In the following skew conformation of ethane, $\mathrm{H} \cdot-\mathrm{C}-\mathrm{C}-\mathrm{H}$" dihedral angle is:

JEE Main · 2019 · 12 Apr 2019 · Shift 2 · General Organic Chemistry

In the following skew conformation of ethane,

H \cdot - C - C - H

" dihedral angle is:

Answer: (C)

In a Newman projection the dihedral angle is simply the angle, read around the

C - C

axis, between the chosen front-carbon bond and the back-carbon bond. Here the front-carbon

C - H^{''}

bond points straight down. Starting from that downward bond and sweeping round the axis to the back-carbon

C - H^{'}

bond in the upper right, the twist is one staggered step of

12 0^{\circ}

plus the extra

2 9^{\circ}

by which

H^{'}

is turned past the neighbouring front-carbon

C - H

bond (the small

2 9^{\circ}

arc marked in the figure). Adding these, the

H^{'} - C - C - H^{''}

dihedral angle is

12 0^{\circ} + 2 9^{\circ} = 14 9^{\circ}

JEE Main · 2019 · 12 Apr 2019 · Shift 2 · Single correct option