01 / 21

Conformational Analysis:
Newman Projections

Learn to visualize molecular rotation around \(\text{C-C}\) single bonds, identify staggered and eclipsed conformations, and predict relative stability using Newman projections.

Organic Chemistry 1

What is a Conformation?

A conformation is a specific arrangement of atoms in a molecule that results from rotation around a single bond. Unlike constitutional isomers or stereoisomers, conformations are not different molecules: they interconvert rapidly at room temperature.

Why Conformations Matter

Reactivity
Molecules react from specific conformations. The preferred conformation affects which products form.
Stability
Different conformations have different potential energies. Molecules spend more time in lower-energy conformations.
Physical Properties
Boiling points, dipole moments, and spectral data are influenced by the distribution of conformations.

The Newman Projection

A Newman projection is a way to visualize the conformation of a molecule by looking straight down the axis of a \(\text{C-C}\) bond. The front carbon is a dot at the center, and the back carbon is a circle behind it.

H H H H H H Staggered Conformation of Ethane Front C Back C

Staggered vs Eclipsed

Staggered Dihedral angle = 60° Eclipsed Dihedral angle = 0° vs

Torsional Strain and Energy

When bonds on adjacent carbons eclipse each other (dihedral = 0°), their electron clouds repel. This repulsion is called torsional strain. The eclipsed conformation of ethane is approximately \(\Delta E \approx 12 \text{ kJ/mol}\) higher in energy than the staggered conformation.

0° 60° 120° 180° 240° 300° 360° Energy Dihedral Angle Eclipsed Eclipsed Eclipsed Eclipsed Staggered Staggered Staggered

The Dihedral Angle

The dihedral angle (also called the torsion angle) is the angle between two bonds on adjacent atoms, measured by looking down the bond axis. It is defined as the angle between the front substituent and the back substituent.

0° Eclipsed
Front and back bonds directly overlap. Maximum torsional strain.
60° Gauche
Staggered but substituents are 60° apart. Some steric strain for large groups.
180° Anti
Staggered with substituents 180° apart. Lowest energy for substituted ethanes.

Anti vs Gauche

\(\text{CH}_3\) \(\text{CH}_3\) Anti (180°) \(\text{CH}_3\) \(\text{CH}_3\) Gauche (60°) vs

Energy Diagram for Butane

Potential Energy Dihedral Angle (\(\text{CH}_3\) to \(\text{CH}_3\)) 0° 60° 120° 180° 240° 300° 360° Totally Eclipsed Gauche Eclipsed Anti (most stable) Eclipsed Gauche Totally Eclipsed

Torsional vs Steric Strain

Torsional Strain Steric Strain
Cause Repulsion between bonding electrons on adjacent atoms Repulsion between electron clouds of bulky groups
When it occurs Bonds are eclipsed (0° dihedral) Large groups are too close (gauche or eclipsed)
Example H eclipses H in ethane \(\text{CH}_3\) near \(\text{CH}_3\) in gauche butane
How to minimize Rotate to staggered conformation Rotate to anti conformation

How to Draw a Newman Projection

1
Choose the \(\text{C-C}\) bond you want to analyze. This becomes the axis you look down.
2
Draw the front carbon as a dot in the center. Attach its three substituents as lines radiating outward at 120° angles.
3
Draw the back carbon as a large circle behind the dot. Attach its three substituents from the circle edge, also at 120° angles.
4
Set the dihedral angle. For staggered: offset back bonds 60° from front bonds. For eclipsed: align them.
5
Label all substituents (H, \(\text{CH}_3\), Cl, etc.) and indicate the dihedral angle if needed.

Example 1: Ethane Conformations

Draw the staggered and eclipsed Newman projections of ethane (\(\text{C}_2\text{H}_6\)), looking down the C1-C2 bond.

H H H H H H Staggered Lowest energy H H H H H H Eclipsed Highest energy vs

Example 2: Butane C2-C3 Bond

Rank the three staggered conformations of butane by stability, looking down the C2-C3 bond.

\(\text{CH}_3\) \(\text{CH}_3\) H H H H Anti Most stable \(\text{CH}_3\) \(\text{CH}_3\) H H H H Gauche +3.8 kJ/mol \(\text{CH}_3\) \(\text{CH}_3\) H H H H Gauche +3.8 kJ/mol

Example 3: Predicting the Lowest Energy

For 2-methylbutane looking down the C2-C3 bond, identify the most stable Newman projection. Front carbon (C2) bears: H, \(\text{CH}_3\), \(\text{CH}_3\). Back carbon (C3) bears: H, H, \(\text{CH}_3\).

Most Stable Conformation \(\text{CH}_3\) \(\text{CH}_3\) H \(\text{CH}_3\) H H

Common Mistakes

Mixing Up Front and Back
The dot is the front carbon, the circle is the back. Placing substituents on the wrong carbon gives a completely different conformation.
Wrong Bond Angles
In a projection, the three bonds on the front carbon should be 120° apart, as should the three on the back. Drawing them with incorrect spacing can misrepresent the conformation.
Ignoring Steric Strain
Thinking all staggered conformations have the same energy. The anti is more stable than gauche when large groups are present.

Summary of Conformations

Conformation Dihedral (\(\text{CH}_3\)-\(\text{CH}_3\)) Relative Energy Strain Type
Anti 180° Lowest None
Gauche 60° Low Steric
Eclipsed (\(\text{CH}_3\), H) 120° High Torsional
Totally Eclipsed 0° Highest Torsional + Steric

Question 1 of 5

In a Newman projection, the front carbon is represented by:

Question 2 of 5

What is the dihedral angle between substituents in a staggered conformation?

Question 3 of 5

Which conformation of butane (looking down the C2-C3 bond) has the lowest potential energy?

Question 4 of 5

The energy difference between staggered and eclipsed ethane is approximately \(12 \text{ kJ/mol}\). This strain is primarily due to:

Question 5 of 5

For butane, the gauche conformation is higher in energy than the anti conformation primarily because of: