Chemistry · General Principles Of Organic Chemistry · NEET
Look straight down the C-C bond of ethane so the front carbon hides the back carbon. Pick one C-H bond on the front carbon and one C-H bond on the back carbon. The angle between these two bonds, seen in this front view, is the dihedral angle (torsion angle). It is NOT the H-C-H angle on one carbon; it measures how much the back carbon is rotated relative to the front carbon.
No, and NEET loves this trap. The bond angle (H-C-H) is about 109.5° and stays fixed because carbon is sp3. The dihedral angle is between bonds on two different carbons across the C-C bond, and it changes freely from 0° to 360° as the molecule rotates. Fixed sp3 bond angle, changing dihedral angle.
The dot in the centre is the front carbon; its three bonds come out from the dot like a Y. The big circle behind is the back carbon; its three bonds come out from the edge of the circle. When front and back bonds line up (overlap) you have the eclipsed form (dihedral 0°). When the back bonds sit in the gaps of the front bonds, you have the staggered form (dihedral 60°).
In staggered ethane the C-H bonds on the two carbons are 60° apart, so the bonding electron pairs are as far apart as possible. This minimises repulsion, so there is no torsional strain and the energy is lowest. In eclipsed ethane the C-H bonds overlap (0° apart), electron pairs repel strongly, giving maximum torsional strain and highest energy. The energy difference is small (about 12.5 kJ/mol), so at room temperature the molecule keeps rotating.
Torsional strain is the small extra energy caused by repulsion between the bonding electron pairs (C-H bonds) on the two carbons when they come close, as in the eclipsed form. It is maximum at 0° dihedral (eclipsed) and zero at 60° dihedral (staggered). This is the reason staggered is the preferred conformation.
The dihedral angle of the least stable conformer of ethane is:
The correct statement regarding the comparison of staggered and eclipsed conformations of ethane is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
As the back carbon rotates around the C-C bond, the dihedral angle changes continuously from 0° to 360°. Staggered forms occur at 60°, 180° and 300°; eclipsed forms occur at 0°, 120° and 240°.
No. Staggered and eclipsed ethane are conformations (conformers), not isomers. They interconvert by simple rotation about the C-C single bond without breaking any bond, so they cannot be separated. This is why they are called conformational isomers only in a loose sense.
No, it is small (about 12.5 kJ/mol). Because it is small, at room temperature the C-C bond rotates freely and the molecule does not stay locked in one form. Even so, at any instant the staggered form is preferred.
A C-C single (sigma) bond has cylindrical symmetry, so rotation does not break the overlap. A C=C double bond has a pi bond formed by sideways overlap of p orbitals; rotating would break this overlap, so rotation is restricted. That is why conformations exist for single bonds but cis-trans isomers exist for double bonds.
