Prismanes

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:

Triprismane, tetraprismane, and pentaprismane have been synthesized and studied experimentally. As of 1994, hexaprismane and higher members have not been successfully synthesized.

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.

Hexaprismane and octaprismane are expected to be kinetically stable.

The prismanes have also been studied in terms of the aromatic or antiaromatic nature of the cage. Hexaprismane in particular would not be thermodynamically stable because it is the dimer of highly stable benzene.