Question

In the following skew conformation of ethane, $H^{\prime }-C-C-H^{\prime \prime }$ dihedral angle is:

Answer 1

A dihedral angle is the angle between two $C-H$ bonds projected on a plane orthogonal to the $C-C$ bond. In the given skew conformation, having Newman's projection the dihedral angle is:
$H^{\prime }-C-C-H^{\prime \prime }=(H^{\prime }-C-C-H_a)+(H_a-C-C-H^{\prime \prime })$
$={29}^o+{120}^o={149}^o$