A bicyclic molecule is a molecule that features two fused rings. Bicyclic molecules occur widely in organic and inorganic compounds.
Fusion of the rings can occur in three ways:
#Across a bond between two atoms - for example, decalin (also known as bicyclo()decane), has a C-C bond shared between two cyclohexane rings
#Across a sequence of atoms (bridgehead) - for example, norbornane (also known as bicyclo()heptane), can be viewed as a pair of cyclopentane rings that share three of the five carbon atoms
#At a single atom (spirocyclic, forming a spiro compound)
Singly fused rings are the most common, and spiro rings are the least common.
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 that 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 that are part of the main ring or main bridge.
A ''dependent secondary bridge'' links at least one bridgehead that is part of a secondary bridge.
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