Question

Draw Sawhorse projection for the eclipsed and staggered conformations of ethane. Which of these conformations is more stable and why?

Answer 1

Hint:To solve this question, we must first understand the concept of Sawhorse projection. Then we need to assess the points leading to the stability of these conformations and then only we can conclude the correct answer.

Complete step-by-step answer:Before we move forward with the solution of this given question, let us first understand some basic concepts about Sawhorse Projection:
Sawhorse Projection is a view of a molecule down a particular carbon-carbon bond, and groups connected to both the front and back carbons are drawn using sticks at ${120^ \circ }$ angles. Sawhorse Projections can also be drawn so that the groups on the front carbon are staggered ( ${60^ \circ }$ apart) or eclipsed (directly overlapping) with the groups on the back carbon. Below are two Sawhorse Projections of ethane.
The structure on the left is staggered, and the structure on the right is eclipsed. These are the simplest Sawhorse Projections because they have only two carbons, and all of the groups on the front and back carbons are identical.
Step 1: The sawhorse projection is:

Step 2: The stability of conformations can be understood by:
Clearly we can observe that, In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule.
Whereas on the other hand, when the staggered form changes into the eclipsed form, the electron clouds of the carbon-hydrogen bonds come closer to each other resulting in increase in electron cloud repulsion.
And in order to check the increased repulsive forces, molecules will have to possess more energy and thus have less stability.

Hence, Staggered form is more stable.

Note:Sawhorse projections are useful for determining if two molecules are enantiomers or diastereomers. They make it easier to see if the structures are mirror images or superimposable.