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A bicyclic molecule is a molecule that features just two rings. Bicyclic molecules occur widely in organic and inorganic compounds.[according to whom?] 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
- in spiro compounds, at a single atom, the spiro atom—for example, the two three-carbon cyclopropane rings of spiro[2.2]pentane connect at the one atom shared by the rings; and
- in compounds containing two isolated rings of any type, e.g., biphenyl and cyclohexylbenzene.
Singly fused rings are the most common, and spiro rings are the least common.[according to whom?]
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Bicyclic molecules have a strict nomenclature.[full citation needed][better source needed] On its simplest level the parent hydrocarbon is the equivalent open-carbon alkane. For bridged compounds, the prefix bicyclo is added, followed by, between brackets, separated by periods, and, in descending order, the number of carbon atoms between each of 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]. For spiro compounds, terms like spiro[2.4] are used, indicating that there are 3- and 5-membered rings meeting at the spiro atom (3 and 5 because the spiro atom itself is not counted).
The following are principle sources for the developing content in this article, that contain further information that may be of interest to readers. In the books listed, the page numbers provided are for some of the chapters or sections of the book that cover spiro compounds.
- A high quality introductory to intermediate textbook of organic chemistry: Clayden, Jonathan ; Greeves, Nick & Warren, Stuart (2012). Organic Chemistry (2nd ed.). Oxford, UK: Oxford University Press. pp. 319f, 432, 604np, 653, 746int, 803ketals, 839, 846f. ISBN 0199270295. Retrieved 2 February 2016.
- The IUPAC documents on naming of fused and bridged fused cyclic compounds: Moss, G.P. and the Working Party of the International Union of Pure and Applied Chemistry [IUPAC], Organic Chemistry Division, Commission on Nomenclature of Organic Chemistry (III.1) (1998). "Nomenclature of fused and bridged fused ring systems (IUPAC Recommendations 1998)" (PDF). Pure Appl. Chem. 70 (1): 143–216. doi:10.1351/pac199870010143. ISSN 1365-3075. Retrieved 7 March 2016. The full author (Working Party) list and a link to a German translation are provided in a corresponding footnote. Also available online at "Extension and Revision of the Nomenclature for Spiro Compounds". London, GBR: Queen Mary University of London., same access date.