Representation of Conformation of Alkanes

We must first study the concept of conformational isomerism, which deals with rotation around single bonds, before moving on to stereochemistry and chirality.

Because of the ‘end-to-end’ (sigma) nature of their orbital overlap, single bonds in organic molecules are free to rotate. Consider the carbon-oxygen bond in ethanol: if you rotate the molecule 180 degrees around this bond, the form changes dramatically:

In the case of ethane, rotation about the carbon-carbon sigma bond leads to a variety of three-dimensional atom configurations.

Conformations are the different arrangements that result from sigma bond rotation in organic chemistry. A conformational isomer, also known as a conformer, is a specific conformation.

Newman’s forecasts

A Newman projection is a depiction of a molecule in which the atoms and bonds are depicted along the axis around which rotation happens.

Ethane projection by Newman

The molecule is displayed in a Newman projection along an axis comprising two bound atoms and the bond between them, along which the molecule can rotate. The “substituents” of each atom comprising the bond, whether hydrogens or functional groups, can be seen both in front of and behind the carbon-carbon bond in a Newman projection. The dihedral angle, also known as the torsion angle, is the angle formed by a substituent on the front atom and a substituent on the rear atom in the Newman projection.

In the case of ethane, we can conceive two “extreme” conformations. The hydrogens on the first carbon line up with or eclipse the hydrogens on the second carbon in one scenario because the dihedral angle is 0°. Ethane has acquired the eclipsed conformation when the dihedral angle is 0° and the hydrogens are exactly aligned (see figure). The staggered conformation occurs when the hydrogens on the first carbon are as far apart as possible from those on the second carbon, which happens at a dihedral angle of 60°.

Ethane conformations – staggered and eclipsed

Because the eclipsed conformation involves unfavourable interactions between hydrogen atoms, the staggered conformation of ethane is more stable and lower in energy than the eclipsed conformation. When the bonds line up, the negatively charged electrons in the bonds repel each other the greatest. As a result, most of the time, ethane is in the more stable staggered structure.

Solid lines protrude from the two carbons to represent the six carbon-hydrogen bonds. In a Newman projection, bonds are not drawn as solid or dashed wedges.

Ethane

The ‘staggered’ conformation of ethane is the lowest energy shape, as seen in the picture above: all dihedral angles are 60o, and the distance between the front and back C-H bonds is maximised.

The molecule is now in the highest energy ‘eclipsed’ conformation, where the dihedral angles are all 0o, if we rotate the front CH3 group 60 degrees clockwise (we stagger the bonds slightly in our Newman projection drawing so that we can see them all).

This causes repulsion between the electrons in the front and back C-H bonds, bringing them closer together. The eclipsed conformation has a higher energy (less stability) than the staggered conformation by about 12 kJ/mol. Because there is a 12 kJ/mol energy barrier to overcome while eclipsing the hydrogens, rotation is not exactly “free.” Despite this, the barrier is low enough that rotation occurs at a rate of 10 10 times per second at room temperature.

Making models of these conformations using your molecular model kit is quite valuable. Ethane’s two major conformations are depicted as ball-and

The molecule returns to a second staggered configuration after another 60° revolution. This procedure can be repeated all the way around the 360° circle, with three eclipsed and three staggered conformations available, as well as an infinite number of conformations in between.

The anti and gauche conformations are clearly more stable than the eclipsed and completely eclipsed conformations. The anti and gauche conformations are where butane spends the most of its time. Butane’s staggered conformations are energy ‘valleys,’ while the eclipsed conformations are energy ‘peaks,’ just like ethane. Butane, on the other hand, has two distinct valleys and two distinct summits. Due to steric strain, which is the repulsive interaction induced by the two bulky methyl groups being pressed too close together, the gauche conformation has a greater energy valley than the anti-conformation. The anti-conformation clearly has less steric strain.

Similarly, steric strain raises the energy of the eclipsed A conformation – where the two methyl groups are as close together as they can be – over the two eclipsed B conformations. This ball-and-stick illustration shows the six primary conformations:

Increased alkanes

Because the anti conformation has the lowest energy (and also because it’s easier to draw), open-chain alkanes are commonly drawn in a zigzag pattern, which suggests anti conformation for all carbon-carbon bonds. A Newman projection looking down the C2-C3 bond of octane is shown in the diagram below.

Conclusion

(Sigma) bonds in alkanes have the property of revolving at the molecular level. In ethane (CH3CH3), one methyl (CH3) group can freely spin around the C-C bond without hitting any obstacles.

It is extremely recommended that you use a molecular model to “see” the bond rotation. With a molecular model, you can rotate one methyl group while holding the other one steady. Rotation around the bond is straightforward since the sp3-sp3 orbitals overlap and the link is cylindrically symmetrical, therefore the molecule seems to remain intact.