Editing Diene

{{short description|Covalent compound that contains two double bonds}}
{{About|organic chemicals|the surname "Diene" used in West Africa|Serer people}}

[[File:1,3-butadiene.svg|thumb|150px|[[1,3-Butadiene|1,3-butadiene]]]]

In [[organic chemistry]], a '''diene''' ({{IPAc-en|ˈ|d|aɪ|iː|n}} {{respell|DY|een}}); also '''diolefin''', {{IPAc-en|d|aɪ|ˈ|oʊ|l|ə|f|ᵻ|n}} {{respell|dy|OH|lə|fin}}) or '''alkadiene''') is a [[covalent compound]] that contains two [[double bond]]s, usually among [[carbon]] atoms.<ref name="IUPAC dienes">{{GoldBookRef|file=D01699|title=dienes}}</ref> They thus contain two [[alkene|alk''ene'']] units, with the standard prefix ''di'' of [[systematic nomenclature]].  As a subunit of more complex molecules, dienes occur in naturally occurring and synthetic chemicals and are used in [[organic synthesis]].  [[Conjugated system|Conjugated]] dienes are widely used as [[monomer]]s in the [[polymer]] industry. [[Polyunsaturated fat]]s are of interest to [[nutrition]].

==Classes==
Dienes can be divided into three classes, depending on the relative location of the double bonds:<ref name="IUPAC dienes"/>
#'''Cumulated dienes''' have the double bonds sharing a common atom. The result is more specifically called an [[allene]].
#'''Conjugated dienes''' have [[Conjugated system|conjugated]] double bonds separated by one single bond. Conjugated dienes are more stable than other dienes because of resonance.
#'''Unconjugated dienes''' have the double bonds separated by two or more single bonds. They are usually less stable than [[isomer]]ic conjugated dienes. This can also be known as an '''isolated''' diene.

[[File:Dienes various examples.png|thumb|center|650px|alt=Structure of various alkadienes (also called dienes or diolefins)|Some dienes: '''A''': 1,2-Propadiene, also known as [[allene]], is the simplest cumulated diene. '''B''': [[Isoprene]], also known as 2-methyl-1,3-butadiene, the precursor to natural rubber. '''C''': [[1,3-Butadiene]], a  precursor to synthetic polymers.  '''D''': [[1,5-Cyclooctadiene]], an unconjugated diene (notice that each double bond is two carbons away from the other). '''E''': [[Norbornadiene]], a strained bicyclic and unconjugated  diene.  '''F''': [[Dicyclopentadiene]].]]

According to the ''[[Gold Book]]'' definition, a "diene" could include one or more [[heteroatom]]s which replace unsaturated carbon atoms, giving structures that could more specifically be called ''heterodienes''.<ref name="IUPAC dienes"/>

Compounds that contain more than two double bonds are called [[polyene]]s. Polyenes and dienes share many properties.

==Synthesis of dienes==

On an industrial scale, butadiene is prepared by [[thermal cracking]] of [[butane]]s. In a similarly non-selective process, [[dicyclopentadiene]] is obtained from [[coal tar]]s.

In the laboratory, more directed and more delicate processes are employed such as [[dehydrohalogenation]]s and [[Condensation (chemistry)|condensations]].  Myriad [[organic reaction|methods]] have been developed, such as the [[Whiting reaction]].  Families of nonconjugated dienes are derived from the [[oligomerization]] and [[Dimer (chemistry)|dimerization]] of conjugated dienes.  For example, 1,5-cyclooctadiene and 4-vinylcyclohexene are produced by dimerization of [[1,3-Butadiene|1,3-butadiene]].

Diene-containing [[fatty acid]]s are [[biosynthesis|biosynthesized]] from [[acetyl CoA]].

α,ω-Dienes have the formula (CH<sub>2</sub>)<sub>n</sub>(CH=CH<sub>2</sub>)<sub>2</sub>.  They are prepared industrially by [[ethenolysis]] of cyclic dienes.  For example, [[1,5-Hexadiene|1,5-hexadiene]] and 1,9-decadiene, useful crosslinking agents and synthetic intermediates, are produced from [[1,5-Cyclooctadiene|1,5-cyclooctadiene]] and [[cyclooctene]], respectively.  The catalyst is derived from Re<sub>2</sub>O<sub>7</sub> on alumina.<ref name=KO>{{cite encyclopedia|title=Metathesis|encyclopedia=Kirk-Othmer Encyclopedia of Chemical Technology|author=Lionel Delaude |author2=Alfred F. Noels |year=2005| doi=10.1002/0471238961.metanoel.a01|place=Weinheim|publisher=Wiley-VCH|isbn=0-471-23896-1}}</ref>

== Reactivity and uses==

===Polymerization===
The most heavily practiced reaction of alkenes, dienes included, is [[polymerization]].  1,3-Butadiene is a precursor to [[Synthetic rubber|rubber]] used in tires, and [[isoprene]] is the precursor to [[natural rubber]].  [[Chloroprene]] is related but it is a synthetic monomer.

===Cycloadditions===
An important reaction for conjugated dienes is the [[Diels–Alder reaction]].  Many specialized dienes have been developed to exploit this reactivity for the synthesis of [[natural product]]s (e.g., [[Danishefsky's diene]]).

===Other addition reactions===
Conjugated dienes add reagents such as [[bromine]] and [[hydrogen]] by both 1,2-addition and 1,4-addition pathways.  Addition of polar reagents can generate complex architectures:<ref>{{OrgSynth | author = Roger Bishop | collvol = 9 | collvolpages = 692 | prep = CV9P0692
| title =9-Thiabicyclo[3.3.1]nonane-2,6-dione}}{{cite journal | title =2,6-Dichloro-9-thiabicyclo[3.3.1]nonane: Multigram Display of Azide and Cyanide Components on a Versatile Scaffold | first4 = M. G. | last4 = Finn | first3 = K. Barry | last3 = Sharpless | journal = [[Molecules (journal)|Molecules]] | first2 = Antonella | year = 2006 | volume = 11 | last2 = Converso | pages = 212–218 | doi = 10.3390/11040212 | author = Díaz, David Díaz | issue = 4| pmid = 17962753 | pmc = 6148556 | doi-access = free }}</ref>
::[[Image:CODSCl2.png|450px|2,6-Dichloro-9-thiabicyclo[3.3.1]nonane, synthesis and reactions]]

===Metathesis reactions===
Nonconjugated dienes are substrates for [[ring-closing metathesis]] reactions.  These reactions require a metal [[catalyst]]:
::[[File:RCM cyclophane example.png|520px]]

===Acidity===
The position adjacent to a double bond is [[acid]]ic because the resulting [[allyl]] anion is stabilized by resonance. This effect becomes more pronounced as more alkenes are involved to create greater stability. For example, deprotonation at position 3 of a 1,4-diene or position 5 of a 1,3-diene give a [[pentadienyl]] anion. An even greater effect is seen if the anion is aromatic, for example, deprotonation of [[cyclopentadiene]] to give the [[cyclopentadienyl anion]].<!-- TODO: chart of pKa for methylene in alkane, singly and doubly allylic, and Cp -->

[[File:HayashiChiralNBD.svg|thumb|left|122px|[[C2-Symmetric ligands|C<sub>2</sub>-symmetric]] diene ligand used in [[asymmetric catalysis]].<ref>{{cite journal|vauthors=Hayashi T, Ueyama K, Tokunaga N, Yoshida K|title=A Chiral Chelating Diene as a New Type of Chiral Ligand for Transition Metal Catalysts: Its Preparation and Use for the Rhodium-Catalyzed Asymmetric 1,4-Addition|journal=J. Am. Chem. Soc.|year=2003|volume=125|issue=38|pages=11508–11509|doi=10.1021/ja037367z|pmid=13129348}}</ref>]]

===As ligands===
Dienes are widely used chelating [[ligand]]s in [[organometallic chemistry]].  In some cases they serve as placeholder ligands, being removed during a catalytic cycle.  For example, the cyclooctadiene ("cod") ligands in [[bis(cyclooctadiene)nickel(0)]] are labile.  In some cases, dienes are spectator ligands, remaining coordinated throughout a catalytic cycle and influencing the product distributions.  [[Chirality|Chiral]] dienes have also been described.<ref>{{Cite journal |last1=Huang |first1=Yinhua |last2=Hayashi |first2=Tamio |date=2022-09-28 |title=Chiral Diene Ligands in Asymmetric Catalysis |url=https://pubs.acs.org/doi/10.1021/acs.chemrev.2c00218 |journal=Chemical Reviews |language=en |volume=122 |issue=18 |pages=14346–14404 |doi=10.1021/acs.chemrev.2c00218 |issn=0009-2665 |pmid= 35972018|url-access=subscription }}</ref>  Other diene complexes include [[(Butadiene)iron tricarbonyl|(butadiene)iron tricarbonyl]], [[cyclobutadieneiron tricarbonyl]], and [[cyclooctadiene rhodium chloride dimer]].

==References==
{{Reflist}}

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[[Category:Dienes| Diene]]