Platonic hydrocarbons are the molecular representation of platonic solid geometries with vertices replaced by carbon atoms and with edges replaced by chemical bonds. Not all platonic solids have an organic molecular counterpart:
- The tetravalency (4-connectedness) of carbon excludes an icosahedron since 5 edges meet at each vertex; pentacoordinate carbon, such as CH5+, is unlikely, although both icosahedrane and octahedrane have been observed in boron compounds.
- angle strain prevents formation of an octahedron, and since 4 edges meet at each corner, there would be no hydrogen atoms, thus this hypothetical octahedral molecule would be an allotrope of elemental carbon, C6, not a hydrocarbon. The existence of octahedrane cannot be ruled out completely, although calculations have shown that it is unlikely.
The following platonic hydrocarbons, on the other hand, have been synthesised:
Tetrahedrane (C4H4) has not yet been synthesized, but it is predicted to be kinetically stable in spite of the acute bond angle and consequent angle strain. Tetrahedral hydrocarbons, including tetrahedrane with tert-butyl substituents and with trimethylsilyl substituents, have been produced.
With increasing number of carbon atoms in the frame, the geometry more closely approximates a sphere, and the space enclosed in the carbon "cage" increases. This trend continues with buckyballs or spherical fullerene (C60). Although not a Platonic hydrocarbon, fullerene has the shape of a truncated icosahedron, an Archimedean solid.
References & notes
- Henning Hopf, Classics in Hydrocarbon Chemistry, Wiley VCH, 2000.
- Lewars, Errol G. (2008). Modeling Marvels: Computational Anticipation of Novel molecules. Springer Science+Business Media. pp. 81–82. doi:10.1007/978-1-4020-6973-4. ISBN 978-1-4020-6972-7. Retrieved January 30, 2012.
- Nevertheless, note the existence of [184.108.40.206]paddlane C(CH2)4C, CAS number 102843-69-6 and the theoretical existence of pyramidane which is a half octahedron with one carbon on a vertex like those of octahedron.
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