Bicyclic molecule
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![spiro[2.2]pentane, a spirocyclic molecule](../../../upload/shared/1/17/Spiro-2.2-pentane.gif)
![bicyclo[2,2,1]heptane](../../../upload/shared/thumb/1/16/Bicyclo2-1-1heptane.gif/180px-Bicyclo2-1-1heptane.gif)
A 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[2.2.1]heptane).
Bicyclic molecules have a strict nomenclature.[1] 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 bicyclo[2,2,1]heptane the 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].
A bridge is an unbranched chain of atoms or an atom or a covalent bond connecting two bridgeheads in a polycyclic compound.
The main bridge is a bridge which connects the two main bridgeheads.
A secondary bridge is any bridge not included in the main ring or the main bridge.
An independent secondary bridge links bridgeheads which are part of the main ring or main bridge.
A dependent secondary bridge links at least one bridgehead which is part of a secondary bridge.
