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Tutorial topics:


Viewing Basics

Molecular Shapes



Substituted Cyclohexane


SN2 Reaction

SN1 Reaction




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) is possible.

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)?

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This page was last updated on September 4, 2001 by Dr. Linfeng Xie.
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