By now you should be familiar with position isomers wherein compounds of the same molecular formula differ because substituents, chain branches, and so on, are not at the same positions in the molecules. 1-Chloropropane and 2-chloropropane are straightforward examples of position isomers. A much more subtle form of isomerism is present when two different compounds have the same molecular formulas, the same substituent and chain-branching positions, and, indeed, even have the same names by all of the nomenclature rules we have given you so far. Such isomers are different because their molecules have different arrangements of the atoms in space. These are stereoisomers and this type of isomerism, called stereoisomerism, is of enormous importance to all areas of organic chemistry and biochemistry.
- 5.2: Configurational Isomers
- We have defined isomers in a very general way as nonidentical molecules that possess the same number and kind of atoms. However, there are several ways in which isomers can be nonidentical. Most, but not all alkenes, have stereoisomers that are not identical because of different spatial arrangements of the component atoms. Stereoisomers that do not interconvert rapidly under normal conditions, and therefore are stable enough to be separated, specifically are called configurational isomers.
- 5.3: Conformational Isomers
- An infinite number of different atomic orientations are possible in many organic molecules, depending on the angular relationship between the hydrogens on each carbon. Two extreme orientations or conformations are the eclipsed conformation and the staggered conformation. It has not been possible to obtain separate samples of ethane that correspond to these or intermediate orientations because actual ethane molecules appear to have essentially "free rotation" about the single bond between carbons
- 5.4: Representation of Organic Structure
- Many problems in organic chemistry require consideration of structures in three dimensions, and it is very helpful to use molecular models to visualize the relative positions of the atoms in space.
- 5.5: The D, L Convention for Designating Stereochemical Configurations
- We pointed out in Chapter 3 the importance of using systematic names for compounds such that the name uniquely describes the structure. It is equally important to be able to unambiguously describe the configuration of a compound. The convention that is used to designate the configurations of chiral carbons of naturally occurring compounds is called the \(D, L\) system. To use it, we view the molecule of interest according to the rules presented here.
- 5.6: Molecules with More Than One Chiral Center. Diastereomers
- We have seen examples of molecules with one chiral center that exist in two mirror-image configurations, which we call enantiomers. What happens when there is more than one chiral center? How many stereoisomers should we expect?
- 5.7: Some Examples of the Importance of Stereoisomerism to Biology. Biological Stereospeciflcity
- Asymmetric or chiral reagents can differentiate between enantiomers, especially by having at least some difference in reactivity toward them. The property of being able to discriminate between diastereomers is called stereospecificity, and this is an especially important characteristic of biological systems.
- 5.E: Stereoisomerism of Organic Molecules (Exercises)
- These are the homework exercises to accompany Chapter 5 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
Contributors and Attributions
John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."