Editing Allenes (section)

==Structure and properties==
===Geometry===
[[File:Allene3D.png|thumb|right|3D view of propadiene (allene)]]
The central carbon atom of allenes forms two [[sigma bond]]s and two [[pi bond]]s. The central carbon atom is [[orbital hybridization#sp hybrids|sp-hybridized]], and the two terminal carbon atoms are [[orbital hybridization#sp2 hybrids|sp<sup>2</sup>-hybridized]]. The bond angle formed by the three carbon atoms is 180°, indicating linear geometry for the central carbon atom. The two terminal carbon atoms are planar, and these planes are twisted 90° from each other. The structure can also be viewed as an "extended tetrahedral" with a similar shape to [[methane]], an analogy that is continued into the stereochemical analysis of certain derivative molecules.

===Symmetry===
[[File:Allene symmetry.png|none|300px]]
The symmetry and isomerism of allenes has long fascinated organic chemists.<ref>{{March6th}}</ref> For allenes with four identical substituents, there exist two twofold axes of rotation through the central carbon atom, inclined at 45° to the CH<sub>2</sub> planes at either end of the molecule. The molecule can thus be thought of as a two-bladed [[propeller]]. A third twofold axis of rotation passes through the C=C=C bonds, and there is a mirror plane passing through both CH<sub>2</sub> planes. Thus this class of molecules belong to the D<sub>2d</sub> [[point group]]. Because of the symmetry, an unsubstituted allene has no net [[bond dipole moment|dipole moment]], that is, it is a non-polar molecule.
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{{Multiple image
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| image1    = Allene chirality.png
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| image2    = Allenes_chirality_depiction.png
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| footer    = ''R'' and ''S'' configurations are determined by precedences of the groups attached to the axial section of the molecule when viewed along that axis. The front plane is given higher priority over the other and the final assignment is given from priority 2 to 3 (i.e. the relationship between the two planes).
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An allene with two different substituents on each of the two carbon atoms will be [[chirality (chemistry)|chiral]] because there will no longer be any mirror planes. The chirality of these types of allenes was first predicted in 1875 by [[Jacobus Henricus van 't Hoff]], but not proven experimentally until 1935.<ref>{{cite journal |title= Experimental Demonstration of the Allene Asymmetry |first1= Peter |last1= Maitland |first2= W. H. |last2= Mills |journal= Nature |volume= 135 |pages= 994 |year= 1935 |issue= 3424 |doi= 10.1038/135994a0 |bibcode= 1935Natur.135Q.994M |s2cid= 4085600 |doi-access= free }}</ref> Where '''A''' has a greater priority than '''B''' according to the [[Cahn–Ingold–Prelog priority rules]], the configuration of the [[axial chirality]] can be determined by considering the substituents on the front atom followed by the back atom when viewed along the allene axis. For the back atom, only the group of higher priority need be considered.

Chiral allenes have been recently used as building blocks in the construction of organic materials with exceptional chiroptical properties.<ref>{{cite journal|title=Allenes in Molecular Materials|last1=Rivera Fuentes|first1=Pablo|last2=Diederich|first2=François|journal=Angew. Chem. Int. Ed. Engl.|date=2012|volume=51|issue=12|pages=2818–2828|doi=10.1002/anie.201108001|pmid=22308109|doi-access=free}}</ref> There are a few examples of drug molecule having an allene system in their structure.<ref>{{Cite journal|last1=Celmer|first1=Walter D.|last2=Solomons|first2=I. A.|title=The Structure of the Antibiotic Mycomycin|date=1952|url=http://dx.doi.org/10.1021/ja01127a529|journal=Journal of the American Chemical Society|volume=74|issue=7|pages=1870–1871|doi=10.1021/ja01127a529|bibcode=1952JAChS..74.1870C |issn=0002-7863|url-access=subscription}}</ref>  Mycomycin, an antibiotic with tuberculostatic properties,<ref>{{Cite journal|last=Jenkins|first=D.E.|date=1950|title=Mycomycin: a new antibiotic with tuberculostatic properties.|journal=J Lab Clin Med|volume=36|issue=5|pages=841–2|pmid=14784717}}</ref> is a typical example. This drug exhibits enantiomerism due to the presence of a suitably substituted allene system.

Although the semi-localized textbook [[Sigma-pi separation|σ-π separation]] model describes the bonding of allene using a pair of localized orthogonal π orbitals, the full molecular orbital description of the bonding is more subtle.  The symmetry-correct doubly-degenerate HOMOs of allene (adapted to the D<sub>2d</sub> point group) can either be represented by a pair of orthogonal MOs ''or'' as twisted helical linear combinations of these orthogonal MOs.  The symmetry of the system and the degeneracy of these orbitals imply that both descriptions are correct (in the same way that there are infinitely many ways to depict the doubly-degenerate HOMOs and LUMOs of benzene that correspond to different choices of eigenfunctions in a two-dimensional eigenspace).  However, this degeneracy is lifted in substituted allenes, and the helical picture becomes the only symmetry-correct description for the HOMO and HOMO–1 of the C<sub>2</sub>-symmetric {{ill|2,3-Pentadiene|lt=1,3-dimethylallene|de|2,3-Pentadien}}.<ref>{{Cite journal|last1=H. Hendon|first1=Christopher|last2=Tiana|first2=Davide|last3=T. Murray|first3=Alexander|last4=R. Carbery|first4=David|last5=Walsh|first5=Aron|date=2013|title=Helical frontier orbitals of conjugated linear molecules|journal=Chemical Science|language=en|volume=4|issue=11|pages=4278–4284|doi=10.1039/C3SC52061G|doi-access=free|hdl=10044/1/41564|hdl-access=free}}</ref><ref>{{Cite journal|last1=Garner|first1=Marc H.|last2=Hoffmann|first2=Roald|last3=Rettrup|first3=Sten|last4=Solomon|first4=Gemma C.|author-link4=Gemma Solomon|date=2018-06-27|title=Coarctate and Möbius: The Helical Orbitals of Allene and Other Cumulenes|url=|journal=ACS Central Science|volume=4|issue=6|pages=688–700|doi=10.1021/acscentsci.8b00086|issn=2374-7943|pmc=6026781|pmid=29974064}}</ref> This qualitative MO description extends to higher odd-carbon cumulenes (e.g., 1,2,3,4-pentatetraene).
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=== Chemical and spectral properties ===
Allenes differ considerably from other alkenes in terms of their chemical properties.  Compared to isolated and conjugated dienes, they are considerably less stable: comparing the isomeric pentadienes, the allenic 1,2-pentadiene has a heat of formation of 33.6 kcal/mol, compared to 18.1 kcal/mol for (''E'')-1,3-pentadiene and 25.4 kcal/mol for the isolated 1,4-pentadiene.<ref>{{Cite journal|last=Informatics|first=NIST Office of Data and|title=Welcome to the NIST WebBook|url=https://webbook.nist.gov/index.html.en-us.en|access-date=2020-10-17|website=webbook.nist.gov|year=1997|doi=10.18434/T4D303|language=en}}</ref>

The C–H bonds of allenes are considerably weaker and more acidic compared to typical vinylic C–H bonds: the bond dissociation energy is 87.7 kcal/mol (compared to 111 kcal/mol in ethylene), while the [[Proton affinity|gas-phase acidity]] is 381 kcal/mol (compared to 409 kcal/mol for ethylene<ref>{{Cite book|last=Alabugin|first=Igor V.|url=http://doi.wiley.com/10.1002/9781118906378|title=Stereoelectronic Effects: A Bridge Between Structure and Reactivity|date=2016-09-19|publisher=John Wiley & Sons, Ltd|isbn=978-1-118-90637-8|location=Chichester, UK|language=en|doi=10.1002/9781118906378}}</ref>), making it slightly more acidic than the propargylic C–H bond of propyne (382 kcal/mol).  

The <sup>13</sup>C NMR spectrum of allenes is characterized by the signal of the sp-hybridized carbon atom, resonating at a characteristic 200-220 ppm. In contrast, the sp<sup>2</sup>-hybridized carbon atoms resonate around 80 ppm in a region typical for alkyne and nitrile carbon atoms, while the protons of a CH<sub>2</sub> group of a terminal allene resonate at around 4.5 ppm — somewhat upfield of a typical vinylic proton.<ref>{{Cite book |last1=Pretsch |first1=Ernö |last2=Bühlmann |first2=Philippe |last3=Badertscher |first3=M. |url=https://www.worldcat.org/oclc/405547697 |title=Structure determination of organic compounds : tables of spectral data |date=2009 |publisher=Springer |isbn=978-3-540-93810-1 |edition=Fourth, Revised and Enlarged |location=Berlin |oclc=405547697}}</ref> 

Allenes possess a rich cycloaddition chemistry, including both [4+2] and [2+2] modes of addition,<ref>{{Cite journal|last1=Alcaide|first1=Benito|last2=Almendros|first2=Pedro|last3=Aragoncillo|first3=Cristina|date=2010-01-28|title=Exploiting [2+2] cycloaddition chemistry: achievements with allenes|url=https://pubs.rsc.org/en/content/articlelanding/2010/cs/b913749a|journal=Chemical Society Reviews|language=en|volume=39|issue=2|pages=783–816|doi=10.1039/B913749A|pmid=20111793|issn=1460-4744|hdl=10261/29537|hdl-access=free}}</ref><ref>{{Cite journal|last=Pasto|first=Daniel J.|date=January 1984|title=Recent developments in allene chemistry|url=https://linkinghub.elsevier.com/retrieve/pii/S004040200191289X|journal=Tetrahedron|language=en|volume=40|issue=15|pages=2805–2827|doi=10.1016/S0040-4020(01)91289-X|url-access=subscription}}</ref> as well as undergoing formal cycloaddition processes catalyzed by transition metals.<ref>{{Cite journal|last1=Alcaide|first1=Benito|last2=Almendros|first2=Pedro|date=August 2004|title=The Allenic Pauson−Khand Reaction in Synthesis|url=http://doi.wiley.com/10.1002/ejoc.200400023|journal=European Journal of Organic Chemistry|language=en|volume=2004|issue=16|pages=3377–3383|doi=10.1002/ejoc.200400023|issn=1434-193X|url-access=subscription}}</ref><ref>{{Cite journal|last1=Mascareñas|first1=José L.|last2=Varela|first2=Iván|last3=López|first3=Fernando|date=2019-02-19|title=Allenes and Derivatives in Gold(I)- and Platinum(II)-Catalyzed Formal Cycloadditions|url= |journal=Accounts of Chemical Research|language=en|volume=52|issue=2|pages=465–479|doi=10.1021/acs.accounts.8b00567|issn=0001-4842|pmc=6497370|pmid=30640446}}</ref>  Allenes also serve as substrates for transition metal catalyzed hydrofunctionalization reactions.<ref>{{Cite journal|last1=Zi|first1=Weiwei|last2=Toste|first2=F. Dean|date=2016-08-08|title=Recent advances in enantioselective gold catalysis|url=https://pubs.rsc.org/en/content/articlelanding/2016/cs/c5cs00929d|journal=Chemical Society Reviews|language=en|volume=45|issue=16|pages=4567–4589|doi=10.1039/C5CS00929D|pmid=26890605|issn=1460-4744|url-access=subscription}}</ref><ref>{{Cite journal|last1=Lee|first1=Mitchell|last2=Nguyen|first2=Mary|last3=Brandt|first3=Chance|last4=Kaminsky|first4=Werner|last5=Lalic|first5=Gojko|date=2017-12-04|title=Catalytic Hydroalkylation of Allenes|journal=Angewandte Chemie International Edition|language=en|volume=56|issue=49|pages=15703–15707|doi=10.1002/anie.201709144|pmid=29052303|doi-access=free}}</ref><ref>{{Cite journal|last1=Kim|first1=Seung Wook|last2=Meyer|first2=Cole C.|last3=Mai|first3=Binh Khanh|last4=Liu|first4=Peng|last5=Krische|first5=Michael J.|date=2019-10-04|title=Inversion of Enantioselectivity in Allene Gas versus Allyl Acetate Reductive Aldehyde Allylation Guided by Metal-Centered Stereogenicity: An Experimental and Computational Study|url= |journal=ACS Catalysis|volume=9|issue=10|pages=9158–9163|doi=10.1021/acscatal.9b03695|pmc=6921087|pmid=31857913}}</ref>

Much like acetylenes, electron-poor allenes are unstable.  Tetrachloroallene polymerizes quantitatively to perchloro(1,2-dimethylenecyclobutane) at −50&nbsp;°C.<ref>{{cite journal|doi=10.1002/ange.19630750113|department=Zuschriften [Letters]|lang=de|title=Perchlor-propadien-(1.2), ein hochreaktives Allen|trans-title=Perchloro-1,2,-propadiene, a highly reactive allene|first1=A.|last1=Roedig|first2=G.|last2=M&auml;rkl|first3=B.|last3=Heinrich|page=88|journal=Angewandte Chemie|volume=75|year=1963|issue=1|bibcode=1963AngCh..75...88R }}</ref>

Cyclic allenes with fewer than 10 ring atoms are [[ring strain|strained]].  Those with fewer than 8 atoms generally only form unstable [[aryne]]-like intermediates.<ref>{{cite journal|journal=Pure Appl. Chem.|volume=78|issue=2|page=451|year=2006|doi=10.1351/pac200678020451|publisher=[[IUPAC]]|title=Synthesis and reactivity of new strained cyclic allene and alkyne precursors|first1=Diego|last1=Peña|first2=Beatriz|last2=Iglesias|first3=Iago|last3=Quintana|first4=Dolores|last4=Pérez|first5=Enrique|last5=Guitián|first6=Luis|last6=Castedo}}</ref><ref name=BicycleThesis>{{cite thesis|url=https://etd.lib.metu.edu.tr/upload/3/12610317/index.pdf|title=Bicyclic strained allenes|institution=[[Middle East Technical University]]|first=Benan|last=Kilba&scedil;|type=PhD|date=Jan 2009}}</ref>{{rp|pp=6&ndash;7}}  The latter are sometimes stabilized by [[non-Kekule molecule|diradical]] or [[ylidic]] resonance structures.<ref name=BicycleThesis/>{{rp|pp=14&ndash;15}}<ref>{{cite journal |last1=Kelleghan |first1=Andrew&nbsp;V. |last2=Tena&nbsp;Meza |first2=Arismel |last3=Garg |first3=Neil&nbsp;K. |date=2023-11-09 |title=Generation and reactivity of unsymmetrical strained heterocyclic allenes |journal=Nat. Synth. |volume=3 |issue=3 |pages=329–336 |doi=10.1038/s44160-023-00432-1 |pmc=11031199 |pmid=38645473}}</ref>