Bicyclic molecule
Encyclopedia
A bicyclic molecule is a molecule that features two fused rings
Cyclic compound
In chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...

. Bicyclic molecules occur widely in organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 and inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

s.

Fusion of the rings can occur in three ways:
  1. Across a bond between two atoms - for example, decalin (also known as bicyclo[4.4.0]decane), has a C-C bond shared between two cyclohexane
    Cyclohexane
    Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...

     rings
  2. Across a sequence of atoms (bridgehead) - for example, norbornane
    Norbornane
    Norbornane is an organic compound and a saturated hydrocarbon with chemical formula C7H12. It is a crystalline compound with melting point 88 °C. The carbon skeleton is a cyclohexane ring bridged by a methylene group in the 1,4- position, and is a bridged bicyclic compound...

     (also known as bicyclo[2.2.1]heptane), can be viewed as a pair of cyclopentane
    Cyclopentane
    Cyclopentane is a highly flammable alicyclic hydrocarbon with chemical formula 510 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occurs as a colorless liquid with a petrol-like odor. Its melting point is −94 °C...

     rings that share three of the five carbon atoms
  3. At a single atom (spirocyclic, forming a spiro compound
    Spiro compound
    A spiro compound is a bicyclic organic compound with rings connected through just one atom. The rings can be different in nature or identical. The connecting atom is also called the spiroatom, most often a quaternary carbon...

    )

Singly fused rings are the most common, and spiro rings are the least common.

Bridges

A bridge is an unbranched chain of atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s or an atom or a covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 connecting two bridgeheads in a polycyclic compound
Polycyclic compound
In organic chemistry, a polycyclic compound is a cyclic compound with more than one hydrocarbon loop or ring structures . In general, the term includes all polycyclic aromatic compounds, including the polycyclic aromatic hydrocarbons, the heterocyclic aromatic compounds containing sulfur,...

.

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.

Nomenclature

Bicyclic molecules have a strict nomenclature
IUPAC nomenclature of organic chemistry
The IUPAC nomenclature of organic chemistry is a systematic method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry . Ideally, every possible organic compound should have a name from which an unambiguous structural formula can be drawn. ...

. On its simplest level the parent hydrocarbon is the equivalent open-carbon alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...

. 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
Heptane
n-Heptane is the straight-chain alkane with the chemical formula H3C5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale...

. 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 (because the spiro atom itself is not counted) meeting at the spiro atom.
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