2/17

Introduction to Newman Projections

  • Carbons in the Molecule: Count the total number of carbons; six carbons mean you can choose six different bonds from different orientations.

  • Indicate Front and Back Carbons: When drawing a Newman projection, identify which carbon is in the front and which is in the back.

    • It’s important for visualizing what groups are attached to each carbon.

Drawing Newman Projections

  • Hints for Drawing:

    • Include hydrogen atoms explicitly using wedges (for groups coming toward you) and dashes (for groups going away).

    • You can abbreviate methyl groups as ME (CH₃) and ethyl groups as ET (C₂H₅) to simplify.

    • Skeletal structures are acceptable as long as they represent the correct number of carbons.

  • Perspective: Look down the bond you’re representing, considering the orientations of groups coming off the front and back carbons.

    • Consider conventional versus unconventional positions; choose the correct projection template for a valid representation.

Understanding Staggered vs. Eclipsed Conformations

  • Staggered Conformation:

    • Lower energy due to the spread of atoms, making it more stable.

  • Eclipsed Conformation:

    • Higher energy due to overlapping atoms; geometric rotations can lead to different conformations.

Cyclohexane and Chair Conformational Analysis

  • Chair Conformations:

    • Cyclohexane is commonly represented as a hexagon but in reality, it’s three-dimensional.

    • Wedges and dashes help indicate how groups are situated in space (up or down).

  • Directional Information:

    • Groups represented on dashes are positioned away from you and those on wedges are positioned towards you.

  • Types of Chair Conformations:

    • Axial Lines: Alternating up and down for adjacent carbons.

    • Equatorial Lines: Positioned more horizontally, giving a different perspective and potential energy state.

Energy Considerations in Conformation

  • Energy Diagrams: Understanding potential energy in different conformations is essential.

  • Stability Factors: Different arrangements can lead to steric strain when atoms or groups are too close, particularly when substituents occupy axial positions.

  • Chair Flip: A process of transitioning from one chair conformation to another, where axial groups switch to equatorial positions and vice versa.

Strategies for Drawing Chair Conformations

  • Practice Drawing the Chair: Focus first on axial lines and then equatorial lines.

    • Visual mnemonic: Think of axial lines as chimneys on a house; they should not point inside the house.

    • Stability is achieved by keeping larger substituents in equatorial positions to minimize steric strain.

  • Common Mistakes:

    • Ensure not to position two axial or two equatorial groups on a single carbon; each must alternate.

    • Check for equatorial lines' correct directional placement based on their carbon's position (left/right).