Chemistry · General Principles Of Organic Chemistry · NEET
Ethane has two main conformations formed by rotating one CH3 group around the C-C single bond. The staggered form is the most stable because its C-H bonds are as far apart as possible (dihedral angle 60 degrees, no torsional strain), while the eclipsed form is least stable because its C-H bonds overlap (dihedral angle 0 degrees, maximum torsional strain). Memory hook: Staggered = Spread out = Stable; Eclipsed = C-H bonds Eclipse (hide) each other = Energy high.
Newman projections of ethane: in the staggered form the rear C-H bonds sit between the front ones (60 degrees apart, minimum repulsion, most stable); in the eclipsed form they line up behind the front bonds (0 degrees, maximum torsional strain, least stable).
Your doubts, answered
Why is the staggered conformation of ethane more stable than the eclipsed?
In the staggered form the six C-H bonds on the two carbons are placed as far apart as possible. This keeps the electron clouds of the C-H bonds at maximum distance, so repulsion between them is minimum. Minimum repulsion means minimum energy and maximum stability. In the eclipsed form the C-H bonds come directly in front of each other, so their electron clouds are closest, repulsion is maximum, energy is high, and stability is least.
What is torsional strain?
Torsional strain is the small extra energy a molecule gains due to repulsion between electron clouds of bonds on two adjacent atoms when they come close during rotation about a single bond. The staggered form of ethane has the least torsional strain (bonds far apart). The eclipsed form has the maximum torsional strain (bonds overlap). The energy difference between them is only about 12.5 kJ/mol, which is very small.
What is the dihedral (torsional) angle for each form?
The dihedral angle is the angle between a C-H bond on the front carbon and a C-H bond on the rear carbon when you look straight down the C-C bond. For the staggered form it is 60 degrees (bonds spread out). For the eclipsed form it is 0 degrees (bonds line up exactly). NEET has directly asked the dihedral angle of the least stable conformer, which is the eclipsed form = 0 degrees.
Can staggered and eclipsed conformers be separated and isolated?
No. The energy barrier between them is very small (about 12.5 kJ/mol), so at room temperature molecular collisions supply enough energy for free rotation about the C-C bond. The molecule keeps flipping between forms extremely fast, so it has never been possible to separate ethane into different conformers. This is why they are called conformers, not different compounds.
Do bond angles and bond lengths change between conformations?
No. During rotation about the C-C single bond, no bond is broken. Only the relative positions of the C-H bonds change. So all H-C-H bond angles and all bond lengths stay exactly the same in every conformation. Only the dihedral angle (and hence the energy) changes. NEET has tested exactly this point.
⚠️ The NEET trap ✗ The eclipsed conformation is more stable because the C-H bonds overlap and appear closer. ✓ The staggered conformation is the most stable because its C-H bonds are farthest apart, giving minimum electron-cloud repulsion and no torsional strain; the eclipsed form is the least stable due to maximum torsional strain. 🧠 Overlapping (eclipsing) means electron clouds crowd together = more repulsion = LESS stable, not more. Never confuse looking close with being stable.
Real NEET questions
2016
The correct statement regarding the comparison of staggered and eclipsed conformations of ethane is:
A · The staggered conformation of ethane is less stable than the eclipsed conformation, because the staggered conformation has torsional strain.
B · The eclipsed conformation of ethane is more stable than the staggered conformation, because the eclipsed conformation has no torsional strain.
C · The eclipsed conformation of ethane is more stable than the staggered conformation even though the eclipsed conformation has torsional strain.
D · The staggered conformation of ethane is more stable than the eclipsed conformation, because the staggered conformation has no torsional strain. ✓
Solution: In the staggered form the C-H bonds on the two carbons are 60 degrees apart (dihedral angle), so electron-cloud repulsion is minimum. This means no (least) torsional strain, minimum energy, and maximum stability. The eclipsed form has C-H bonds aligned (0 degrees dihedral), giving maximum torsional strain and highest energy. So staggered is more stable because it has no torsional strain. Answer (D).
2021
The dihedral angle of the least stable conformer of ethane is:
A · 60 degrees
B · 0 degrees ✓
C · 120 degrees
D · 180 degrees
Solution: The least stable conformer of ethane is the eclipsed form (maximum torsional strain). In the eclipsed form the front and rear C-H bonds overlap exactly, so the dihedral (torsional) angle between them is 0 degrees. The staggered conformer (60 degrees) is the most stable. Answer (B).
Solved General Principles Of Organic Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The staggered conformation. Its C-H bonds are as far apart as possible (dihedral angle 60 degrees), giving minimum repulsion, minimum energy, and no torsional strain.
Which conformation of ethane is the least stable?
The eclipsed conformation. Its C-H bonds overlap (dihedral angle 0 degrees), giving maximum electron-cloud repulsion and maximum torsional strain, so it has the highest energy.
How many conformations does ethane have?
Ethane can in principle have an infinite number of conformations because rotation about the C-C bond is continuous. But only two are named for exams: the staggered (most stable) and the eclipsed (least stable) extremes.
What is the energy difference between staggered and eclipsed ethane?
About 12.5 kJ/mol. This barrier is small, so rotation is almost free at room temperature and the conformers cannot be separated.
What allows free rotation in ethane?
The C-C bond in ethane is a sigma bond whose electron cloud is symmetrical about the internuclear axis. Rotating one carbon does not disturb this cylindrical overlap, so rotation about the C-C single bond is essentially free.
