Bicyclic molecule

A bicyclic molecule (from bi 'two' and cycle 'ring') is a molecule that features two joined rings.[1] Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (all of the ring atoms are carbons), or heterocyclic (the rings' atoms consist of at least two elements), like DABCO.[2] Moreover, the two rings can both be aliphatic (e.g. decalin and norbornane), or can be aromatic (e.g. naphthalene), or a combination of aliphatic and aromatic (e.g. tetralin).

Three modes of ring junction are possible for a bicyclic compound:[3]

  • In spiro compounds, the two rings share only one single atom, the spiro atom, which is usually a quaternary carbon.[4] An example of a spirocyclic compound is the photochromic switch spiropyran.
  • In fused/condensed[5][6] bicyclic compounds, two rings share two adjacent atoms. In other words, the rings share one covalent bond, i.e. the bridgehead atoms are directly connected (e.g. α-thujene and decalin).
  • In bridged bicyclic compounds, the two rings share three or more atoms, separating the two bridgehead atoms by a bridge containing at least one atom. For example, norbornane, also known as bicyclo[2.2.1]heptane, can be viewed as a pair of cyclopentane rings each sharing three of their five carbon atoms. Camphor is a more elaborate example.
  1. ^ Smith, Michael B. (2011). Organic Chemistry: An Acid—Base Approach. CRC Press. ISBN 978-1-4200-7921-0.
  2. ^ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Heterocyclic compounds". doi:10.1351/goldbook.H02798
  3. ^ Sorrell, Thomas N. (2006). Organic Chemistry. University Science Books. ISBN 978-1-891389-38-2.
  4. ^ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Spiro compounds". doi:10.1351/goldbook.S05881
  5. ^ Wang, Zhendi; Yang, Chun; Yang, Zeyu; Brown, Carl E.; Hollebone, Bruce P.; Stout, Scott A. (2016). "Petroleum biomarker fingerprinting for oil spill characterization and source identification". Standard Handbook Oil Spill Environmental Forensics. pp. 131–254. doi:10.1016/B978-0-12-803832-1.00004-0. ISBN 978-0-12-803832-1.
  6. ^ Speight, James G. (2017). "Organic Chemistry". Environmental Organic Chemistry for Engineers. pp. 43–86. doi:10.1016/B978-0-12-804492-6.00002-2. ISBN 978-0-12-804492-6.
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