Organic Chemistry: Carbocycles: Introduction to Cycloalkanes

Nomenclature

Carbocycles are organic molecules that contain one or more rings, chains of atoms that loop back on themselves. The simplest cyclic molecules are the cycloalkanes, which have molecular formulas C_nH₂n. Cycloalkanes are named after their corresponding linear alkanes with the prefix -cyclo. Cycloalkanes can be drawn as regular polygons using line-angle representations.

Substituted cycloalkanes are named similarly to linear alkanes, as the following examples illustrate. The positions on the ring are numbered in such a way that substituents receive the lowest possible numberings. Since all positions on the ring are equivalent except for the attachment of substituents, numbers are only indicated in the name of the compound when more than one substituent is present.

Figure %: Nomenclature of substituted cycloalkanes.

Cis-trans Isomerism in Cycloalkanes

Like alkenes, cycloalkanes are capable of cis-trans isomerism. A cycloalkane has two distinct faces, and any substituent on a ring lies toward one of two faces. When two substituents on a ring point to the same face, they are cis. When the two substituents point to opposite faces, they are trans. Like the cases of cis-trans isomerism in alkenes, these isomers have the same atomic connectivities but differ in their spatial arrangement of atoms. Hence, they are stereoisomers.

Figure %: *Cis*- and *trans*-1,2-dimethylcyclohexane

Ring Strain

The heat of formation of a molecule is the energy change that occurs when a molecule is assembled from its component atoms. Heats of formations typically have negative signs, indicating that the molecule is more stable than its component atoms. First, consider the heats of formations of the n-alkanes, which advance regularly by -4.95 kcal/mol for each increase in chain length. Since each unbranched alkane differs from the next in the series by a methylene (-CH₂-) group, we infer that - 4.95 is the heat of formation associated with each methylene group. The cycloalkanes, which have molecular formulas of (CH₂)_n, consist of methylene groups arranged in a ring. Hence we might expect the heat of formation of any n carbon cycloalkane to be n times -4.95.

Figure %: Expected and actual heats of formation for some cycloalkanes.

In every case except cyclohexane, the actual heat of formation is less negative than the predicted value. That is, cycloalkanes are less stable than their straight-chain counterparts due to ring strain, unfavorable energetics caused by ring formation. Rings strains can be calculated from the difference between actual and expected heats of formation. Both cyclopropane and cyclobutane have large ring strains of 27 kcal/mol and 26 kcal/mol, respectively. Cyclopentane has much less ring strain at 6.5 kcal/mol. Cyclohexane is the only cycloalkane that has no ring strain. Cycloheptane and higher cycloalkanes tend to have modest amounts of ring strain (although strain diminishes for very large rings, where the length of the ring allows atoms to arrange themselves in low-energy conformations).

Bridged ring systems are particularly rigid due to ring strain. In a bridged system, bridgehead carbons are the points at which the two cycles meet. These carbons are nearly always singly bonded, or sp³-hybridized. Forming a Π bond would require sp² hybridization and a trigonal planar geometry that would be terribly strained in the context of the ring constraints. This concept is summarized by Bredt's Rule: No bridgehead alkenes.