Since the isomeric 2-butencs diiTer from one another only in the way the atoms
are oriented in space (but are like one another with respect to which atoms are
attached to which other atoms), they belong to the general class we have called
stereoisomers (Sec. 4.1). They are not, however, mirror images of each other, and
hence are not enantiomers. As we have already said, stereoisomers that are not
mirror images of each other are called diascereomers.
The particular kind of diastereomers that owe their existence to hindered
rotation about double bonds are called geometric isomers. The isomeric 2-butenes,
then, are diastereomers, and more specifically, geometric isomers.
We recall that the arrangement of atoms that characterizes a particular
stereoisomer is called its configuration. The configurations of the isomeric
2-butenes are the structures I and II. These configurations are differentiated in
their names by the prefixes cis- (Latin: on this side) and trans- (Latin: across),
which indicate that the methyl groups are on the same side or on opposite sides of
the molecule. In a way that we are not prepared to take up at this time, the isomer
of b.p. +4 has been assigned the cis configuration and the isomer of b.p. + 1 the
trans configuration.
There is hindered rotation about any carbon-carbon double bond, but it
gives rise to geometric isomerism only if there is*a certain relationship among the
groups attached to the doubly-bonded carbons. We can look i^or this isomerism
by drawing the possible structures (or better yet, by constructing them from molecular
models), and then seeing if these are indeed isomeric, or actually identical.
On this basis we find that propylene, 1-butene, and isobutvlene should not show
isomerism; this conclusion agrees with the facts. Many higher alkenes may, of
course, show geometric isomerism
If we consider compounds other than hydrocarbons, we find that 1,1-dichloroand
1,1-dibromoethene should not show isomerism, whereas the 1,2-dichloro- and
1,2-dibromoethenes should. In every case these predictions have been found correct.
Isomers of the following physical properties have been isolated.
As we soon conclude from our examination of these structures, geometric
isomerism cannot exist if either carbon carries two identical groups. Some possible
combinations are shown below.
The phenomenon of geometric isomerism is a general one and can be encountered
in any class of compounds that contain carbon-carbon double bonds
(or even double bonds of other kinds).
The prefixes cis and trans work very well for disubstituted ethylenes and some
trisubstituted ethylenes. But how are we to specify configurations like these?
Which groups are our reference points? Looking at each doubly-bonded carbon
in turn, we arrange its two atoms or groups in their Cahn^ngoldUPrelog sequence.
We then take the group of higher priority on the olne carbon and the group of
higher priority on the other carbon, and tell whether they are on the same side of
the molecule or on opposite sides. So that it will be clear that we are using this
method of specification, we use the letter Z to mean on the same side, and the letter
E to mean on opposite sides. (From the German: zusanfmen, together, and
entgegen, opposite.)
In so far as chemical and physical properties are concerned, geometric isomers
show the same relationship to each other as do the other diastereomers we have
encountered (Sec. 4.17). They contain the same functional groups and hence show
similar chemical properties. Their chf "nical properties are not identical, however,
since their structures are neither idei ical nor mirror images; they react with the
same reagents, but at different rates.
As the examples above illustrate, geometric isomers have different physical
properties: different melting points, boiling points, refractive indices, solubilities,
densities, and so on. On the basis of these different physical properties, they can
be distinguished from each other and, once the configuration of each has been
determined, identified. On the basis of these differences in physical properties they
can, in principle at least, be separated. (See Sec. 4.17.)
When we take up the physical properties of the alkenes (Sec. 5.9), we shall
discuss one of the ways in which we can tell whether a particular substance is the
els- or /raws-isomer, that is, one of the ways in which we assign configuratio
A pair of geometric isomers are, then, diastereomers. Where do they fit into the
other classification scheme, the one based on how the stereoisomcrs are interconverted
(Sec. 4.20)? We shall discuss this question in more detail later (Sec. 7.1), but for the
moment we can say this. In the important quality of isolability, geometric isomers resemble
configurational isomers, and for a very good reason: in both cases interconversion
requires bond breaking a TT bond in the case of geometric isomers.