three lines attached to it corresponding to three hydrogen atoms. The lines are inclined at an angle of ° to each other. Sawhorse projections of eclipsed and staggered conformations of ethane are depicted in Fig. .
. . Newman projections In this projection, the molecule is viewed at the C–C bond head on. The carbon atom nearer to the eye is represented by a point.
Three hydrogen atoms attached to the front carbon atom are shown by three lines drawn at an angle of ° to each other. The rear carbon atom (the carbon atom away from the eye) is represented by a circle and the three hydrogen atoms are shown attached to it by the shorter lines drawn at an angle of ° to each other. The Newman’s projections are depicted in Fig. .
. Fig. . Sawhorse projections of ethane Fig.
. Newman’s projections of ethane Fig. . Orbital picture of ethene depicting σ bonds only Relative stability of conformations: As mentioned earlier, 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. 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 repulsions. To check the increased repulsive forces, molecule will have to possess more energy and thus has lesser stability. As already mentioned, the repulsive interaction between the electron clouds, which affects stability of a conformation, is called torsional strain .
Magnitude of torsional strain depends upon the angle of rotation about C–C bond. This angle is also called dihedral angle or torsional angle . Of all the conformations of ethane, the staggered form has the least torsional strain and the eclipsed form, the maximum torsional strain . Therefore, staggered conformation is more stable than the eclipsed conformation.
Hence, molecule largely remains in staggered conformation or we can say that it is preferred conformation. Thus it may be inferred that rotation around C–C