Cycloalkanes: Introduction, Structure and Physical Properties

Cycloalkanes: Polycyclic Alkanes

contain one or more rings. If the rings are fused, the compounds are called bi-, tri-, tetra- or polycyclic alkanes representing condensed and bridged systems as well as spiro compounds. Clicking on the structures in the following table will lead to three-dimensional representations of these compounds.

Condensed ring
Bridged ring
Spiro compound

In condensed and bridged ring systems, two rings share two carbon atoms which are called bridgehead atoms.

Fig.1

The nomenclature for bicyclic alkanes is similar to that of cycloalkanes in that the total number of carbon atoms determines the basic name. For example, decalin has ten carbon atoms and the basic name therefore is decane.

Fig.2
Decalin

Numbering of the molecule is based on the number of atoms in each of the linkages connected to the two bridgehead atoms which in the case of decalin are 4,4 and 0.

Fig.3

The systematic name of decalin is arrived at by adding the prefix bicyclo to the basic name. Additionally, the number of atoms in each of the linkages is shown in square brackets and placed between prefix and basic name. The number of carbon atoms in the bridge between the bridgehead atoms is represented by the third number in the bracket . Since the bridge in decalin shown in green does not contain a C atom, the systematic name for decalin is bicyclo[4.4.0]decane.

A characteristic feature of polycyclic alkanes is their large structural diversity. Some of these compounds are of special interest.

Fig.4
Adamantane possesses a diamond-like structure in the . This is the most stable configuration for a $sp3$-hybridized carbon. Dodecahedrane is one of the platonic hydrocarbons. It was synthesized only in 1982 and the synthesis involved over 23 steps. The melting point of 430°C is very high for a C20 hydrocarbon.