Hydrocarbons 1 Question 1
1. In the following skew conformation of ethane, $\mathrm{H}^{\prime}-\mathrm{C}-\mathrm{C}-\mathrm{H}^{\prime \prime}$ dihedral angle is
(2019 Main, 12 April II)
(a) $58^{\circ}$
(b) $149^{\circ}$
(c) $151^{\circ}$
(d) $120^{\circ}$
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Solution:
- A dihedral angle is the angle between two $\mathrm{C}-\mathrm{H}$ bonds projected on a plane orthogonal to the $\mathrm{C}-\mathrm{C}$ bond. In the given skew conformation, having Newman’s projection the dihedral angle is
$$ \begin{aligned} & \mathrm{H}^{\prime}-\mathrm{C}-\mathrm{C}-\mathrm{H}^{\prime \prime} \ &=\left(\mathrm{H}^{\prime}-\mathrm{C}-\mathrm{C}-\mathrm{H}{a}\right)+\left(\mathrm{H}{a}-\mathrm{C}-\mathrm{C}-\mathrm{H}^{\prime \prime}\right) \ &=29^{\circ}+120^{\circ}=149^{\circ} \end{aligned} $$
