This rotational barrier is not large enough to prevent rotation except at extremely cold temperatures.
![rotate label in rapid sketch rotate label in rapid sketch](https://www.mdpi.com/energies/energies-14-07444/article_deploy/html/images/energies-14-07444-g002-550.jpg)
Rotate label in rapid sketch free#
The carbon-carbon bond is not completely free to rotate – the 3 kcal/mol torsional strain in ethane creates a barrier to rotation that must be overcome for the bond to rotate from one staggered conformation to another.
![rotate label in rapid sketch rotate label in rapid sketch](https://bioone.org/ContentImages/Journals/brad/1988/6/brad.n6.1988.a1/graphic/f01_01.jpg)
Unhindered (Free) Rotations Do Not Exist in Ethane We will focus on the staggered and eclipsed conformers since they are, respectively, the lowest and highest energy conformers. This process can be continued all around the 360° circle, with three possible eclipsed conformations and three staggered conformations, in addition to an infinite number of variations in between. Torsional strain (or eclipsing strain) is the name give to the energy difference caused by the increased electrostatic repulsion of eclipsing bonds.Īnother 60° rotation returns the molecule to a second eclipsed conformation. The energy of the eclipsed conformation is approximately 3 kcal/mol (12 kJ/mol) higher than that of the staggered conformation. This is the highest energy conformation because of unfavorable electrostatic repulsion between the electrons in the front and back C-H bonds. If we now rotate the front CH 3 group 60° clockwise, the molecule is in the highest energy ‘eclipsed' conformation, and the hydrogens on the front carbon are as close as possible to the hydrogens on the back carbon. Maximizing the distance between the electrons decreases the electrostatic repulsion between the electrons and results in a more stable structure. In this conformation, the distance between the bonds (and the electrons in them) is maximized. This angle between a sigma bond on the front carbon compared to a sigma bond on the back carbon is called the dihedral angle. In the staggered conformation, all of the C-H bonds on the front carbon are positioned at an angle of 60° relative to the C-H bonds on the back carbon. The lowest energy conformation of ethane, shown in the figure above, is called the ‘staggered’ conformation. Figure 3.6.1: A 3D Model of Staggered Ethane.