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Conformations look easy in the textbook and go wrong in the exam: rear bonds drawn in the wrong place, staggered and eclipsed swapped, or conformers treated as separate compounds. This post shows how to draw a Newman projection, follows one full rotation of ethane step by step, and puts numbers on the energy barrier.
Where is this in the syllabus?
The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus both list "Conformations: Sawhorse and Newman projections (of ethane)". The JEE Advanced 2026 syllabus asks for ethane and butane, "Newman projections only". The base text is section 9.2.4 of the NCERT Class 11 unit on hydrocarbons. Check the current year's syllabus before you plan.
What is a conformation?
NCERT defines conformations as spatial arrangements "which can be converted into one another by rotation around a C-C single bond". Rotation is not perfectly free: it is hindered by a small barrier of 1 to 20 kJ/mol, and the repulsion behind it is called torsional strain.
Two extremes matter. In the eclipsed form, the hydrogens on the two carbons are as close as possible. In the staggered form, they are as far apart as possible. Anything in between is a skew conformation. NCERT stresses that bond angles and bond lengths stay the same in all of them; only the angle of rotation changes.
How do you draw a Newman projection?
Look straight down the C–C bond.
Draw the front carbon as a point, with three bonds meeting at it, 120° apart.
Draw the rear carbon as a circle, with its three bonds starting from the edge of the circle, also 120° apart.
For staggered, place the rear bonds between the front ones. For eclipsed, place them behind the front ones; in practice you draw them slightly offset so they stay visible.
In a Sawhorse projection you look at the bond from an angle instead. NCERT draws the front carbon at the lower end of the C–C line and the rear carbon at the upper end.
The angle between a front C–H and a rear C–H is the dihedral (torsional) angle. Staggered forms have 60° between neighbouring front and rear bonds; eclipsed forms have 0°.
Worked example: one full turn
Mark one hydrogen on the front carbon, H(a), and one on the rear carbon, H(b). Start with them eclipsed, at 0°, and turn the rear carbon in 60° steps.
0°: eclipsed
60°: staggered
120°: eclipsed
180°: staggered, with H(a) and H(b) directly opposite
240°: eclipsed
300°: staggered
360°: eclipsed, back to the start
So one turn passes through three eclipsed and three staggered forms. The abstract of a 2001 Nature paper by Pophristic and Goodman describes the same thing: during one rotation, ethane alternates "three times" between eclipsed and staggered. Because all six hydrogens are identical, the three staggered forms are identical, and so are the three eclipsed ones. In butane, which JEE Advanced adds, they are not, which is why butane has more named conformations.
How big is the barrier?
NCERT gives the energy difference between staggered and eclipsed ethane as about 12.5 kJ/mol. Two quick calculations show what that means.
Per eclipsing pair. An eclipsed form has three H–H pairs lined up. If you share the barrier equally among them (a simplification), each costs 12.5 ÷ 3 ≈ 4.2 kJ/mol.
Against thermal energy. At 298 K, RT = 8.314 × 298 ≈ 2478 J/mol ≈ 2.5 kJ/mol. The barrier is 12.5 ÷ 2.48 ≈ 5 times RT. That is small for a chemical barrier, which is why NCERT says molecules gain enough energy through collisions to cross it, and rotation is "almost free for all practical purposes".
Why is the staggered form more stable?
NCERT's answer, which is what boards and entrance exams expect, is electron-cloud repulsion: in the eclipsed form the C–H bond electrons come closer, raising torsional strain. Know that chemists have argued about the cause. Pophristic and Goodman's computational study concluded that the staggered preference comes from stabilisation by hyperconjugation, and that removing repulsion did not change which form was preferred. For your exam, write the NCERT explanation; the debate is useful background if you go on to study chemistry. Our post on inductive, resonance and hyperconjugation effects explains hyperconjugation.
Which mistakes cost marks?
Swapping the names. Eclipsed means lined up (0°); staggered means in between (60°).
Drawing rear bonds from the centre. Rear bonds start at the circle.
Saying rotation is completely free. It is almost free, with a small barrier.
Calling conformers structural or geometrical isomers. They differ only by rotation about a single bond. See our post on stereoisomerism for the real isomer types.
Changing bond angles between conformations. They stay the same.
A practice routine
Draw staggered and eclipsed ethane in both Newman and Sawhorse form, from memory, three times.
Repeat the full-turn table above with a different starting angle, such as 60°.
Sketch energy against dihedral angle from 0° to 360°: three peaks and three troughs.
If you are taking JEE Advanced, repeat steps 1 to 3 for butane and note why its forms are not all equal.
Three-dimensional rotation is easier to see than read about. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, and a physical model kit helps too.
Today, draw the six positions of one full turn and label each one before you look back at this page.
