We learned in the previous chapters that isomers are defined as
different compounds with same molecular formula. Those isomers that
differ in bond connectivity are called Constitutional Isomers
or Structural Isomers. The isomers created from single bond
rotation or ring-flip are called Conformational Isomers. In
this tutorial, we will learn a new type of isomers: isomers with same
bond connectivity but differerent orientation in space. (with this
broad definition, Conformational Isomers are also stereoisomers.)
Compounds that are mirror images of each other but non-superimposable
are called enantiomers. Thus enantiomers are one type of stereoisomers
that relate to each other as mirror images, as shown below.
Note that they are mirror images of each other. Now rotate
the two models and decide if they are superimposable. Given two molecules,
how do we know if the two compounds are enantiomers? There are two
conditions that must be met if they are enantiomers: 1) the two compounds
must be mirror image of each other; and 2) they must not
be superimposable on each other. (If the second condition fails, then
the two molecules represent the same compound)
Now let's look at two other molecules shown below:
It is obvious that these two models are mirror images
of each other, but are they enantiomers? Test youself using the definition!
(Hint: rotate them!)
Diastereomers are another type of stereoisomers that
are not mirror images of each other. For two given compounds to
be diastereomers, two conditions must be met: 1) They must not
be mirror images of each other; and 2) They must differ only
in spatial orientation of their atoms or groups. (i.e. they must
be stereoisomers to start with)
We have seen a pair of diastereomers before, cis-2-butene
and trans-2-butene. Recall that cis and trans
isomers are called geometric isomers, but they are also diastereomers
by definition. Let's look at their "live" models:
Thus geometric isomers are one type of diastereomers
(and are, of course, stereoisomers as well).
Another kind of diastereomers arises when a molecule
contains two or more chiral (stereo) centers. The following are the
models for four stereoisomers of 1-chloro-2-fluoro-1,2-diiodoethane.
Because it has two chiral carbons, a total of four stereoisomers (maximum)
Now test youselve on these questions:
a) What is the relationship between (1R, 2R) and (1S,
2S)? What is the relationship between (1R, 2S) and (1S, 2R)?
b) What is the relationship between (1R, 2R) and (1R,2S)?
(1R, 2R) and (1S, 2R)? (1S, 2S) and (1R, 2S)? (1S, 2S) and (1S, 2R)?
c) Can you determine the configurations for all the chiral
centers (R or S)?