CONFORMATIONS
Alkanes can have infinite number of conformations due to (mostly) free rotations about the internuclear axis.
However, it should be remembered that the single bond in an alkane is hindered by a small energy barrier of 1-20 kJ mol–1 due to weak repulsive interaction between the adjacent bonds. Such a type of repulsive interaction is called torsional strain.
For example, ethane has infinite number of conformers. However, there are two extreme cases. One such conformation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed conformation and the other in which hydrogens are as far apart as possible is known as the staggered conformation. Any other intermediate conformation is called a skew conformation.
In all the conformations, the bond angles and the bond lengths remain the same.
Eclipsed and the staggered conformations can be represented by sawhorse and newman projections.
Relative stability of conformations: 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.
NOTE: Thus, it can be said that rotation about carbon-carbon single bond in ethane is almost free for all practical purposes.
NOTE: It has not been possible to separate and isolate different conformational isomers of ethane.
