Cyclopentane and Stereoisomerism

In many ways the least complicated cyclic hydrocarbon is cyclopentane. If the carbon atoms in this molecule lie in a plane, they form a nearly perfect pentagon (Fig. 1-24); in fact, the usual symbol for cyclopentane is a pentagon. The internal angles of a regular pentagon are, 108°, almost the normal tetrahedral angle (109°28').

Cyclopentane and n-pentane are similar, except that the cyclic compound has two less hydrogen atoms.

Although both are nearly inert chemically, they react with halogens in the presence of light through the substitution of one or more hydrogen atoms. Since the cyclic structure confers a high degree of symmetry on the molecule, only one monochlorocyclopentane is possible. Here cyclopentane differs from n-pentane, which can produce three monochloro derivatives.

Figure 1-24. Cyclopentane hardly differs chemically from alkanes, as shown here by its reaction with chlorine and light. The symbol for a cyclic hydrocarbon is a regular polygon with as many corners as there are atoms in the ring. Unless otherwise specified, enough hydrogen atoms are understood to be attached to these carbon atoms so that each carbon has a valence of four.

Cyclic molecules provide a nice opportunity to illustrate a new type of isomerism, called stereo isomerism. Stereoisomers are different compounds having the same structural formula, which means the same kinds of atoms connected in the same sequences. Stereoisomers differ in the arrangement of the atoms in space. Consider, for example, the possible isomers of dichlorocyclopentane. Both of the chlorine atoms may become attached to the same carbon atom to give 1,1-dichlorocyclopentane, or they may be on adjacent carbon atoms as in 1,2-dichlorocyclopentane, or they may be in the 1,3-position. Now notice that in the 1,2-case the two chlorines may be on the same side of the ring (cis) or on opposite sides (trans) (Fig. 1-25). The same is true of 1,3-dichlorocyclopentane. The two forms of 1,2-dichlorocyclopentane are stereoisomers of one another, and the two forms of 1,3-dichlorocyclopentane are stereoisomers of one another. This type of isomerism should be clearly distinguished from structural isomerism, in which the isomers differ from one another in the order in which the atoms are joined together. For example, 1,1-dichlorocyclopentane is a structural isomer of any of the other four dichlorocyclopentanes.

Stereoisomers are much more than an obscure curiosity. They occur in all but the simplest structures, and are of particularly great importance in organic molecules of biological origin. We shall see other types of stereoisomers in later chapters.

Figure 1-25. As well as structural isomerism, the dichlorocyclopentanes exhibit stereoisomerism, since both the 1,2-and 1,3-isomers can exist in cis and trans forms.

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