Cyclopentadiene: Difference between revisions

{{Chembox

| Verifiedfields = changed

| verifiedrevid = 448739996

| ImageFileL1 = Cyclopentadiene.png

| ImageNameL1 = Skeletal formula of cyclopentadiene

| ImageFileR1 = Cyclopentadiene-3D-vdW.png

| ImageNameR1 = Spacefill model of cyclopentadiene

| ImageFile2 = Cyclopentadiene-3D-balls.png

| 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_Ref = {{fdacite|correct|FDA}}

| EINECS = 208-835-4

| MeSHName = 1,3-cyclopentadiene

| ChEBI_Ref = {{ebicite|correct|EBI}}

| RTECS = GY1000000

| Beilstein = 471171

| Gmelin = 1311

| StdInChI = 1S/C5H6/c1-2-4-5-3-1/h1-4H,5H2

| InChI = 1/C5H6/c1-2-4-5-3-1/h1-4H,5H2

| 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/cm³

| 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 |url=https://www.worldcat.org/oclc/930681942 |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=cm³/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/m³)<ref name=PGCH>{{PGCH|0170}}</ref>

| IDLH = 750 ppm<ref name=PGCH/>

| REL = TWA 75 ppm (200&nbsp;mg/m³)<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]] C₅H₆.<ref name=scha1965>LeRoy H. Scharpen and Victor W. Laurie (1965): "Structure of cyclopentadiene". ''The Journal of Chemical Physics'', volume 43, issue 8, pages 2765–2766. {{doi|10.1063/1.1697207}}.</ref> It is often abbreviated '''CpH''' because the [[cyclopentadienyl anion]] is abbreviated Cp^&minus;.

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^&minus;), 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>

[[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 (chemistry)|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.

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>{{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|Diene-selective Diels–Alder reaction in Corey's total synthesis of prostaglandin F2α]]

Even more [[fluxional molecule|fluxional]] are the derivatives C₅H₅E(CH₃)₃ (E = [[silicon|Si]], [[germanium|Ge]], [[tin|Sn]]), wherein the heavier element migrates from carbon to carbon with a low activation barrier.

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>

{{main|Cyclopentadienyl anion}}

The compound is unusually [[acid]]ic (p''K''_a&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.

{{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 MC₅H₅ 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>

: NiCl₂ + 2&nbsp;NaC₅H₅ → Ni(C₅H₅)₂ + 2&nbsp;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. }}</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}}

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''-Bu₃C₅H₃, 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.

== References ==

*[https://www.cdc.gov/niosh/npg/npgd0170.html NIOSH Pocket Guide to Chemical Hazards]

{{Cycloalkenes}}

{{Annulenes}}

{{Cyclopentadiene complexes}}

[[Category:Cyclopentadienes| ]]

[[Category:Annulenes]]

[[Category:Five-membered rings]]