Rotate the bond and watch the strain energy curve draw itself.
Newman Projection simulator
Drag across the molecule to rotate the back carbon
This simulation needs a modern browser with canvas support. The measurements and formula below still describe the physics.
Live measurements
Controls
Graph
Formula
E(φ) = E torsional + E steric
- E Strain energy
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- E tors Torsional strain
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- E ster Steric strain
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Observations
What's happening
Real-world examples
Ethane
Shrink both large groups to hydrogen and the steric term disappears. What is left is a 12 kJ/mol barrier from three eclipsing H/H pairs, and three staggered conformers no molecule prefers over the others.
Anti butane
At 180 degrees the two methyls sit as far apart as the bond allows. This is the global minimum, and about two thirds of butane molecules are in it at room temperature.
Gauche butane
At 60 degrees the molecule is still staggered, yet the two methyls are close enough to cost 3.8 kJ/mol. There are two of these wells, so gauche keeps a real share of the population.
Fully eclipsed
At 0 degrees the methyls are directly behind one another, which costs 19 kJ/mol and is the highest point on the curve. Nothing sits here; it is the wall between the two gauche wells.
Uses of newman projection
- Predicting the reactive conformer for an E2 elimination
- Explaining why polyethylene chains prefer an extended zig-zag
- Checking a conformer drawing before an organic chemistry exam
newman projection pitfalls
- Measuring the dihedral between the wrong pair of front and back bonds
- Calling conformers isomers that can be separated
- Assuming ethane has steric strain because it has a rotation barrier
newman projection questions (8)
What is a Newman projection?
A Newman projection is what one carbon-carbon bond looks like end on. The front carbon is the point where three bonds meet. The back carbon is the circle behind it, and its three bonds start at the circumference. Drawing the bond this way makes the angle between front and back groups the only thing you have to read.
What is the dihedral angle in a Newman projection?
The dihedral angle, also called the torsion angle, is the angle between a chosen front bond and a chosen back bond. Zero degrees means the two point the same way, so the groups eclipse. One hundred and eighty degrees means they point opposite ways, which is the anti arrangement. Pick the two highest-priority groups and measure between those, or you will read a different number for the same molecule.
What is the difference between staggered and eclipsed conformers?
Staggered means every front bond sits between two back bonds, at 60, 180 and 300 degrees. Eclipsed means front and back bonds line up, at 0, 120 and 240 degrees. Staggered conformers are the minima on the energy curve and eclipsed conformers are the maxima. Nothing rests at an eclipsed angle, because there is no well there to rest in.
Why is anti butane more stable than gauche butane?
Both are staggered, so their torsional strain is identical. The difference is distance between the two methyl groups. Anti holds them 180 degrees apart and gauche holds them 60 degrees apart, close enough for their electron clouds to push against each other. That crowding costs 3.8 kJ/mol, which is the whole gap.
What is the rotation barrier of ethane?
Ethane costs 12 kJ/mol to rotate through its eclipsed conformation. That number is three eclipsing hydrogen pairs at about 4 kJ/mol each. Set the substituent size slider to zero and the simulator becomes ethane exactly, with the steric readout pinned at zero and the barrier at 12.
How do you calculate the percentage of anti and gauche conformers?
Compare Boltzmann factors. The gauche to anti ratio is 2 x e^(-delta E / RT), where delta E is 3.8 kJ/mol for butane and the two counts both gauche wells. At 298 K that works out near 64 percent anti. Raise the temperature slider and watch the split even out, because RT climbs toward the size of the gap.
Can conformers be separated from each other?
No, and this is the most common misconception about them. A 12 to 19 kJ/mol barrier is small enough that a bond rotates through it billions of times a second at room temperature. Conformers are a population, so a bottle of butane holds every conformer at once in the proportions the Boltzmann factors set.
Why does the gauche minimum sit slightly off 60 degrees?
Steric repulsion keeps falling as the methyls move apart, so it pulls the minimum toward anti. Torsional strain pushes back once the bonds start to eclipse again. The balance lands a degree or two past 60 rather than exactly on it, which real conformational energy surfaces also show.