreviated DCPD, is a chemical compound with formula C10H12. At room temperature, it is a white brittle wax, although lower purity samples can be straw coloured liquids. The pure material smells somewhat of soy wax or camphor ...
via a
Diels–Alder reaction In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a peric ...
. This dimer can be
restored ''Restored'' is the fourth studio album by American contemporary Christian music musician Jeremy Camp. It was released on November 16, 2004 by BEC Recordings. Track listing Standard release Enhanced edition Deluxe gold edition Standard ...
by heating to give the monomer. The compound is mainly used for the production of
cyclopentene Cyclopentene is a chemical compound with the formula . It is a colorless liquid with a petrol-like odor. It has few applications, and thus is mainly used as a minor component of gasoline, present in concentrations of less than 1%. It is one of t ...
and its derivatives. It is popularly used as a precursor to the
cyclopentadienyl anion In chemistry, the cyclopentadienyl anion or cyclopentadienide is an aromatic species with a formula of and abbreviated as Cp−. It is formed from the deprotonation of the molecule cyclopentadiene. Properties The cyclopentadienyl anion i ...
(Cp), an important
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
in
cyclopentadienyl complex A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (''η''5-) bonding mode. The metal–cyclopentadien ...
es in
organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
.


Production and reactions

Cyclopentadiene production is usually not distinguished from
dicyclopentadiene Dicyclopentadiene, abbreviated DCPD, is a chemical compound with formula C10H12. At room temperature, it is a white brittle wax, although lower purity samples can be straw coloured liquids. The pure material smells somewhat of soy wax or camphor ...
since they interconvert. They are obtained from coal tar (about 10–20 g/tonne) and by steam cracking of
naphtha Naphtha ( or ) is a flammable liquid hydrocarbon mixture. Mixtures labelled ''naphtha'' have been produced from natural gas condensates, petroleum distillates, and the distillation of coal tar and peat. In different industries and regions ''n ...
(about 14 kg/tonne). To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to around 180 °C. The monomer is collected by distillation, and used soon thereafter. It advisable to use some form of
fractionating column A fractionating column or fractional column is an essential item used in the distillation of liquid mixtures to separate the mixture into its component parts, or fractions, based on the differences in volatilities. Fractionating columns are used in ...
when doing this, to remove refluxing uncracked dimer.


Sigmatropic rearrangement

The hydrogen atoms in cyclopentadiene undergo rapid ,5sigmatropic shifts. The hydride shift is however sufficiently slow at 0 °C to allow alkylated derivatives to be manipulated selectively. Even more
fluxional In chemistry and molecular physics, fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in so ...
are the derivatives C5H5E(CH3)3 (E = Si, Ge, Sn), wherein the heavier element migrates from carbon to carbon with a low activation barrier.


Diels–Alder reactions

Cyclopentadiene is a highly reactive
diene In organic chemistry a diene ( ) (diolefin ( ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nomenclature. ...
in the
Diels–Alder reaction In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a peric ...
because minimal distortion of the diene is required to achieve the envelope geometry of the transition state compared to other dienes. Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20 °C.


Deprotonation

The compound is unusually
acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
ic (p''K''a = 16) for a
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
, a fact explained by the high stability of the
aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
cyclopentadienyl anion, .
Deprotonation Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju.ed ...
can be achieved with a variety of bases, typically
sodium hydride Sodium hydride is the chemical compound with the empirical formula Na H. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and H− ions, in co ...
, sodium metal, and
butyl lithium Butyllithium may refer to one of 5 isomeric organolithium reagents of which 3 are commonly used in chemical synthesis: * ''n''-Butyllithium, abbreviated BuLi or nBuLi * ''sec''-Butyllithium, abbreviated ''sec''-BuLi or sBuLi, has 2 stereoisomers, ...
. Salts of this anion are commercially available, including
sodium cyclopentadienide Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pi ...
and
lithium cyclopentadienide Lithium cyclopentadienide is an organolithium compound with the formula C5H5Li. The compound is often abbreviated as LiCp, where Cp− is the cyclopentadienide anion. Lithium cyclopentadienide is a colorless solid, although samples often are p ...
. They are used to prepare
cyclopentadienyl complex A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (''η''5-) bonding mode. The metal–cyclopentadien ...
es.


Metallocene derivatives

Metallocenes and related cyclopentadienyl derivatives have been heavily investigated and represent a cornerstone of
organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
owing to their high stability. The first metallocene characterised,
ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...
, was prepared the way many other metallocenes are prepared: by combining alkali metal derivatives of the form MC5H5 with dihalides of the
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s: As typical example,
nickelocene Nickelocene is the organonickel compound with the formula Ni( ''η''5-C5H5)2. Also known as bis(cyclopentadienyl)nickel or NiCp2, this bright green paramagnetic solid is of enduring academic interest, although it does not yet have any known pra ...
forms upon treating
nickel(II) chloride Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemi ...
with sodium cyclopentadienide in
THF Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
. :NiCl2 + 2 NaC5H5 → Ni(C5H5)2 + 2 NaCl Organometallic complexes that include both the cyclopentadienyl anion and cyclopentadiene itself are known, one example of which is the
rhodocene Rhodocene is a chemical compound with the formula . Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compo ...
derivative produced from the rhodocene monomer in
protic solvent In chemistry, a protic solvent is a solvent that has a hydrogen atom bound to an oxygen (as in a hydroxyl group ), a nitrogen (as in an amine group or ), or fluoride (as in hydrogen fluoride). In general terms, any solvent that contains a Labile# ...
s.


Organic synthesis

It was the starting material in
Leo Paquette Leo Armand Paquette ( – January 21, 2019) was an American organic chemist. Biography He was born on July 15, 1934 to parents Armand and Clarice with roots in Quebec (great-grandfather Edmund was born in Contrecoeur, Quebec) and he received h ...
's 1982 synthesis of
dodecahedrane Dodecahedrane is a chemical compound, a hydrocarbon with formula , whose carbon atoms are arranged as the vertices (corners) of a regular dodecahedron. Each carbon is bound to three neighbouring carbon atoms and to a hydrogen atom. This compound ...
. The first step involved reductive dimerization of the molecule to give dihydrofulvalene, not simple addition to give dicyclopentadiene.


Uses

Aside from serving as a precursor to cyclopentadienyl-based catalysts, the main commercial application of cyclopentadiene is as a precursor to
comonomer In polymer chemistry, a comonomer refers to a polymerizable precursor to a copolymer aside from the principal monomer. In some cases, only small amounts of a comonomer are employed, in other cases substantial amounts of comonomers are used. Furthe ...
s. Semi-hydrogenation gives
cyclopentene Cyclopentene is a chemical compound with the formula . It is a colorless liquid with a petrol-like odor. It has few applications, and thus is mainly used as a minor component of gasoline, present in concentrations of less than 1%. It is one of t ...
. Diels–Alder reaction with
butadiene 1,3-Butadiene () is the organic compound with the formula (CH2=CH)2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two viny ...
gives
ethylidene norbornene Ethylidene norbornene (ENB) is an organic compound that consists of an ethylidene (CH3C(H)=) group attached to norbornene. It is a colorless liquid. The molecule consists of two sites of unsaturation, one of which participates in the copolymer ...
, a comonomer in the production of EPDM rubbers.


Derivatives

Cyclopentadiene can substitute one or more hydrogens, forming derivatives having covalent bonds: * Bulky cyclopentadienes *
Calicene Calicene or triapentafulvalene is a hydrocarbon of the fulvalene class with chemical formula C8H6, composed of a cyclopentadiene ring and a cyclopropene ring linked by a double bond. Its name is derived from the Latin ''calix'' meaning "goblet", ...
*
Cyclopentadienone Cyclopentadienone is an organic compound with molecular formula C5H4O. The parent cyclopentadienone is rarely encountered, because it rapidly dimerizes. Many substituted derivatives are known, notably tetraphenylcyclopentadienone. Such compounds ...
* Di-''tert''-butylcyclopentadiene *
Methylcyclopentadiene Methylcyclopentadiene is any of three isomeric cyclic dialkenes with the formula C5MeH5 (Me = CH3). These isomers are the organic precursor to the methylcyclopentadienyl ligand (C5H4Me, often denoted as Cp′), commonly found in organometallic che ...
*
Pentamethylcyclopentadiene 1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diene with the formula C5Me5H (Me = CH3). 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand ''1,2,3,4,5-pentamethylcyclopentadienyl'', which is often denoted Cp* (C5Me5) and read ...
*
Pentacyanocyclopentadiene Pentacyanocyclopentadiene is a derivative of cyclopentadiene with five cyano groups with the molecular formula C5H(CN)5. The corresponding anion, pentacyanocyclopentadienide, is a ligand with the molecular formula . In contrast to other anions b ...
Most of these substituted cyclopentadienes can also form
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s, and join
cyclopentadienyl complex A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (''η''5-) bonding mode. The metal–cyclopentadien ...
es.


See also

*
Aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to saturate ...


References


External links


International Chemical Safety Card 0857
{{Cyclopentadiene complexes Annulenes Five-membered rings