Understanding Rotation about a C=C Double Bond   

Susan E. Barrows and Thomas H. Eberlein

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

Abstract

In this article, twisting about the C=C double bond and the consequential pyramidalization of sp2 carbon atoms in alkenes were examined in a molecular modeling study using trans-2-butene as a model system. According to our trans-2-butene model and other similar work, most of the strength of a π bond is retained upon twisting, even for remarkably large C–C=C–C dihedral angles (up to 90°). The phenomenon of sp2 carbon atom pyramidalization and preservation of π bond strength upon twisting a C=C double bond is well established in the literature, but is rarely discussed in introductory textbooks. This absence is noteworthy because profound manifestations of this effect do occur in compounds that are covered in an introductory organic chemistry curriculum. We present a simple method of introducing the concept of a flexible C=C π bond into beginning organic chemistry courses. We report the energetic demands of partial twisting about the C=C bond in 2-butene as calculated using DFT, LMP2, and MCSCF methods. Finally, using the results of these calculations, we assessed the degree of strain introduced by the twisted nature of the C=C bond in trans cycloalkenes.

Journal of Chemical Education, 2005, 82, 1329            Full text (PDF)

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