A Newman projection looks straight down one C–C bond: the front carbon is the dot where three bonds meet; the back carbon is the circle. Drawn here, not just described.
Single bonds rotate freely, so each molecule flickers between rotational conformations — here are the ones we'll look down.
Structures drawn live (RDKit skeletal formulas).
1. A Newman Projection Looks Straight Down One C–C Bond
Sight along the bond; the one variable that matters is the dihedral angle — the twist between a front bond and the nearest back bond.
2. Staggered Is Low Energy; Eclipsed Is High Energy
Staggered puts the back bonds in the gaps between front bonds (60° offset, low energy); eclipsed lines them up (0°, high energy), and the two alternate every 60°.
3. Ethane Has Just One Staggered and One Eclipsed Form
With only hydrogens, every staggered form is identical; the rotation barrier is about 2.9 kcal/mol, from three eclipsing H–H interactions.
4. Butane Splits Staggered Into Anti and Gauche
Down C2–C3, the two methyls make staggered split into anti (180° apart, most stable) and gauche (60° apart, ~0.9 kcal/mol higher); eclipsed peaks at methyl-on-methyl.
5. Two Strains Set the Heights: Torsional and Steric
Torsional strain comes from bonds eclipsing bonds; steric strain comes from bulky groups crowding — both stack at once in fully eclipsed methyl-on-methyl butane.
6. The Energy-vs-Angle Curve Repeats Every 120°
Minima fall at the staggered angles (60°, 180°, 300°) and maxima at the eclipsed ones; butane's wave is lumpy, the same logic behind the cyclohexane chair conformation.
7. Summary
View down one C–C bond · front dot, back circle · dihedral angle is the variable · staggered low, eclipsed high · butane splits into anti (low) and gauche · energy vs angle is one repeating wave.
Quiz yourself
Tap a question to reveal the answer — free, no login.
The front (near) carbon is a dot where three bonds meet at 120° — a "Y". The back (far) carbon is a large circle with three bonds coming off its rim, also 120° apart. You are looking straight down the C–C bond that joins them.
Staggered is lower. Its bonds sit 60° apart, so the bonding electron pairs on the front and back carbons are as far from each other as possible, minimizing torsional strain. Eclipsed lines the bonds up at 0°, forcing electron pairs together — an energy maximum.
Lowest is anti — the two methyls 180° apart (staggered, minimal steric strain). Highest is the fully eclipsed methyl-on-methyl form (0°), where torsional and steric strain stack, about 4.5–6 kcal/mol above anti.
No. Conformations interconvert by simple rotation about a sigma bond — no bonds are broken. Anti and gauche butane are the same molecule caught in different rotational poses, flickering back and forth billions of times per second.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.