Cyclohexane is the prototypical example of a cyclic organic molecule taking on interchanging conformations. In cyclohexane, the lowest-energy conformer is called chair (perhaps due to its faint resemblance to a lounging chaise). In a chair conformation, there are 6 axial substituents (pointing perpendicular to the ring plane), and 6 equatorial substituents (pointing along the ring plane). This means that cyclohexane has, at any given moment, two distinct types of hydrogens.
However, we cannot differentiate between these two types of hydrogens, because cyclohexane conformations are highly fluxional and interconvert quickly at room temperature between two energetically-equivalent chair conformations, a process often called the “cyclohexane chair flip”. From any chair conformation, C-C bond rotation leads first to the half-chair conformer, which is a minimum on the potential energy surface but around 25 kJ/mol (6 kcal/mol) less stable than the “chair” conformer. From the half-chair, another rotation can yield a different “chair” conformation, where the previously axial substituents are now equatorial, and the previously equatorial substituents are now axial. Another well-known conformer of cyclohexane is the “boat” conformer, however it is a transition structure between half-chairs.
Indeed, passing through a boat conformer is not mandatory to flip from one chair conformer of a cyclohexane to another. See below for the two animations for fluorocyclohexane to see how the boat conformer is likely avoided.
Fluorocyclohexane chair flip (through boat conformer)
Fluorocyclohexane chair flip (without boat conformer)
Download the data
- Intrinsic reaction coordinate (IRC) for the chair-to-chair flip of cyclohexane. (.xyz)
- IRC file for the chair-to-chair flip of fluorocyclohexane through the boat conformer. (.xyz)
- IRC file for the chair-to-chair flip of fluorocyclohexane without the boat conformer. (.xyz)
Fluorocyclohexane animations created and shared by Michel Gravel (University of Saskatchewan).