Editing Allenes (section)

=== 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}}</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}}</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}}</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|p=88|journal=Angewandte Chemie|volume=75|year=1963|issue=1}}</ref>