A Newman projection of ethane is drawn looking along the C–C bond. The front carbon is the point where three C–H bonds meet at 120° to one another; the rear carbon is a circle whose three C–H bonds lie directly behind those of the front carbon, a dihedral angle of 0°. Which statement is correct?
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With the rear bonds directly behind the front ones, the C–H bond pairs repel most, so this is the eclipsed conformation. It is less stable than the staggered form by about 12.5 kJ mol⁻¹, too small a difference to allow the two to be isolated.
- A.
The eclipsed form has maximum repulsion between the bonds and is the least stable.
- B.
The rotational barrier of about 12.5 kJ mol⁻¹ is so small that the conformations interconvert rapidly and cannot be separated.
- D.
In the staggered form the rear C–H bonds lie midway between the front ones, at 60°, not directly behind them.
NCERT: Class 11 Chemistry, Chapter 9