also rotate plane-polarized light; however, knowing the specific rotation of one diastereomer gives
no indication of the specific rotation of another diastereomer. This is a stark contrast from
enantiomers, which will always have equal-magnitude rotations in opposite directions.
Cis–Trans Isomers
Cis–trans isomers (formerly called geometric isomers) are a specific subtype of diastereomers in
which substituents differ in their position around an immovable bond, such as a double bond, or
around a ring structure, such as a cycloalkane in which the rotation of bonds is greatly restricted. In
simple compounds with only one substituent on either side of the immovable bond, we use the
terms cis and trans. If two substituents are on the same side of the immovable bond, the molecule is
considered cis. If they are on opposite sides, it is considered trans, as shown in Figure 2.8 earlier. For
more complicated compounds with polysubstituted double bonds, (E)/(Z) nomenclature is used
instead, as described in the next section.
MCAT EXPERTISE
While the MCAT is up-to-date with science, it is still possible to see older terms for some
concepts on Test Day. Thus, it’s important to know not only the current name cis–trans
isomers, but also the older name, geometric isomers.Meso Compounds
For a molecule to have optical activity, it must not only have chiral centers within it, but must also
lack a plane of symmetry. Thus, if a plane of symmetry exists, the molecule is not optically active
even if it possesses chiral centers. This plane of symmetry can occur either through the chiral center
or between chiral centers. A molecule with chiral centers that has an internal plane of symmetry is
called a meso compound, an example of which is shown in Figure 2.14.