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
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.
Supplement
Supplemental materials include the guided-inquiry Student Exercise. There are also Instructor Notes describing the implementation of the exercise, details regarding the choice of molecular modeling techniques, and answers for the student exercise.
Additional online materials, including files of the optimized geometries of the cycloalkene models, may be found at the authors' Web site (accessed Jun 2004). Three file formats are provided: *.xyz for use with the Chemscape Chime plug-in; *.pdb for use with Chem 3D; and *.sxf for use with Spartan, SpartanView, or Titan.
Contents
Folder JCE2004p1529W containing instructor.doc and student.doc (Microsoft Word)
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Citation
Barrows, Susan E.; Eberlein, Thomas H. J. Chem. Educ.2004 81 1529.
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