Chem 245 2:11

Confirmations of Alkanes

• Transition from Gen Chem: Topic Four shifts focus from general chemistry to organic molecules and spatial awareness in 3D.

• Representation Methods: Learn various molecular representation methodologies:

  • Skeletal or Bond Line Structure: Carbon at corners and ends, allows hidden hydrogen counting.

  • Lewis Structure: Displays every bond and atom clearly; more detailed but complex.

  • Wedge and Dash Notation: Illustrates three-dimensional shape with wedges (coming towards viewer) and dashes (pointing away).

Understanding Newman Projections

• Purpose: To view and understand molecule orientation by looking down a carbon-carbon bond.

• Front and Back Carbons Representation:

  • Front Carbon: Shown as a dot.

  • Back Carbon: Shown as a circle.

• Bond Angles: Bond angles associated with tetrahedral geometry are around 109.5 degrees, typically represented in 3D models.

Drawing Newman Projections

• Steps to Create a Newman Projection:

  1. Identify the front and back carbon in the molecule.

  2. Draw the respective structures, keeping in mind the hydrogen orientation based on molecular perspective.

  3. Fill in the remaining atoms based on their orientation (wedge vs. dash).

  4. Remember to consider how rotations of bonds affect the conformation.

• Types of Conformations:

  • Staggered: Lowest energy, atoms spaced apart.

  • Eclipsed: Higher energy due to overlap of atoms.

• Torsional Strain: Energy difference between staggered and eclipsed stated as 12 kJ/mol.

Energy Considerations in Conformations

• Staggered vs. Eclipsed: Staggered is preferable in terms of energy efficiency - typically holds a 99% presence in any ethane sample.

• Understanding Conformational Energy Profiles: As carbon-carbon single bonds rotate, they alternate between staggered and eclipsed states. The energy profile is key to understanding these changes.