Bicyclic molecule
A bicyclic molecule is a molecule that features just two rings. Bicyclic molecules occur widely in organic and inorganic compounds. Among organic compounds, the two rings can be fully carbocyclic (all atoms in each ring all carbons), or one or both can be heterocyclic (at least one atom in the rings not a carbon atom). Moreover, the two rings can both be aliphatic, or can be aromatic, or a combination of aliphatic and aromatic (cf. decalins and naphthalenes, norbornanes and cyclophanes, aliphatic spiro compounds and spiroaromatics, and biphenyl and cyclohexylbenzene, as aromatic and mixed examples).
Joining of the rings can occur in four ways:
in fused ring compounds, across a bond between two atoms—for example, decalin, or bicyclo[4.4.0]decane, has a carbon-carbon bond in common between the two "fused" cyclohexane rings ;
in bridged compounds, across a series of atoms, or even one atom or a valence bond, that span between "bridgehead" atoms—for example, norbornane, or bicyclo[2.2.1]heptane, can be thought of as a pair of cyclopentane rings that share three of each of their five carbon atoms; or