Organic Chemistry Lesson of the Day – The 4 Conformational Isomers of Butane

In a previous Chemistry Lesson of the Day, I introduced the simplest case of conformational isomerism – the staggered and eclipsed conformations of ethane.  The next most complicated case of conformational isomerism belongs to butane.  Here are the Newman’s projections of the 4 possibilities.

butane conformers

Modified image courtesy of Avitek from Wikimedia.

The conformational isomers are named with respect to the proximity of the 2 methyl groups.  The dihedral angle between the 2 methyl groups, θ, is below each Newman projection.  From left to right, the conformational isomers are:

  • fully eclipsed (θ = 0 degrees)
  • gauche (θ = 60 degrees)
  • eclipsed (θ = 120 degrees)
  • anti (θ = 180 degrees)

Clearly, the fully eclipsed conformation has the most steric strain* between the 2 methyl groups, so its internal energy is highest.

Clearly, the anti conformation has the lowest steric strain between the 2 methyl groups, so its internal energy is lowest.

The gauche conformation has less steric strain than the eclipsed conformation, so its internal energy is the lower of the two conformations.

From lowest to highest internal energy, here is the ranking of the conformation isomers:

  1. anti
  2. gauche
  3. eclipsed
  4. fully eclipsed

This can be visualized by the following energy diagram.

butane energy diagram

Image courtesy of Mr.Holmium from Wikimedia.

*As mentioned in my previous Chemistry Lesson of the Day on the 2 conformational isomers of ethane, there is some controversy about what really causes the internal energy to increase in eclipsed conformations.  Some chemists suggest that hyperconjugation is responsible.

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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.