Chair Flips

by James

Cyclohexane only looks like a hexagon from the top. When I was first learning ochem it took me awhile to get used to the fact that it wasn’t perfectly flat. Why isn’t it flat? Because if it were, all the CH2 groups would be eclipsed – and we now know that’s bad. 

 Cyclohexane actually spends most of its lifetime as a “chair”: that is, one carbon points “up” (the headrest) and another group points down (the footrest). The other four carbons are in the same plane. 

Some fun facts about chairs:

  • In a chair, all adjacent carbons are perfectly staggered. 
  • There is no ring strain. All bond angles at 109 degrees, as happy as can be. 
  • Groups that point straight up (or straight down) are called axial. Groups that point “somewhat up” or “somewhat down” are called equatorial
  • If you look around the ring note how the axial groups alternate: axial up, axial down, axial up, axial down and so on.
  • The equatorial position is less sterically hindered than the axial position, since in the axial position bulky groups are in close proximity to the other two axial position
  • Chairs can “flip” . It’s this point that causes a lot of confusion.

When a chair “flips”, the “headrest” rotates down so that it becomes a “footrest”, while the footrest rotates “up” so that it becomes a “headrest”.

Like this:

 

Facts about chair flips:

  • All axial groups become equatorial. 
  • All equatorial groups become axial. 
  • All groups that are “up” remain “up”. 
  • All groups that are “down” remain down. 
  • The chair which has the fewest overall steric interactions will be the most stable, and thus favored in equilibrium. This can be calculated if one is given the values of how much each unfavorable interaction “costs”. Bulky groups can prefer the equatorial position by a large margin (favored by 4.9 kcal/mol for t-butyl!) 

Finally, learning how to draw a chair is important. A video says more than an email could, so if drawing chairs is difficult for you, here’s a 6 minute video on how to draw a cyclohexane chair using the parallelogram technique.

Thanks for reading! James

P.S. In the process of a chair flip there is briefly a conformation where the two ends  are close together: this is called a “boat” (or “twist-boat, to be more precise). The energy barrier for this is about 12 kcal/mol.

P.P.S. further reading: The Ups and Downs of Cyclohexane