View source for Cyclopentadiene

{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 448739996
| ImageFileL1 = Cyclopentadiene.png
| ImageClassL1 = skin-invert 
| ImageNameL1 = Skeletal formula of cyclopentadiene
| ImageFileR1 = Cyclopentadiene-3D-vdW.png
| ImageClassR1 = bg-transparent  
| ImageNameR1 = Spacefill model of cyclopentadiene
| ImageFile2 = Cyclopentadiene-3D-balls.png
| ImageClass2 = bg-transparent  
| ImageSize2 = 100
| ImageName2 = Ball and stick model of cyclopentadiene
| PIN = Cyclopenta-1,3-diene
| OtherNames = 1,3-Cyclopentadiene<ref name=PGCH/><br />Pyropentylene<ref>{{cite book |author = William M. Haynes |title = CRC Handbook of Chemistry and Physics |publisher = CRC Press/Taylor and Francis |date = 2016 |isbn = 978-1498754286 |volume=97 |page=276 (3-138) |trans-title=Physical Constants of Organic Compounds}}</ref>
|Section1={{Chembox Identifiers
| Abbreviations = CPD, HCp
| CASNo = 542-92-7
| CASNo_Ref = {{cascite|correct|CAS}}
| PubChem = 7612
| ChemSpiderID = 7330
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| UNII = 5DFH9434HF
| UNII_Ref = {{fdacite|correct|FDA}}
| EINECS = 208-835-4
| MeSHName = 1,3-cyclopentadiene
| ChEBI = 30664
| ChEBI_Ref = {{ebicite|correct|EBI}}
| RTECS = GY1000000
| Beilstein = 471171
| Gmelin = 1311
| SMILES = C1C=CC=C1
| StdInChI = 1S/C5H6/c1-2-4-5-3-1/h1-4H,5H2
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| InChI = 1/C5H6/c1-2-4-5-3-1/h1-4H,5H2
| StdInChIKey = ZSWFCLXCOIISFI-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| InChIKey = ZSWFCLXCOIISFI-UHFFFAOYAI
}}
|Section2={{Chembox Properties
| C = 5
| H = 6
| Appearance = Colourless liquid
| Odor = irritating, [[terpene]]-like<ref name=PGCH/>
| Density = 0.802 g/cm<sup>3</sup>
| MeltingPtK = 183
| BoilingPtK = 312 to 316
| pKa = 16
| ConjugateBase = [[Cyclopentadienyl anion]]
| Solubility = insoluble<ref name=PGCH/>
| VaporPressure = {{convert|400|mmHg|kPa|abbr=on}}<ref name=PGCH/>
| RefractIndex = 1.44 (at 20&nbsp;°C)<ref name="CRC97">{{Cite book |title=CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data. |date=2016 |editor1=William M. Haynes |editor2=David R. Lide |editor3=Thomas J. Bruno |isbn=978-1-4987-5428-6 |edition=2016-2017, 97th |location=Boca Raton, Florida |publisher=CRC Press |oclc=930681942}}</ref>
| MagSus = {{val|-44.5e-6|u=cm<sup>3</sup>/mol}}
}}
|Section3={{Chembox Structure
| MolShape = Planar<ref>{{cite journal | title = Ab initio G2 and DFT calculations on electron affinity of cyclopentadiene, silole, germole and their 2,3,4,5-tetraphenyl substituted analogs: structure, stability and EPR parameters of the radical anions | first1= Valery I. |last1=Faustov |first2=Mikhail P. |last2=Egorov |first3=Oleg M. |last3=Nefedov |first4=Yuri N. |last4=Molin | journal = Phys. Chem. Chem. Phys. | year = 2000 | volume = 2 | pages = 4293–4297 | doi = 10.1039/b005247g | issue = 19| bibcode= 2000PCCP....2.4293F }}</ref>
| Dipole = 0.419 [[Debye|D]]<ref name="CRC97"/>
}}
|Section4={{Chembox Thermochemistry
| Entropy = 182.7&nbsp;J/(mol·K)
| HeatCapacity = 115.3&nbsp;J/(mol·K)
| DeltaHform = 105.9&nbsp;kJ/mol<ref name="CRC97"/>
}}
|Section5={{Chembox Hazards
| FlashPtC = 25
| PEL = TWA 75 ppm (200&nbsp;mg/m<sup>3</sup>)<ref name=PGCH>{{PGCH|0170}}</ref>
| IDLH = 750 ppm<ref name=PGCH/>
| REL = TWA 75 ppm (200&nbsp;mg/m<sup>3</sup>)<ref name=PGCH/>
| LC50 = 14,182 ppm (rat, 2&nbsp;[[hour|h]])<br/>5091 ppm (mouse, 2&nbsp;h)<ref>{{IDLH|542927|Cyclopentadiene}}</ref>
| AutoignitionPtC = 640
| NFPA-H = 2
| NFPA-F = 3
| NFPA-I = 0
}}
|Section6={{Chembox Related
| OtherFunction_label = [[hydrocarbon]]s
| OtherFunction = [[Benzene]]<br/>[[Cyclobutadiene]]<br/>[[Cyclopentene]]
| OtherCompounds = [[Dicyclopentadiene]]
}}
}}

'''Cyclopentadiene''' is an [[organic compound]] with the [[chemical formula|formula]] C<sub>5</sub>H<sub>6</sub>.<ref name=scha1965>{{cite journal | last1=Scharpen | first1=LeRoy H. | last2=Laurie | first2=Victor W. | title=Structure of Cyclopentadiene | journal=The Journal of Chemical Physics | volume=43 | issue=8 | date=1965-10-15 | issn=0021-9606 | doi=10.1063/1.1697207 | pages=2765–2766 | bibcode=1965JChPh..43.2765S | url=https://pubs.aip.org/jcp/article/43/8/2765/78386/Structure-of-Cyclopentadiene | url-access=subscription }}</ref> It is often abbreviated '''CpH''' because the [[cyclopentadienyl anion]] is abbreviated Cp<sup>&minus;</sup>.

This colorless liquid has a strong and [[unpleasant odor]]. At room temperature, this cyclic [[diene]] [[dimer (chemistry)|dimerizes]] over the course of hours to give [[dicyclopentadiene]] via a [[Diels–Alder reaction]]. This dimer can be [[retro-Diels–Alder reaction|restored]] by heating to give the monomer.

The compound is mainly used for the production of [[cyclopentene]] and its derivatives. It is popularly used as a precursor to the [[cyclopentadienyl anion]] (Cp<sup>&minus;</sup>), an important [[ligand]] in [[cyclopentadienyl complex]]es in [[organometallic chemistry]].<ref>{{cite book |last=Hartwig |first= J. F. |title=Organotransition Metal Chemistry: From Bonding to Catalysis |publisher=University Science Books |location=New York, NY |date=2010 |isbn=978-1-891389-53-5}}</ref>

==Production and reactions==
[[File:AW Cyclopentadiene.jpg|thumb|left|Cyclopentadiene monomer in an ice bath]]
Cyclopentadiene production is usually not distinguished from [[dicyclopentadiene]] since they interconvert.  They are obtained from coal tar (about 10–20&nbsp;g/[[tonne|t]]) and by [[steam cracking]] of [[Petroleum naphtha|naphtha]] (about 14&nbsp;kg/t).<ref name=Ullmann/> To obtain cyclopentadiene monomer, commercial dicyclopentadiene is cracked by heating to around 180&nbsp;°C. The monomer is collected by distillation and used soon thereafter.<ref>{{OrgSynth | title = Cyclopentadiene and 3-Chlorocyclopentene | prep = cv4p0238 | collvol = 4 | collvolpages = 238 | first= Robert Bruce |last=Moffett | year = 1962}}</ref> It advisable to use some form of [[fractionating column]] when doing this, to remove refluxing uncracked dimer. 

===Sigmatropic rearrangement===
The hydrogen atoms in cyclopentadiene undergo rapid [[sigmatropic reaction|[1,5]-sigmatropic shifts]]. The hydride shift is, however, sufficiently slow at 0&nbsp;°C to allow alkylated derivatives to be manipulated selectively.<ref name=CF2a>{{cite journal |last1=Corey |first1=E. J. |last2=Weinshenker |first2=N. M. |last3=Schaaf |first3=T. K. |last4=Huber |first4=W. |year=1969 |title=Stereo-controlled synthesis of prostaglandins F-2a and E-2 (dl)|journal=Journal of the American Chemical Society |volume=91 |issue=20 |pages=5675–5677 |doi=10.1021/ja01048a062 |pmid=5808505}}</ref> 
[[File:Prostaglandin Diels-Alder Corey (cropped2).png|400 px|thumb|center|The [[methoxy group]] ends up only on the methylene bridge, because [[Diels-Alder]] addition at &minus;55&nbsp;°C occurs much faster than the sigmatropic shift (excerpted from [[E. J. Corey|Corey]]'s total synthesis of [[prostaglandin F2α|prostaglandin F<sub>2α</sub>]])<ref name=CF2a/>]]

Even more [[fluxional molecule|fluxional]] are the derivatives C<sub>5</sub>H<sub>5</sub>E(CH<sub>3</sub>)<sub>3</sub> (E = [[silicon|Si]], [[germanium|Ge]], [[tin|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 the [[Diels–Alder reaction]] because minimal distortion of the diene is required to achieve the envelope geometry of the transition state compared to other dienes.<ref>{{cite journal |first1=Brian |last1=Levandowski |first2=Ken |last2=Houk |date=2015 |title=Theoretical Analysis of Reactivity Patterns in Diels–Alder Reactions of Cyclopentadiene, Cyclohexadiene, and Cycloheptadiene with Symmetrical and Unsymmetrical Dienophiles |doi=10.1021/acs.joc.5b00174 |pmid=25741891 |journal=[[J. Org. Chem.]] |volume=80 |issue=7 |pages=3530–3537}}</ref> Famously, cyclopentadiene dimerizes. The conversion occurs in hours at room temperature, but the monomer can be stored for days at −20&nbsp;°C.<ref name=Ullmann>{{Ullmann|first1=Dieter |last1=Hönicke |first2=Ringo |last2=Födisch |first3=Peter |last3=Claus |first4=Michael |last4=Olson |title=Cyclopentadiene and Cyclopentene |doi=10.1002/14356007.a08_227}}</ref>

===Deprotonation===
{{main|Cyclopentadienyl anion}}
The compound is unusually [[acid]]ic (p''K''<sub>a</sub>&nbsp;= 16) for a [[hydrocarbon]], a fact explained by the high stability of the [[aromatic]] cyclopentadienyl anion, {{chem|C|5|H|5|−}}. [[Deprotonation]] can be achieved with a variety of bases, typically [[sodium hydride]], sodium metal, and [[butyl lithium]]. Salts of this anion are commercially available, including [[sodium cyclopentadienide]] and [[lithium cyclopentadienide]]. They are used to prepare [[cyclopentadienyl complex]]es.

===Metallocene derivatives===
{{main|Metallocene}}
Metallocenes and related [[Cyclopentadienyl complex|cyclopentadienyl derivatives]] have been heavily investigated and represent a cornerstone of [[organometallic chemistry]] owing to their high stability. The first metallocene characterised, [[ferrocene]], was prepared the way many other metallocenes are prepared by combining alkali metal derivatives of the form MC<sub>5</sub>H<sub>5</sub> with dihalides of the [[transition metal]]s:<ref>{{cite book |author1-link=Gregory S. Girolami |author3-link=Robert Angelici |last1=Girolami |first1=G. S. |last2=Rauchfuss |first2=T. B. |last3=Angelici |first3=R. J. |title=Synthesis and Technique in Inorganic Chemistry |year=1999 |publisher=University Science Books |location=Mill Valley, CA |isbn=0-935702-48-2}}</ref> As typical example, [[nickelocene]] forms upon treating [[nickel(II) chloride]] with sodium cyclopentadienide in [[tetrahydrofuran|THF]].<ref>{{cite book |last1=Jolly |first1=W. L. |title=The Synthesis and Characterization of Inorganic Compounds |url=https://archive.org/details/synthesischaract0000joll |url-access=registration |year=1970 |publisher=Prentice-Hall |location=Englewood Cliffs, NJ |isbn=0-13-879932-6}}</ref>

: {{chem2|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]] derivative produced from the rhodocene monomer in [[protic solvent]]s.<ref>{{cite journal |title = Permethylmetallocene: 5. Reactions of Decamethylruthenium Cations |year = 1985 |last1 = Kolle |first1 = U. |last2 = Grub |first2 = J. |journal = [[Journal of Organometallic Chemistry|J. Organomet. Chem.]] |volume = 289 |issue = 1 |pages = 133–139 |doi =10.1016/0022-328X(85)88034-7 }}</ref>

===Organic synthesis===
It was the starting material in [[Leo Paquette]]'s 1982 synthesis of [[dodecahedrane]].<ref>{{cite journal |title= Domino Diels–Alder reactions. I. Applications to the rapid construction of polyfused cyclopentanoid systems |journal= [[J. Am. Chem. Soc.]] |year= 1974 |volume= 96 |issue= 14 |pages= 4671–4673 |doi= 10.1021/ja00821a052 |author1-link=Leo Paquette |last1=Paquette |first1= L. A. |last2= Wyvratt |first2= M. J. |bibcode= 1974JAChS..96.4671P }}</ref> The first step involved [[redox|reductive]] dimerization of the molecule to give [[Fulvalene|dihydrofulvalene]], not simple addition to give dicyclopentadiene.

[[File:DodecahedranePrecursorSynthesis.png|thumb|center|400px|The start of Paquette's 1982 dodecahedrane synthesis. Note the dimerisation of cyclopentadiene in step&nbsp;1 to dihydrofulvalene.]]
{{Clear left}}

==Uses==
Aside from serving as a precursor to cyclopentadienyl-based catalysts, the main commercial application of cyclopentadiene is as a precursor to [[comonomer]]s.  Semi-hydrogenation gives [[cyclopentene]].  Diels–Alder reaction with [[butadiene]] gives [[ethylidene norbornene]], a comonomer in the production of [[EPDM rubber]]s.

==Derivatives==
[[File:(t-Bu)3C5H3.png|thumb|right|144 px|Structure of ''t''-Bu<sub>3</sub>C<sub>5</sub>H<sub>3</sub>, a prototypical [[bulky cyclopentadiene]]<ref>{{cite book |doi=10.1002/9781119477822.ch8 |title=Inorganic Syntheses |year=2018 |last1=Reiners |first1=Matthis |last2=Ehrlich |first2=Nico |last3=Walter |first3=Marc D. |chapter=Synthesis of Selected Transition Metal and Main Group Compounds with Synthetic Applications |volume=37 |page=199 |isbn=978-1-119-47782-2 |s2cid=105376454}}</ref>]]

Cyclopentadiene can substitute one or more hydrogens, forming derivatives having covalent bonds:
* [[Bulky cyclopentadiene]]s
* [[Calicene]]
* [[Cyclopentadienone]]
* [[Di-tert-butylcyclopentadiene|Di-''tert''-butylcyclopentadiene]]
* [[Methylcyclopentadiene]]
* [[Pentamethylcyclopentadiene]]
* [[Pentacyanocyclopentadiene]]

Most of these substituted cyclopentadienes can also form [[anion]]s and join [[cyclopentadienyl complex]]es.

==See also==
*[[Aromaticity]]

== References ==
{{Reflist}}

==External links==
*[http://www.inchem.org/documents/icsc/icsc/eics0857.htm International Chemical Safety Card 0857]
*[https://www.cdc.gov/niosh/npg/npgd0170.html NIOSH Pocket Guide to Chemical Hazards]

{{Cycloalkenes}}
{{Annulenes}}
{{Cyclopentadiene complexes}}

[[Category:Cyclopentadienes| ]]
[[Category:Annulenes]]