Bluffton University

Organic Chemistry

Molecular Models:
Conformation and Newman projections
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Unsubstituted cycloalkanes, if planar, would not only have significant angle strain (caused by abnormal bond angles in the ring) but also considerable torsional strain (from eclipsing of adjacent hydrogens). Cyclopropane and cyclobutane cannot avoid having large amounts of strain in their structures, but cyclopentane and cyclohexane can easily adopt conformations in which not only the angles have normal values but eclipsing (thus torsional strain) is minimized.
cyclopropane   cyclobutane
 
  Cyclobutane's lowest-energy conformer is slightly puckered -- angle strain is increased in order to relieve torsional strain!

Planar cyclopentane has almost-normal bond angles of 108°. Nevertheless, to avoid torsional strain the molecule bends into the so-called envelope conformation, in which one of the carbon atoms is bent out-of-plane. Notice that this staggers the hydrogens on that atom relative to those on adjacent carbon atoms.

 

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Copyright © 1998, 1999, 2000, 2003, 2007 by Daniel J. Berger. This work may be copied without limit if its use is to be for non-profit educational purposes. Such copies may be by any method, present or future. The author requests only that this statement accompany all such copies. All rights to publication for profit are retained by the author.