Bicyclo2-1-1heptane.gifA bicyclic molecule contains two fused aliphatic rings. Fusion can occur at a single atom (spirocyclic), at two mutually bonded atoms or across a sequence of atoms (bridgehead). All these systems occur frequently in naturally-occurring organic compounds. An example is norbornane (bicyclo^*heptane).

Bicyclic molecules have a strict nomenclature.IUPAC nomenclature for bicyclics On its simplest level the parent hydrocarbon is the equivalent open carbon alkane. In front of it is added the prefix bicyclo and between brackets and in descending order the number of carbon atoms in each of the rings not counting the bridgeheads. For example in bicyclothe carbon frame contains a total of 7 atoms hence the parent name heptane. This molecule has three bridges having 2, 2 and 1 carbon atoms hence the prefix bicyclo[2,2.1.

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