Diastereomers
There are two forms; the cis and trans isomers. The form in which the substituent hydrogen
atoms are on the same side of the bond that doesn’t allow rotation is calledcis; the form
in which the substituent hydrogens are on opposite sides of the bond is calledtrans. An
example of a small hydrocarbon displaying cis-trans isomerism is 2-butene.
Cis isomersandtrans isomersof a substance have different physical properties. Trans
isomers generally have higher boiling points and lower densities. This is because the trans
isomers molecules can line up and fit together better than the cis form. Two isomers with
very different properties are maleic acid and fumaric acid. The names are two trivial names
for 2-butenedioic acid and repectively the cis and trans isomer.
Cycloalkanes and similar compounds can also display cis-trans isomerism. As an example
of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane. These
compounds can be named more rigorously using R/S notation^1.
Figure 103cis-1,2-dichlorocyclohexane Figure 104trans-1,2-dichlorocyclohexane
1(R),2(S)-dichlorocyclohexane 1(S),2(S)-dichlorocyclohexane37.2 E/Z notation
Main article: E-Z System^2
The trans/cis system for naming isomers breaks down when there are more than two dif-
ferent substituents on a double bond. (The cis/trans system should only be used when
the carbon atoms involved each have a hydrogen atoms attached). The E/Z notation is
unamibiguous. Z (from the Germanzusammen) means together and usually corresponds
to the termcis; E (from the Germanentgegen) means opposite and usually corresponds
to the termtrans.
Usually, E isomers are more stable than Z isomers because of steric effects. When two large
groups are closer to each other, as they often are with Z, they interfere more with each
other and have a higher potential energy than with E, where the large groups are farther
apart and interfere less with each other.
1 Chapter 41 on page 163
2 Chapter41.2on page 164