Stereochemistry: Classification of Isomers

Discussion: The structures of organic molecules vary widely. Within this variety there are similarities in structure which result in similarities in chemical and/or physical properties. If one can predict the properties of one compound based in its molecular structure, a compound with similar molecular structure is expected to have similar properties. For example, we know that the opiate alkaloid morphine is an analgesic, so we would expect compounds with similar chemical structure to also have analgesic properties.

This is true for codeine and heroin; note the structural similarities to morphine. The differences in the structures of these compounds lead to their differing levels of biological activity.  This power to predict properties of a substance based on its similarity to other substances of known structure is a very powerful tool for organic chemists, and especially so for discovering new pharmaceuticals. It is therefore useful for the student of organic chemistry to learn to recognize various categories of structural relationships. We have already defined the terms for the major structural categories. Figure 4.1 (page 121 of the text) shows the relationship of these categories. An expanded flowchart (below) can be used to categorize structures.

Example 1: Using the flowchart given above, categorize each pair of structures using all the terms that apply: identical, isomers, constitutional isomers, stereoisomers, enantiomers, or diastereomers. Molecular models may be useful in some cases.

     

Solution 1: a. These two molecules have the same chemical formula (C4H10O), so they are isomers. Examining the order of atomic attachment, we see they have the same connectivity: a four-carbon chain with a hydroxyl group on the second carbon. They are therefore stereoisomers. To determine if the structures are mirror images, it may be necessary to construct molecular models. When constructing these models, pay careful attention to the three-dimensional arrangement of the given structures. Comparison of these molecular models show that they are mirror images, so the molecules are enantiomers.  Thus, this pair of structures are isomers, stereoisomers, and enantiomers.

b. These two molecules have the same chemical formula (C4H6O6), so they are isomers. The atoms are attached in the same sequence: a two carbon chain, with each carbon having one hydrogen, one hydroxyl group, and one carboxylic acid group, so the compounds are stereoisomers. Comparison of molecular models shows the compounds are not mirror images. One stereocenter of each molecule is mirror image, while the other is not. (When examining models for mirror image relationships, it is often necessary to consider a wide variety of conformations. Try to rotate the molecule so that obvious groups can be moved into a mirror image position if possible. In this case, you might use the two carboxylic acid groups for this purpose.) Because these molecules are not mirror images, the are diastereomers.
 

Exercises: Using the flowchart given above, categorize each pair of structures using all the terms that apply: identical, isomers, constitutional isomers, stereoisomers, enantiomers, or diastereomers. Molecular models may be useful in some cases.







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