Alkane Conformation

Section IIc: Conformations of Alkanes & Cycloalkanes

Structural formulas are useful for showing the attachment of atoms, and three-dimensional drawings are useful for showing molecular shapes. Neither of these, however, conveys much information regarding the dynamics of molecular conformations and the role that these play in controlling equilibrium shapes and reactivity of organic molecules.

As mentioned previously, there is generally free rotation around carbon-carbon single bonds. At room temperature, this rotation can be quite rapid and can occur with a rate constant of

10⁸ sec^-1. For ethane, this rotation has only a small intrinsic energy barrier since the van der Waals radius of the hydrogen atoms on the adjacent carbons is sufficiently small so that overlap is minimal. A movie file demonstrating this rotation is shown below:

(Click on the Play Button to view the movie; use the BACK button to return to this page)

This can be contrasted, however, with rotation around the central carbon-carbon bond in butane, shown in the movie panel below, in which two methyl groups clearly overlap during a single rotation (the van der Waals radii of the methyl hydrogen atoms clearly overlap).

The rotation around the single bond in ethane, while not obviously hindered, does generate conformational isomers having different potential energies. As shown above, as the dihedral angle between the ethane hydrogen atoms changes from 60

(a staggered conformation) to 120

(an eclipsed conformation), the potential energy of the molecule increases by about 3 kcal/mole. As the methyl group continues to rotate towards 180

, the potential energy again drops and rises again as the next eclipsed structure is formed.

The effect of rotation on the potential energy of butane around the central carbon-carbon bond is more significant, as shown above. The structure shown at 0

is fully eclipsed, that is, both methyl groups are aligned and are interacting maximally. As the front methyl group is rotated 60

, a gauche conformation is produced in which the methyl group is nestled between the back methyl and the adjacent hydrogen atom. Another 60

rotation produces an eclipsed version of the gauche conformation which is approximately 2.4 kcal/mole less stable. At 180

, the anti conformation is formed in which the two methyl groups are on opposite faces of the molecule and no groups are eclipsed. This is the most stable confomer and it differs from the fully eclipsed confomer by about 5 kcal/mole in potential energy. Further rotations regenerate an equivalent eclipsed gauche conformer (at 240

), another gauche form (300

) and finally, the eclipsed form at 360

. These rotations are best seen in a movie clip which can be accessed by clicking on the icon below:

(Click on the Play Button to view the movie)

Rotations such as these are not possible in cycloalkanes, where the ring constrains the movements around the carbon-carbon single bonds. Cyclopropane rings are generally flat and have little conformational flexibility. The flexibility of four- and five-membered rings is significantly greater and these molecules exist as a dynamic equilibrium among various "puckered" conformations, as shown below.

The dynamic flexibility of a five-membered ring is best visualized in the movie clip (below) which simulates the equilibrium interconversion of the various conformational isomers.

(Click on the Play Button to view the movie)

The conformational flexability of cyclohexane is somewhat unique in that two equivalent structures are involved which are linked by a process termed "ring inversion". As shown in the figure above, the lowest energy conformation of cyclohexane is one in which each end of the molecule is "puckered", relative to the plane of the ring. This form is commonly called the "chair conformation", as it somewhat resembles a reclined lawn chair. Inspection of this structure shows that there are two types of hydrogens in the molecule; a set that is perpendicular to the plane of the ring (axial hydrogens) and a set which are more-or-less in the plane of the ring (equatorial hydrogens). The chemical reactivity of cyclohexane, however, is inconsistent with two types of hydrogens in a stable form of the molecule (for example, there is only one monochlorocyclohexane, not two, as would be predicted if axial and equatorial hydrogens could be replaced independently). The explanation for this fact is that the flexability of cyclohexane allows for rapid ring inversion, in which one chair conformation is replaced by a second. Intermediate between these two chair forms is an unstable conformation called "boat cyclohexane", in which both ends of the molecule are puckered in the same direction. The important thing to note about the process of ring inversion is that during ring inversion, all axial substituents are converted to equatorial substituents, and all equatorial substituents become axial. This process is difficult to visualize, initially, without the use of molecular models, but can be seen easily in the movie clip below.

(Click on the Play Button to view the movie)

The axial hydrogens in cyclohexane experience a slight amount of steric repulsion. More bulky groups, however, can interact strongly with other axial substituents, making it energetically unfavorable for these groups to occupy axial positions. These unfavorable interactions can be seen below in the equatorial and axial representations of bromocyclohexane.

In the equatorial conformation, the bromine is "sticking out" from the plane of the ring and is experiencing only minimal steric interactions with neighboring groups. In the axial conformation, however, the van der Waals radii of the bromine significantly overlaps with that of the two axial hydrogens. This type of steric interaction can also be clearly seen in the models for ethylcyclohexane, shown below.

The full rotation of the ethyl group is also shown in the movie clip shown below.

(Click on the Play Button to view the movie)

As stated above, steric interactions tend to make conformations containing axial substituents energetically unfavorable, relative to placing these substituents in equatorial positions. Since axial and equatorial groups in cyclohexane are linked via equilibria involving ring inversion, the net effect is to force the equilibrium towards the more stable form in which the bulky substituents are in equatorial positions, as shown below. For very large substituents (i.e., the tert-butyl group) this equilibrium is so strongly shifted so that ring inversion essentially never occurs. Such groups are said to "lock" the ring into the energetically favorable conformation.