Conformations of Ethane: Staggered vs Eclipsed (Dihedral Angle)
Chemistry · Hydrocarbons · NEET
Ethane can rotate around its C-C single bond, giving different shapes called conformations. The two important ones are staggered (H atoms far apart, dihedral angle 60°, most stable) and eclipsed (H atoms overlap, dihedral angle 0°, least stable). Memory hook: "Staggered = Spread out = Stable; Eclipsed = Erect on top = Energy high."
Newman projections: front carbon (dot) and back carbon (circle). In staggered ethane the back H atoms (red) sit between the front H atoms (dihedral 60°, low energy). In eclipsed ethane the back H atoms hide directly behind the front ones (dihedral 0°), causing torsional strain and higher energy.
Your doubts, answered
Is the staggered or eclipsed conformation of ethane more stable?
The staggered conformation is more stable. In staggered form the C-H bonds of the front and back carbon are as far apart as possible, so the electron clouds repel each other the least. Less repulsion means less energy and more stability. The eclipsed form is the least stable because its C-H bonds line up directly and repel strongly. This exact idea was asked in NEET 2016.
What is the dihedral angle for staggered and eclipsed ethane?
Dihedral angle (also called torsional angle) is the angle between a front C-H bond and the nearest back C-H bond when you look down the C-C bond. For the eclipsed form it is 0° (bonds overlap). For the staggered form it is 60° (bonds are spread evenly). NEET 2021 asked the dihedral angle of the least stable conformer, which is the eclipsed form at 0°.
Why is the eclipsed conformation less stable than staggered?
Because of torsional strain. In the eclipsed form the front and back C-H electron clouds come very close and repel each other. This repulsion raises the energy, making it unstable. The staggered form has no torsional strain, so it sits at the lowest energy. Do not confuse this with steric strain, which matters more in bigger molecules like butane.
Do bond length and bond angle change between conformers of ethane?
No. When ethane rotates about the C-C single bond, no bond is broken. Only the dihedral angle (the twist) changes. Both the H-C-H bond angles and all the bond lengths stay exactly the same. This is why NEET 2017 answer is 'both bond angle and bond length remain same.' This is the most common trap in this topic.
What is the difference between conformation and configuration?
Conformations differ only by rotation around a single bond, and no bonds are broken, so you cannot separate them. Configurations (like cis-trans) differ in the actual arrangement of atoms and you must break a bond to change one into another. Ethane's staggered and eclipsed forms are conformations, not configurations, so they cannot be isolated at room temperature.
⚠️ The NEET trap ✗ Eclipsed ethane is more stable because its bonds line up neatly. ✓ Staggered ethane is more stable because it has no torsional strain; eclipsed has maximum torsional strain and is least stable. 🧠 Lined-up (eclipsed) does NOT mean happy. Overlapping C-H clouds repel, raise energy, and lower stability. Spread out (staggered) always wins.
Real NEET questions
NEET 2016 Phase 1
The correct statement regarding the comparison of staggered and eclipsed conformations of ethane, is:
A · The staggered conformation of ethane is less stable than eclipsed conformation, because staggered conformation has torsional strain.
B · The eclipsed conformation of ethane is more stable than staggered conformation, because eclipsed conformation has no torsional strain.
C · The eclipsed conformation of ethane is more stable than staggered conformation even though the eclipsed conformation has torsional strain.
D · The staggered conformation of ethane is more stable than eclipsed conformation, because staggered conformation has no torsional strain. ✓
Solution: In the staggered form the C-H electron clouds are as far apart as possible, giving minimum repulsion (least torsional strain) and maximum stability. The eclipsed form has maximum torsional strain and is least stable. So option D is correct: staggered is more stable because it has no torsional strain.
NEET 2017
With respect to the conformers of ethane, which of the following statements is true?
A · Bond angle remains same but bond length changes
B · Bond angle changes but bond length remains same
C · Both bond angle and bond length change
D · Both bond angles and bond length remain same ✓
Solution: Conformers of ethane interconvert only by rotation about the C-C single bond. No bond is broken; only the dihedral (torsional) angle changes. Therefore both bond angles and bond lengths stay unchanged. Correct answer is D.
NEET 2021
The dihedral angle of the least stable conformer of ethane is:
A · 60°
B · 0° ✓
C · 120°
D · 180°
Solution: The least stable conformer of ethane is the eclipsed form (maximum torsional strain). In it, each front C-H bond directly overlaps a back C-H bond, so the dihedral (torsional) angle between them is 0°. Correct answer is B.
Solved Hydrocarbons NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the twist angle between a front C-H bond and the nearest back C-H bond when you look straight down the C-C bond of ethane.
What is the energy difference between the two forms?
The eclipsed form is about 12.5 kJ/mol higher in energy than the staggered form. This barrier is small, so rotation is almost free at room temperature.
Can staggered and eclipsed ethane be separated in the lab?
No. The energy barrier is too low, so the molecule keeps rotating. The two conformers cannot be isolated as separate compounds.
Which projection is best to show conformations?
The Newman projection is easiest. You look down the C-C bond: the front carbon is a dot with three lines, and the back carbon is a circle with three lines.
How many staggered and eclipsed forms does ethane have during one full turn?
During a 360° rotation, ethane passes through three staggered forms (every 120°) and three eclipsed forms in between.
