25.2. Hydrocarbon Rings http://www.ck12.org
carbon atom is above and below the plane. Illustrated below (Figure25.10) are two possibilities for the puckered
cyclohexane molecule. Each of the structures is called a conformation. The conformation on the left is called a boat
conformation, while the one on the right is called a chair conformation.
FIGURE 25.10
The cyclohexane molecule adopts vari-
ous puckered conformations in order to
reduce ring strain. The most stable one
is called the chair conformation and is
shown on the right. The boat conforma-
tion on the left is less stable than the
chair, but it is still preferable to a planar
molecule.While both conformations reduce the ring strain compared to a planar molecule, the chair is preferred. This is
because the chair conformation results in fewer repulsive interactions between the hydrogen atoms. However,
interconversion readily occurs between the two conformations.
Larger cycloalkanes also exist, but five- and six-membered rings are among the most common structures found in
naturally occurring organic compounds. Cyclic hydrocarbons may also be unsaturated. Acycloalkeneis a cyclic
hydrocarbon with at least one carbon-carbon double bond. Acycloalkyneis a cyclic hydrocarbon with at least one
carbon-carbon triple bond. Shown below (Figure25.11) are the simplified structural formulas for cyclohexene and
cyclooctyne.
FIGURE 25.11
Cyclohexene (left) and cyclooctyne
(right).Aromatic Hydrocarbons
Benzene is the parent compound of a large family of organic molecules known as aromatic compounds. Although
benzene was discovered in 1826, it was many years before its structure was fully understood. In 1895, German
chemist August Kekulé (1829-1896) realized that the benzene molecule could be best represented by a six-membered
carbon ring with one hydrogen atom attached to each of the carbon atoms. Unlike cyclohexane, benzene is unsatu-
rated; each carbon atom participates in one double bond. Two different resonance structures with alternating single
and double bonds around the ring can be written for benzene.