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 (CH), 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, and many higher members of the series have been studied using computer models. The first several members do indeed have the geometry of a regular prism, with flat n-gon bases. As n becomes increasingly large, however, modeling experiments find that highly symmetric geometry is no longer stable, and the molecule distorts into less-symmetric forms. One series of modelling experiments found that starting with [11]prismane, the regular-prism form is not a stable geometry. For example, the structure of [12]prismane would have the cyclobutane chain twisted, with the dodecagonal bases non-planar and non-parallel.