Newman Projection

How to draw one

Pick a C–C single bond. Imagine you're an electron sitting on the front carbon, looking through the bond toward the back carbon. The front carbon's other three substituents hang off you at 120°. The back carbon's three substituents stick out from a disk you can't see through.

3D ball-and-stick                Newman projection
                                       H
   H   H                               |
    \ /                              H ─•─ H
     C — C                              ●
    / \  \                            H ─ H
   H   H  H                              |
                                        H

(looking from C1 toward C2 of ethane,    front C as dot,
60° staggered conformation)              back C as circle

Draw the front carbon as a small dot (or just an intersection point) with three lines going out at 12, 4, and 8 o'clock. Then draw a slightly larger circle around the dot — that's the back carbon — with three lines going out at 6, 10, and 2 o'clock. Substituents at the end of each line. The dihedral angle between any front line and any back line tells you the conformation.

Ethane: staggered vs eclipsed

Two limiting conformations exist as you rotate ethane through 360°:

Staggered (dihedral = 60°)         Eclipsed (dihedral = 0°)
       H                                  H/H
       |                                   |
   H ─•─ H                             H ─•─ H
       ●                                   ●
   H ─ ─ H                             H ─ H
       |                                   |
       H                                  H/H

Energy: 0 (reference)              Energy: +12 kJ mol⁻¹

Plot energy vs. dihedral and you get a sine-like curve with three minima (staggered, every 120°) and three maxima (eclipsed, every 120°). At room temperature ethane crosses the 12 kJ barrier ~10¹¹ times per second, so it's not stuck in any one conformation — but it spends >95 % of its time near the staggered minima.

Butane: gauche vs anti

Butane has two methyl groups instead of two extra hydrogens. Looking down the C2–C3 bond gives a richer energy profile:

Anti staggered (180°)           Gauche staggered (60°)
       CH₃                              CH₃
       |                                |
   H ─•─ H                          H ─•─ H
       ●                                ●
   H ─ ─ H                          H ─CH₃
       |                                |
      CH₃                               H

Energy: 0 (most stable)         Energy: +3.8 kJ mol⁻¹

Eclipsed conformations have higher peaks: methyl-methyl eclipsed at 0° dihedral is ~25 kJ mol⁻¹, hydrogen-methyl eclipsed at 120° is ~16 kJ mol⁻¹. The full butane curve has three minima (one anti at 180°, two gauche at ±60°) and three maxima (one syn-methyl-methyl at 0°, two H-methyl eclipsed at ±120°). At equilibrium ~70 % of butane sits in the anti conformation, ~30 % combined in the two gauche wells.

Why staggered beats eclipsed

Two contributions, in roughly the proportions modern theory has settled on:

1. Hyperconjugation (~9 of the 12 kJ). In the staggered conformation, each C–H bonding orbital (σ_CH) is aligned anti-periplanar to a C–H antibonding orbital (σ*_CH) on the neighboring carbon. Electron density delocalizes from σ into σ*, lowering the energy. In eclipsed geometry the σ-σ* alignment is gone.
2. Steric repulsion (~3 of the 12 kJ). Eclipsed hydrogens are ~2.30 Å apart, inside the sum of their van der Waals radii (~2.40 Å). The Pauli-driven repulsion is small for H-H but grows fast for H-CH₃, CH₃-CH₃, or H-OH eclipsing.

Older organic textbooks attributed the entire 12 kJ to steric repulsion. The hyperconjugation explanation was settled by 2001 NBO (Natural Bond Orbital) calculations and is now standard.

Newman vs other projections

	Newman	Sawhorse	Wedge-dash	Fischer
What it shows	One bond, head-on	One bond, oblique	Whole molecule, 3D	Stereocenters as crosses
Dihedral readable?	Yes, instant	Yes, with effort	Yes, with effort	No (eclipsed lie)
R/S readable?	No	Sometimes	Yes (with CIP)	Indirect
Best for	Conformations, E2 alignment	Visualizing both ends	General structure	Sugars, amino acids
Front/back convention	Dot front, circle back	3D oblique	Wedge front, dash back	Horizontal front, vertical back
Implies geometry	Real (one snapshot)	Real	Real	Idealized eclipsed
Multi-bond chains	One per bond	One per bond	Whole molecule	Whole chain stacked

Newman is the projection you reach for when the question is about rotation, torsion, or anti-periplanar geometry. For absolute stereochemistry, switch to wedge-dash. For sugars, switch to Fischer or Haworth.

Worked example: butane energy profile

Sweep the C2–C3 dihedral of butane from 0° to 360° and tabulate the energies:

Dihedral CH₃–C–C–CH₃	Conformation	Relative energy (kJ mol⁻¹)	Population at 298 K
0°	Eclipsed (CH₃-CH₃ syn)	~+25	negligible
60°	Gauche	+3.8	~14 % (each gauche well)
120°	Eclipsed (CH₃-H)	~+16	negligible
180°	Anti	0 (reference)	~70 %
240°	Eclipsed (CH₃-H)	~+16	negligible
300°	Gauche	+3.8	~14 %
360°	Eclipsed (CH₃-CH₃ syn)	~+25	negligible

The Boltzmann ratio between anti (0 kJ) and gauche (3.8 kJ) at 298 K is exp(-3 800 / 8.314 × 298) ≈ 0.21, multiplied by 2 (two gauche wells) ≈ 0.42. Normalize and you get ~70 % anti, 30 % gauche, in line with experimental NMR coupling-constant analysis on neat liquid butane.

Newman projections in cyclohexane

Cyclohexane in the chair conformation is six staggered-ethane Newmans glued together. Sight down any C–C bond of chair cyclohexane and you see a perfectly staggered Newman; sight down a bond of boat cyclohexane and you see an eclipsed Newman. That is exactly why chair is ~30 kJ mol⁻¹ more stable than boat — six bonds × ~5 kJ of avoided eclipsing per bond.

For a cyclohexane substituent, the axial-vs-equatorial choice is also a Newman question: an equatorial methyl is anti to two ring carbons, while an axial methyl is gauche to both. The 7.6 kJ mol⁻¹ A-value of methylcyclohexane is exactly two gauche-butane interactions × 3.8 kJ each.

Newman projections and E2 elimination

The E2 reaction needs the β-hydrogen and the leaving group to be anti-periplanar — dihedral 180° around the C–C bond. A Newman projection makes that requirement obvious:

Anti-periplanar E2 transition state    Syn-periplanar (forbidden in classical E2)
       H                                       H, X eclipsed
       |                                          |
   R ─•─ R                                  R ─•─ R
       ●                                          ●
   R ─ ─ R                                  R ─ R
       |                                          |
       X                                          X (180° below)

(180° dihedral H-C-C-X)                  (0° dihedral H-C-C-X)

If the molecule cannot reach a 180° dihedral (e.g., locked into a ring with the wrong axial/equatorial pattern), E2 simply doesn't happen — Newman analysis is how you predict that.

Variants and conventions

• Reverse Newman — sometimes drawn looking from C2 to C1 instead of C1 to C2; same molecule, reflected diagram. Always check which atom is the dot.
• Three-bond Newman — for propane or longer, students sometimes nest two Newmans, but that loses spatial coherence; sawhorse is better.
• Sawhorse projection — an oblique 3D version that shows the same dihedral but keeps both end carbons visible. Easier for visualizing the molecule's chirality, harder for reading the dihedral precisely.
• Newman of a heteroatom bond — works fine for C–O (ethanol's OH gauche/anti to a methyl) and C–N (amino-alcohol conformations); used in carbohydrate computational analysis.

Pitfalls

• Confusing the dot and the circle. The dot is the front carbon and its three substituents radiate from the center; the circle is the back carbon and its three substituents radiate from the rim. Drawing all six substituents from a single dot is a rookie error.
• Dihedral confusion. The dihedral is between a front substituent and a back substituent, not between two on the same carbon. Two substituents on the same end of the bond are at 120°, not a dihedral.
• Forgetting which way you're looking. A Newman of butane viewed C1 → C4 is different from C4 → C1; keep the orientation labeled.
• Drawing eclipsed when you mean staggered. Eclipsed projections have the back substituents directly behind the front ones; rookies often draw all six lines at 60° spacing, accidentally producing a staggered diagram while labeling it eclipsed.
• Using Newman for chirality. Newman projections suppress the connectivity beyond the chosen bond; you can't read R/S from a Newman without first identifying the priorities. Use wedge-dash if absolute configuration is the question.