Prismanes

{{Short description|Class of prism-shaped hydrocarbons}} {{distinguish|text=Prismane C8}}

The '''prismanes''' are a class of hydrocarbon compounds consisting of prism-like polyhedra of various numbers of sides on the polygonal base. Chemically, it is a series of fused cyclobutane rings (a ladderane, with all-cis/all-syn geometry) that wraps around to join its ends and form a band, with cycloalkane edges. Their chemical formula is (C₂H₂)_n, where ''n'' is the number of cyclobutane sides (the size of the cycloalkane base), and that number also forms the basis for a system of nomenclature within this class. The first few chemicals in this class are:

{|class="wikitable" |+ !Structures !80px !80px !80px !80px |- !Common name |Prismane<br>[3]Prismane<br>Triprismane |Cubane (preferred)<br>[4]Prismane<br>Tetraprismane | [5]Prismane<br>Pentaprismane | [6]Prismane<br>Hexaprismane |- !Chemical formula | (C₂H₂)₃<br />C₆H₆ | (C₂H₂)₄<br />C₈H₈ | (C₂H₂)₅<br />C₁₀H₁₀ | (C₂H₂)₆<br />C₁₂H₁₂ |- !IUPAC nomenclature |tetracyclo<wbr>[2.2.0.0^2,6.0^3,5]<wbr>hexane |pentacyclo<wbr>[4.2.0.0^2,5.0^3,8.0^4,7]<wbr>octane |hexacyclo<wbr>[4.4.0.0^2,5.0^3,9.0^4,8.0^7,10]<wbr>decane |heptacyclo<wbr>[6.4.0.0^2,7.0^3,6.0^4,11.0^5,10.0^9,12]<wbr>dodecane |- !3D models (PubChem) |[https://pubchem.ncbi.nlm.nih.gov/compound/Prismane#section=3D-Conformer&fullscreen=true triprismane] |[https://pubchem.ncbi.nlm.nih.gov/compound/Cubane#section=3D-Conformer&fullscreen=true cubane] |[https://pubchem.ncbi.nlm.nih.gov/compound/138295#section=3D-Conformer&fullscreen=true pentaprismane] |[https://pubchem.ncbi.nlm.nih.gov/compound/53438605#section=3D-Conformer&fullscreen=true hexaprismane] |}

Triprismane, tetraprismane, and pentaprismane have been synthesized and studied experimentally. {{As of|1994|}}, hexaprismane and higher members are have not been successfully synthesized.<ref name="[n]prismanes">{{cite journal|doi=10.1080/00304949409458427|publisher=Taylor & Francis|journal=Organic Preparations and Procedures International|title=The synthesis and reactions of prismanes|first1=Mark&nbsp;A.|last1=Forman|first2=William&nbsp;P.|last2=Dailey|volume=26|issue=3|year=1994}}</ref>

The geometries of the unknown members have been studied using computer models. Initially, they do have the geometry of a regular prism, with flat ''n''-gon bases. As ''n'' becomes increasingly large, however, the highly symmetric geometry is expected to be unfavorable, with the molecule distorting into less-symmetric forms. One series of models suggests that the transition occurs at [11]prismane. For example, the structure of [12]prismane would have the cyclobutane chain twisted, with the dodecagonal bases non-planar and non-parallel.<ref>{{cite journal |title= Quantum-chemical investigation of structure and stability of [''n'']-prismanes and [''n'']-asteranes |first1= T. N. |last1= Gribanova |first2= R. M. |last2= Minyaev |first3= V. I. |last3= Minkin |journal= Russian Journal of Organic Chemistry |volume= 43 |issue= 8 |year= 2007 |pages= 1144–1150 |doi= 10.1134/S107042800708009X |s2cid= 97458519 }}</ref><ref>{{cite book |title= CRC Handbook of Organic Photochemistry and Photobiology, Volumes 1 & 2, Second Edition |editor-first1= William H. |editor-last1= Horspool |editor-first2= Francesco |editor-last2= Lenci |publisher= CRC Press |year= 2003 |isbn= 9780203495902 |chapter= 23. Photochemical Approaches to the Synthesis of [n]Prismanes |first1= Terou |last1= Shinmyozu |first2= Riki |last2= Nogita |first3= Motoki |last3= Akita |first4= Chultack |last4= Lim }}</ref> <ref>{{cite journal |title= The geometries of pentaprismane and hexaprismane insights from molecular mechanics |first1= Norman L. |last1= Allinger |authorlink1= Norman Allinger |first2= Philip E. |last2= Eaton |authorlink2= Philip Eaton |journal= Tetrahedron Letters |volume= 24 |issue= 35 |year= 1983 |pages= 3697–3700 |doi= 10.1016/S0040-4039(00)94512-X }}</ref>

Hexaprismane and octaprismane are expected to be kinetically stable.<ref>{{cite journal |last1= Shostachenko |first1= S. A. |last2= Maslov |first2= M. M. |last3= Prudkovskii |first3= V. S. |last4= Katin |first4= K. P. |title= Thermal stability of hexaprismane C12H12 and octaprismane C16H16 |journal= Physics of the Solid State |date= 2015 |volume= 57 |issue= 5 |pages= 1023–1027 |doi= 10.1134/S1063783415050261 }}</ref>

The prismanes have also been studied in terms of the aromatic or antiaromatic nature of the cage.<ref>{{cite journal |last1= Alonso |first1= Mercedes |last2= Poater |first2= Jordi |last3= Solà |first3= Miquel |title= Aromaticity changes along the reaction coordinate connecting the cyclobutadiene dimer to cubane and the benzene dimer to hexaprismane |journal= Structural Chemistry |date= 2007 |volume= 18 |issue= 6 |pages= 773–783 |doi= 10.1007/s11224-007-9240-4 |doi-access= free }}</ref> Hexaprismane in particular would not be thermodynamically stable because it is the dimer of highly stable benzene.

== Nonconvex prismanes == {{multiple image |total_width= 300 |image1= helvetane.png |width1= 580 |height1= 580 |image2= Israelane.png |width2= 559 |height2= 593 |footer= Helvetane (left) and Israelane }} For large base-sizes, some of the cyclobutanes can be fused anti to each other, giving a non-convex polygon base. These are geometric isomers of the prismanes. Two isomers of [12]prismane that have been studied computationally are named helvetane and israelane, based on the star-like shapes of the rings that form their bases.<ref>{{cite journal |title= Structures and Properties of Closed Ladderanes C₂₄H₂₄, Laddersilanes Si₂₄H₂₄, and Their Nitrogen-Containing Isoelectronic Equivalents: A G3(MP2) Investigation |first1= Xin |last1= Wang |first2= Kai-Chung |last2= Lau |first3= Wai-Kee |last3= Li |journal= J. Phys. Chem. A |year= 2009 |volume= 113 |issue= 14 |pages= 3413–3419 |doi= 10.1021/jp900161s |pmid= 19296633 |bibcode= 2009JPCA..113.3413W }}</ref> This was explored computationally after originally being proposed as an April fools joke. Their names refer to the shapes found on the flags of Switzerland and Israel, respectively. {{clear}}

== Polyprismanes == [[File:Poly-n-prismane.png|thumb|left|Bi[5]prismane (left) and tri[4]prismane]] The polyprismanes consist of multiple prismanes stacked base-to-base.<ref>{{cite journal |title= Poly[''n'']prismanes: A Family of Stable Cage Structures with Half-Planar Carbon Centers |first1= Ruslan M. |last1= Minyaev |first2= Vladimir I. |last2= Minkin |first3= Tatyana N. |last3= Gribanova |first4= Andrey G. |last4= Starikov |first5= Roald |last5= Hoffmann |authorlink5= Roald Hoffmann |journal= J. Org. Chem. |year= 2003 |volume= 68 |issue= 22 |pages= 8588–8594 |doi= 10.1021/jo034910l |pmid= 14575490 }}</ref> The carbons at each intermediate level—the ''n''-gon bases where the prismanes fuse to each other—have no hydrogen atoms attached to them. {{clear}}

==Related structures== {{multiple image |total_width= 300 |image1= 3Asterane.png |width1= 478 |height1= 515 |image2= 4Asterane.png |width2= 466 |height2= 370 |footer= [3]Asterane (left) and [4]asterane }} The asteranes contain a methylene group bridge on each edge between the two ''n''-gon bases. Each side is thus a cyclohexane rather than a cyclobutane.

A substituted compound based on hexasilaprismane, the all-silicon analog of triprismane, has been synthesized.<ref>{{Cite journal| doi = 10.1021/ja00066a075| title = Chemistry of Organosilicon Compounds. 303. The "Missing" Hexasilaprismane: Synthesis, X-Ray Analysis and Photochemical Reactions| year = 1993| last1 = Sekiguchi | first1 = A.| last2 = Yatabe | first2 = T.| last3 = Kabuto | first3 = C.| last4 = Sakurai | first4 = H.| journal = Journal of the American Chemical Society| volume = 115| issue = 13| pages = 5853–5854 }}</ref> Theoretical analysis suggests that the prismane form of Si₆H₆ is more stable than the aromatic-ring or Dewar benzene-like isomers of Si₆H₆.<ref>{{cite journal |last1= Sax |first1= Alexander |last2= Janoschek |first2= Rudolf |title= Si₆H₆: Is the Aromatic Structure the Most Stable One? |journal= Angewandte Chemie International Edition in English |date= 1986 |volume= 25 |issue= 7 |pages= 651–652 |doi= 10.1002/anie.198606511 }}</ref><ref>{{cite journal |last1= Janoschek |first1= R. |title= Si6H6 revisited: The hexasilaprismane-to-hexasilabenzene and hexasila-dewarbenzene interconversion |journal= Journal of Inorganic and Organometallic Polymers |date= 1995 |volume= 5 |issue= 2 |pages= 155–161 |doi= 10.1007/BF01058143 }}</ref>

{{clear}}

==References== {{reflist|2}}

Category:Molecular geometry Category:Polycyclic nonaromatic hydrocarbons