How Newman Projections Work

Quick Answer

A Newman projection shows one carbon-carbon bond viewed end on, so you can read the angle between the groups on either end. That angle is the dihedral angle, and the energy it costs is E(phi) = E torsional + E steric. Staggered angles at 60, 180 and 300 degrees are the minima; eclipsed angles at 0, 120 and 240 degrees are the maxima. For butane the anti conformer at 180 degrees is the global minimum, gauche at 60 degrees costs 3.8 kJ/mol, and rotating through the fully eclipsed conformation costs 19 kJ/mol. Drag the molecule here and the energy curve fills in underneath it.

How Do You Read A Newman Projection?

The point in the middle is the front carbon, and the circle behind it is the back carbon. Three bonds meet at the point and three more start at the circumference. Everything else about the drawing is the angle between one front bond and one back bond. Read that angle and you have named the conformer.

• Front bonds run all the way to the centre; back bonds stop at the circle.
• Measure the dihedral between the two highest-priority groups, one on each carbon.
• Zero degrees is eclipsed and 180 degrees is anti, whichever molecule you are drawing.

What Is The Difference Between Torsional And Steric Strain?

Torsional strain is the cost of lining bonds up behind each other, and it appears only near an eclipsed angle. Steric strain is two bulky groups pushing against each other, and it never fully switches off. The simulator reports the two separately because they answer different questions. One explains why staggered beats eclipsed, and the other explains why anti beats gauche.

• Torsional strain in ethane is about 4 kJ/mol per eclipsing hydrogen pair.
• Steric strain scales with how big the two groups are, so it grows as you widen the size slider.

Why Is Anti Butane More Stable Than Gauche Butane?

Both conformers are staggered, so neither pays any torsional penalty. The methyl groups are what differ: 180 degrees apart in anti, 60 degrees apart in gauche. Crowding at 60 degrees costs 3.8 kJ/mol, and that is the entire anti vs gauche energy gap. Set the size slider to zero and the gap vanishes, because hydrogens are too small to crowd.

• For example, drag from 180 to 60 degrees and the strain readout climbs from 0.0 to 3.8 kJ/mol while the torsional readout stays at zero.
• That is the anti vs gauche difference isolated: same staggering, different distance.

How Do You Find The Most Stable Conformer Of Butane?

Rotate to every staggered angle and compare. Butane has three: 60, 180 and 300 degrees. Anti at 180 degrees reads 0.0 kJ/mol and the other two read 3.8 kJ/mol each, therefore anti is the most stable conformer. The Newman projection simulator does the comparison for you, because the strain curve underneath marks all three wells at once.

• The minima are always staggered. If your answer is an eclipsed angle, you have found a maximum instead.
• Ties are real: ethane has three identical minima, so no single conformer is most stable.

How Do You Work Out Conformer Populations?

A strain gap becomes a population split through the Boltzmann factor e^(-delta E / RT). Butane has two gauche wells against one anti well, so the ratio carries a factor of two. At 298 K the arithmetic lands near 64 percent anti. Raise the temperature and the split flattens, because RT grows toward the size of the gap.

• RT at 298 K is about 2.5 kJ/mol, which is the same order as the 3.8 kJ/mol gauche penalty.
• A gap of 3.8 kJ/mol is worth a two to one split; a gap of 12 kJ/mol leaves almost nothing in the higher well.

Ethane vs Butane: What Actually Changes?

Ethane has three identical staggered conformers and one barrier height. Butane splits that symmetry: one anti well, two shallower gauche wells, and two different barrier heights depending on which groups eclipse. Slide the substituent size from zero up to 25 percent and watch the flat ethane curve break into butane in front of you.

• Ethane barrier: 12 kJ/mol, from three eclipsing hydrogen pairs.
• Butane barriers: 16 kJ/mol through the methyl and hydrogen eclipse, 19 kJ/mol through the methyl and methyl eclipse.

Where Does Conformational Analysis Actually Matter?

Conformational analysis decides which shape a molecule reacts from, not merely which shape it prefers. An E2 elimination needs the leaving group and the hydrogen anti-periplanar, so the reactive conformer is a specific dihedral rather than the most populated one. Polyethylene chains pack into an extended zig-zag for the same reason butane prefers anti, and that packing is what makes the material stiff. For an exam, the practical use is checking a conformer drawing before you commit to it.

• E2 elimination: the reactive conformer can be a minor one, because reaction rate depends on both population and geometry.
• Polymers: repeat the anti preference along a chain and you get the extended zig-zag that sets how the solid packs.

Common Mistakes