Tell whether each of the following molecules has a double-bond stereoisomer. If so, identify its stereocenters.

Question

[latex]\H_3C\hspace{30 pt} CH_2CH_3\hspace{30 pt} H_3C\hspace{30 pt} CH_3 \ \qquad \diagdown \qquad \diagup \hspace{30 pt} \hspace{30 pt} \qquad \diagdown \qquad \diagup \\qquad \; C=C\hspace{30 pt} \hspace{30 pt} \hspace{30 pt} C=C \\qquad \diagup\qquad \; \diagdown \hspace{30 pt} \hspace{30 pt} \quad \; \diagup\qquad \; \diagdown \\; H\hspace{30 pt}\qquad CH_3 \hspace{30 pt} \; H_3C\hspace{30 pt}\quad H \\hspace{30 pt} \qquad A\hspace{30 pt} \hspace{30 pt} \hspace{30 pt} \qquad B[/latex]

Accepted Answer

Apply the definition of double-bond stereoisomers as illustrated in Eq. 4.2. That is, interchange the positions of the two groups at either carbon of the double bond. This process will give one of two results: either the resulting molecule will be congruent to the original—that is, superimposable on the original atom-for-atom—or it will be different. If it’s different, it can only be a stereoisomer, because its connectivity is the same.

In molecule A, interchanging the two groups at either carbon of the double bond gives different molecules. In the original, the methyl groups are trans; after the interchange, the methyl groups are cis. Hence, A has a doublebond stereoisomer:

(You should verify that exchanging the two groups at the other carbon of the double bond gives the same result.) The two carbons of the double bond are both stereocenters.

In the case of structure B, interchanging the two groups at either carbon of the double bond gives back an identical molecule.

You may have found that the structure you obtained from interchanging the two groups doesn’t look identical to the one on the left, but it is. You can demonstrate their identity by flipping either structure 180° about a horizontal axis (green dashed line)—in other words, by turning it over, as shown in Eq. 4.4. But if you have difficulty seeing this, you must build molecular models of both structures and convince yourself that the two can be superimposed atom-for-atom. There is no substitute for model building when it comes to the spatial aspects of organic chemistry! After a little work with models on issues like this, you will develop the ability to see these relationships without models. Study Guide Link 4.1 offers more insights about how to achieve facility in relating alkene structures.

Because interchanging two groups in Eq. 4.4 does not give stereoisomers, this alkene contains no stereocenters.

Question 4.1: Tell whether each of the following molecules has a double-bo...

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