The Foundations of Chemistry

Cycloalkanes

The cyclic saturated hydrocarbons, or cycloalkanes,have the general formula CnH 2 n. The

cycloalkanes (and other ring compounds that we will encounter later) are often shown in

simplified skeletal form in which each intersection of two lines represents a C atom; we

mentally add enough H atoms to give each carbon atom four bonds. The first four unsub-

stituted cycloalkanes and their simplified representations are

In some of these structures, the bond angles are somewhat distorted from the ideal

tetrahedral angle of 109.5°, the most severe distortions being 60° in cyclopropane and 90°

in cyclobutane. As a result, these rings are said to be “strained,” and these two compounds

are unusually reactive for saturated hydrocarbons. Cyclopentane is stable with a nearly

flat ring, because the bond angles in a regular pentagon (108°) are near the tetrahedral

angle (109.5°).

Cyclohexane is not stable as a flat ring. The bond angles of the flat ring would be 120°,

not the 109.5°, of an sp^3 -hybridized carbon. To avoid the strain, the ring “puckers” and

becomes nonplanar (Figures 27-7 and 27-8).

CH 2

CH 2

CH 2

CH 2

H 2 C

H 2 C

CH 2

H 2 C CH 2

CH 2 H 2 CCHH 2 C 2

CH 2

H 2 C

CH 2

CH 2

H 2 C

cyclopropane cyclobutane cyclopentane cyclohexane

Noncyclic compounds are sometimes
referred to as acyclic compounds.

104 8CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

NAMING SATURATED HYDROCARBONS

It is important to realize that many compounds (and their names) were so familiar to

chemists before the development of the IUPAC system (beginning about 1890) that they

continued to be called by their common, or “trivial,” names. In this and the next chapter,

IUPAC names of compounds appear in blue type, and their common alternative names

are shown in black type.

27-2

Figure 27-7 Stereoview of a nonplanar form of cyclohexane.

Figure 27-8 A model showing a

nonplanar arrangement of cyclohexane.

In skeletal representations, each
intersection represents a carbon with
any attached hydrogens. (See Section
27-3 for more details on writing
skeletal representations.)