Organic Chemistry Lecture: Newman Projections

Introduction to Newman Projections

• Newman projections represent free rotation about carbon-carbon single bonds in alkanes.
• They illustrate different rotational conformations and associated energy levels.
• Analogous to rotating your arms in different positions, where some positions are easier (lower energy) than others.

Example: Butane

• Focus on the C2 to C3 bond of butane.
• Sigma bonds can rotate freely, unlike pi bonds.
• The molecule can be visualized in different conformations by rotating this bond.

Visualizing Newman Projections

• Looking down the C2 to C3 bond, C2 and C3 appear aligned.
• C3 has a methyl group and two hydrogens; C2 also has similar groups.
• Draw a Newman projection with C2 as a dot (front carbon) and C3 as a large circle (back carbon).

Conformations

• Staggered Conformation:
  • Atoms are 60 degrees apart.
  • More stable due to reduced electron repulsion.
• Eclipsed Conformation:
  • Atoms overlap each other.
  • Less stable due to increased electron repulsion.

Rotation and Energy Levels

• Rotate the C3 carbon 60 degrees at a time relative to C2.
• Staggered Conformation:
  • More stable.
  • Two methyl groups are far apart (anti-conformation).
• Eclipsed Conformation:
  • Less stable.
  • Bonds overlap, increasing electron repulsion.
• Gauche Conformation:
  • Methyl groups are 60 degrees apart.

Energy Diagram for Butane

• Six conformations possible, rotating through 360 degrees.
• Staggered conformations are lower in energy than eclipsed.
• Anti-conformation (largest groups 180 degrees apart) is the most stable.
• Energy ranking: Anti < Staggered < Gauche < Eclipsed.

Considerations for Complex Molecules

• Consider the size of substituents (e.g., isopropyl vs. ethyl) when drawing projections.
• Larger groups create more electron repulsion and influence conformation stability.

Conclusion

• Understanding Newman projections is crucial for visualizing molecular rotations and conformations.
• Practice by drawing and analyzing different molecules.
• Attend discussion sections for further examples and clarification.