An organic compound that contains one or more closed rings of carbon atoms. The term alicyclic specifically excludes carbocyclic compounds with an array of π-electrons characteristic of aromatic rings. Compounds with one to five alicyclic rings of great variety and complexity are found in many natural products such as steroids and terpenes. By far the majority of these have six-membered rings. See also Aromatic hydrocarbon; Steroid.
Structures and nomenclature
The bonding in cyclic hydrocarbons is much the same as that in open-chain alkanes and alkenes. An important difference, however, is the fact that the atoms in a ring are part of a closed loop. Complete freedom of rotation about a carbon-carbon bond (CC) is not possible; the ring has faces or sides, like those of a plate.
Simple monocyclic hydrocarbons are usually represented as bond line structures; for example, cyclopropane (1a) is usually represented as structure (1b) and methylcycloheptane as structure (2). These hydrocarbons are named by adding the prefix
In bicyclic compounds the rings can be joined in three ways: spirocyclic, fused, and bridged, as illustrated in the structures for spiro[4.5]decane (5), bicyclo[4.3.0]nonane (6), and bicyclo[2.2.1]heptane (7). In each case the name indicates the total
Any of these bicyclic systems can be transformed to a tricyclic array by introduction of another bond between nonadjacent carbons, as in structure (8), or an additional ring, as in structure (9).
The boiling points, melting points, and densities of cycloalkanes are all higher than those of their open-chain counterparts, reflecting the more compact structures and greater association in both liquid and solid. Geometrical constraints in the smaller rings have significant effects on reactions of cycloalkanes and derivatives. Because of ring strain, ring-opening reactions of cyclopropane, such as isomerization to propene, take place under conditions that do not affect alkanes or larger-ring cycloalkanes.
Comparison of reaction rates of compounds with different ring size or cis-trans configuration has provided important insights about reaction mechanism and conformational analysis. See also Organic reaction mechanism.
Ring-forming reactions
A number of useful methods have been devised that lead to alicyclic rings. These reactions are of three types: C-C bond formation between atoms in an open-chain precursor, cycloaddition or cyclooligomerization, and expansion or contraction of a more readily available ring. In cycloaddition, two molecules react with formation of two bonds; in cyclooligomerization, three or more molecules combine to form three or more bonds.
Two alicyclic compounds are manufactured in large volume; both are products of the petroleum industry. Cyclopentadiene (12) is formed from various alkylcyclopentanes in naphtha fractions during the refining process. It is a highly reactive diene and spontaneously dimerizes in a 4 + 2 cycloaddition [see reaction below]; it is used as a copolymer in several resins.
Cyclohexane is produced in large quantity by hydrogenation of benzene. The principal use of cyclohexane is conversion by oxidation in air to a mixture of cyclohexanol and the ketone, which is then oxidized further to adipic acid for the manufacture of nylon. See also Organic synthesis.
