Newman Projections: Drawing Ethane

Introduction

A Newman projection shows the arrangement around a chosen bond more clearly than a zigzag line drawing. Ethane is the simplest example: looking along its C–C bond makes the difference between staggered and eclipsed conformations visible at once. This view will also help with substituted carbon chains.

Core explanation

Choose a viewing direction along the bond from one carbon toward the other. The front carbon is drawn as a dot or small central point, with three bonds radiating 120° apart. The rear carbon is represented by a circle surrounding the front point, with three bonds extending outward from the circle. The carbon–carbon bond itself points toward the viewer and is not drawn as a side spoke. In staggered ethane, each rear C–H bond lies halfway between adjacent front C–H bonds: the nearest H–C–C–H dihedral angle is 60°. In eclipsed ethane, each rear bond lines up with a front bond, giving 0° dihedral angles. Rotating the rear group by 60° switches between a staggered and an eclipsed drawing. Continuing through a full 360° rotation gives three equivalent staggered minima and three equivalent eclipsed maxima because the three H atoms at each end are interchangeable. The projection is a viewpoint, not a different kind of molecule; reversing the viewing direction changes which carbon is drawn at the front but leaves the actual conformation unchanged. When substituents differ, label every spoke before rotating, because accidentally moving a group from front to rear changes the representation rather than the bond angle.

Step-by-step reasoning

Select the C–C bond and choose which carbon is nearer the eye. Put its three C–H bonds on rays from the centre at about 120° intervals. Draw a circle for the rear carbon and put its three bonds either aligned with the front rays for eclipsed or midway between them for staggered. Mark a front–rear H dihedral angle.

Visual explanation

Picture one three-spoked wheel placed in front of another. When spokes line up, they hide behind one another and the Newman drawing is eclipsed. Turning the rear wheel by 60° places its spokes in the gaps and creates a staggered drawing.

Real-world analogy

Looking down a row of two ceiling fans gives a similar view. The near fan's blades are attached to a central hub; the far fan's blades seem to emerge from a ring around it. When blades overlap in sight, the projected positions are eclipsed.

Real-world example

Conformational diagrams help chemists predict the accessible shape of flexible molecules in a reaction. Ethane is the teaching model because all six substituents are hydrogen; in substituted ethanes the same Newman method shows steric interactions among larger groups.

Why?

The Newman projection strips away distracting chain geometry and exposes dihedral angles. Eclipsing brings adjacent bonds into aligned orientations with greater torsional strain; offsetting them lowers energy. The drawing thus links a geometric angle to a physical energy difference.

Common misconception

The rear carbon is not the circle's centre dot; the centre dot denotes the front carbon and the surrounding circle denotes the rear one. Also, rotating the paper does not change the dihedral angle, whereas rotating only one carbon group does.

Worked example

Question: Draw an eclipsed Newman projection of ethane viewed down C1–C2, then rotate the rear carbon 60°. Reasoning: Begin with each rear C–H spoke directly behind a front C–H spoke. Turning only the rear spokes by 60° places them midway between the front spokes. Answer: The first structure is eclipsed at 0°; the second is staggered with a 60° nearest dihedral angle.

Quick check

1. What symbol normally represents the rear carbon in a Newman projection? Answer: A circle surrounding the front carbon's central dot.

Exam focus

State the viewing bond and direction. Keep front and rear atoms distinct, and label substituents before any rotation. Remember that ethane has three equivalent staggered and three equivalent eclipsed orientations over a full turn.

Advanced insight

A Newman projection can be converted into a sawhorse drawing by tilting the viewed bond back into the plane and restoring its length. Both encode the same dihedral relationships; neither alone supplies a new absolute configuration.

Summary

A Newman projection looks down a selected bond. The front carbon is the central point and the rear carbon the circle. Ethane is staggered when rear C–H bonds lie between front bonds and eclipsed when they align. A 60° rotation switches the two, showing conformational change without bond breaking.

Practice questions

1. In a Newman projection, which carbon is indicated by the central point? Answer: The front carbon, nearer the observer.

2. What is the nearest H–C–C–H dihedral angle in staggered ethane? Answer: 60°, because the rear spokes are halfway between front spokes.

3. How many equivalent staggered positions occur in a full ethane rotation? Answer: Three, at intervals of 120°, because each carbon has three equivalent hydrogens.

4. Does rotating the entire page by 60° turn staggered ethane into eclipsed ethane? Answer: No. Paper rotation moves front and rear spokes together; their relative dihedral angle stays unchanged.