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STEREOISOMERISM OF ORGANIC MOLECULES – Configurational Isomers

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.

To understand stereoisomerism of carbon compounds, we must understand the ways in which the bonds to carbon atoms are arranged in space. As shown in Section 2-2A, this depends on whether the carbon atoms form single, double, or triple bonds to another atom. Thus, four single bonds to a carbon form a tetrahedral arrangement; two single bonds and one double bond to a carbon give a planar array with bond angles near 120°, while one single bond and one triple bond (or two double bonds) to a carbon are arranged linearly:


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Finally, if you have not studied the material already, you may wish to return to the last part of Chapter 3 and become acquainted with the nomenclature of cycloalkanes, alkenes, cycloalkenes, and alkynes.

 

5-1 CONFIGURATIONAL ISOMERS

5-1A Geometric Isomerism

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. Among the alkenes, 1- and 2-butene are position isomers, because in these compounds the double bond has a different position in the carbon chain:

Most, but not all alkenes, also have stereoisomers that are not identical because of different spatial arrangements of the component atoms. Thus there are two stereoisomers of 2-butene that differ in the geometric arrangement of the groups attached to the double bond. In one isomer, both methyl groups are on the same side of the double bond (cis-2-butene) and in the other, the methyl groups are on opposite sides of the double bond {trans-2-butene):

 

The two isomers clearly have the same structural framework but they differ in the arrangement of this framework in space —hence the designation stereoisomers. They owe their separate existence to the fact that the double bond is rigid and the parts of the molecule are not free to rotate with respect to each other about this bond. Therefore the isomers do not interconvert without breaking the double bond, and they exist as different compounds, each with its own chemical and physical properties. Ball-and-stick models of cis- and trans-2-butene are shown in Figure 5-1, and the rigidity of the double bond is simulated in the model by a pair of stiff springs or bent sticks connecting the two carbons of the double bond.

It should be clear to you that there will be no cis-trans isomers of al kenes in which one end of the double bond carries identical groups. Thus we do not expect there to be cis-trans isomers of 1-butene or 2-methylpropene, and

 

Figure 5-1 Ball-and-stick models of cis-2-butene

 

 

Figure 5-1 Ball-and-stick models of trans-2-butene

 

 

indeed none are known:

 

You may wish to verify this by making ball-and-stick models of these substances.

Ring formation also confers rigidity on molecular structure such that rotation about the ring bonds is prevented. As a result, stereoisomerism of the cis-trans type is possible. For example, 1,2-dimethylcyclopropane exists in two forms that differ in the arrangement of the two methyl groups with respect to the ring. In the cis isomer, the methyl groups both are situated above (or below) the

Figure 5-2 Ball-and-stick models of cis and trans isomers of 1,2-dimethylcyclopropane

 

plane of the ring and in the trans isomer they are situated one above and one below, as shown in Figure 5-2. Interconversion of these isomers does not occur without breaking one or more chemical bonds.

Stereoisomers that do not interconvert rapidly under normal conditions, and therefore are stable enough to be separated, specifically are called configurational isomers. Thus cis- and trans-2-butene are configurational isomers, as are cis- and trans-1,2-dimethylcyclopropane. The terms cis-trans isomerism or geometric isomerism commonly are used to describe configurational isomerism in compounds with double bonds and rings. When referring to the configuration of a particular isomer, we mean to specify its geometry. For instance, the isomer of 1,2-dichloroethene shown below has the trans configuration; the isomer of 1,3-dichlorocyclobutane has the cis configuration:

 

Cis-trans isomerism is encountered very frequently. By one convention, the configuration of a complex alkene is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound is trans-4-ethyl-3-methyl-3-heptene, despite the fact that two identical groups are cis with respect to each other, because  the longest continuous chain is trans as it passes through the double bond:

 

 

Notice that cis-trans isomerism is not possible at a carbon-carbon triple bond, as for 2-butyne, because the bonding arrangement at the triply bonded carbons is linear:

 

Many compounds have more than one double bond and each may have the potential for the cis or trans arrangement. For example, 2,4-hexadiene has three different configurations, which are designated as trans-trans, cis-cis, and trans-cis. Because the two ends of this molecule are identically substituted, the trans-cis becomes identical with cis-trans:

 

Isomer Examples & Stereoisomers video

 

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