Organic Chemistry Lesson of the Day – The 2 Conformational Isomers of Ethane

The simplest case of conformational isomerism belongs to ethane, C2H6.

ethane conformers

Newman projections of the 2 conformational isomers of ethane.

Image courtesy of Mr.Holmium via Wikimedia.

In the Newman projections above, you can see that the dihedral angle between any 2 vicinal hydrogens plays a key role in the stability of ethane.  In particular, there are 2 extrema in that plot of the change in Gibbs free energy vs. the dihedral angle:

  • The minimum is attained when the dihedral angle is 180 \times (2n + 1) \div 3 degrees, where n is any integer (n = 0, \pm 1, \pm 2, \pm 3, ...).  In other words, the vicinal hydrogens are as far apart from each other as possible.  This conformation is called the staggered conformation.
  • The maximum is attained when the dihedral angle is 180 \times (2n) \div 3 degrees, where n is any integer (n = 0, \pm 1, \pm 2, \pm 3, ...).  In other words, the vicinal hydrogens are as close to each other as possible.  This conformation is called the eclipsed conformation.

The stability of ethane is dependent on this dihedral angle.

  • If the vicinal hydrogens are far part from each other (in a staggered conformation, for example), then there is less torsional strain* between the 2 carbon-hydrogen bonds, resulting in more stability.
  • If the vicinal hydrogens are close to each other (in an eclipsed conformation, for example), then there is greater torsional strain* between the 2 carbon-hydrogen bonds resulting in less stability.

*In my undergraduate education, I learned that the greater stability in the staggered conformation is due to less torsional (steric) strain.  However, Vojislava Pophristic & Lionel Goodman (2001) argued that the effect is actually due to the stabilizing effect of hyperconjugation.  Song et al. (2005) and Mo and Yao (2007) rebutted this argument in separate publications.  Read these articles as searched under “ethane hyperconjugation steric strain” on Google Scholar for more information.

References

  • Pophristic, V., & Goodman, L. (2001). Hyperconjugation not steric repulsion leads to the staggered structure of ethane. Nature, 411(6837), 565-568.
  • Song, L., Lin, Y., Wu, W., Zhang, Q., & Mo, Y. (2005). Steric strain versus hyperconjugative stabilization in ethane congeners. The Journal of Physical Chemistry A, 109(10), 2310-2316.
  • Mo, Y., & Gao, J. (2007). Theoretical analysis of the rotational barrier of ethane. Accounts of chemical research, 40(2), 113-119.

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