Cis and Trans Isomerization in Cyclic Alkenes: A Topic for Discovery Using the Results of Molecular Modeling  

Susan E. Barrows and Thomas H. Eberlein

School of Science, Engineering, and Technology, Penn State Schuylkill, The Capital College, Schuylkill Haven, PA 17972   

Abstract

Beginning organic chemistry students are often taught the generalizations that alkenes are planar and that cis alkenes are less stable than their trans isomers. An exception to these rules occurs in cycloalkenes of fewer than eleven members, wherein cis isomers are more stable then their trans counterparts. This article describes an activity in which students are led to discover the fundamental reasons behind the unusual instability of the trans isomers in medium-sized cycloalkenes by using the results of molecular modeling. Notably, students will make the unexpected discovery that twisting about π bonds is perhaps more facile than they had been led to believe. Furthermore, such twisting is invariably accompanied by pyramidalization of the ostensibly "planar" sp² carbon atoms, which helps to maintain p-orbital overlap and π-bond strength. Thus, this molecular modeling-based exercise helps drive home the point that alkenes need not be planar.

Journal of Chemical Education, 2004, 81, 1529            Full text (PDF) | Supplement

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