What is the energy minimum of cyclopentane conformational diagram: the halfchair or envelope? How does cyclopentane flip?


As you mention, there are two main conformations that cyclopentane may adopt.

conformations of cyclopentane

In the absence of substituents around the ring, there is actually only a $\pu{0.5 kcal/mol}$ energy difference between envelope and half-chair (envelope slightly lower in energy) meaning cyclopentane itself (computationally at least) is in rapid equilibration with no 'major' conformation as is observed with cyclohexane. (Note that the energy of the totally planar conformation is about $\pu{5 kcal/mol}$ higher than that of the both bent conformations, and so is not observed to any great degree).

With a single substituent on the ring however, such as a methyl group, the envelope conformation becomes slightly more favourable (to the tune of around $\pu{3.5 kcal/mol}$) giving a barrier to the ring flipping and meaning there is a dominant conformation.

  • $\begingroup$ "In the absence of substituents around the ring, there is actually only a 0.5 kcal/mol energy difference between envelop and half-chair meaning cyclopentane itself" In favor of which one? $\endgroup$ – EJC Jul 24 '16 at 12:45
  • $\begingroup$ Are in the envelope conformation the C atoms in a plane, except the rightmost one? $\endgroup$ – EJC Jul 24 '16 at 12:47
  • $\begingroup$ Yes. So in the envelope conformation just one carbon is bent out of the plan $\endgroup$ – NotEvans. Jul 24 '16 at 12:51
  • $\begingroup$ Why do you show two C$_s$ envelope conformations? Is not the left one the same as the right after a rotation. As all the 5 possible envelope-half-chairs are equally probable surely only one envelope and half chair is needed ? As thermal energy is approx 2.4 kJ/mol interconversion should be very rapid, I'm guessing but $10^8$ s$^{-1}$ would not be unreasonable. $\endgroup$ – porphyrin Jul 24 '16 at 12:55
  • 1
    $\begingroup$ The left envelope is equal to the right. I think the diagram was just intending to show that to get between two different envelope conformations you have to go via the half chair, which, for unsubstituted cyclopentane has little barrier is fine. $\endgroup$ – NotEvans. Jul 24 '16 at 12:59

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