Let's solve **SA.5**, focusing on the step-by-step drawing of **Newman projections** for a 360° rotation around the specified bond, and then sketch the **strain energy vs. angle** graph. I'll also briefly explain the process so you can apply it to any similar molecule. --- # SA.5: Newman Projections for 360° Rotation ## **a) Propane down C2–C3** ### **Step 1: Draw the Structure** Propane: CH₃–CH₂–CH₃ We’re looking **down the C2–C3 bond**. ### **Step 2: Identify Groups** - **Front carbon (C2):** Left CH₃, up H, down H - **Back carbon (C3):** Right CH₃, up H, down H ### **Step 3: Draw Newman Projections at Key Angles** Draw projections at **°, 60°, 120°, 180°, 240°, 300°, 360°**. #### **° (Eclipsed)** - Front: CH₃ (up), H (left), H (right) - Back: CH₃ (up, eclipsing front CH₃), H (left, eclipsing front H), H (right, eclipsing front H) #### **60° (Staggered)** - Back groups rotated 60° clockwise #### **180° (Eclipsed)** - CH₃ groups are eclipsed with H (Repeat for all angles.) ### **Step 4: Graph Strain Energy vs. Angle** - **Eclipsed** conformations (°, 120°, 240°, 360°): **highest energy** - **Staggered** (60°, 180°, 300°): **lowest energy** Graph: A sinusoidal curve with **peaks at eclipsed** and **valleys at staggered**. --- ## **b) 2-Methylbutane down C2–C3** ### **Structure:** CH₃–CH(CH₃)–CH₂–CH₃ Looking down the C2–C3 bond: - **Front (C2):** CH₃, H, CH₃ (methyl) - **Back (C3):** H, CH₂CH₃, H ### **Key Angles:** Draw Newman projections at 60° intervals. #### **Lowest Energy (Anti)** - Large groups (CH₃ and CH₂CH₃) are opposite #### **Highest Energy (Eclipsed)** - Large groups are eclipsed **Repeat as above, but note:** - **Anti:** CH₃ (front) opposite CH₂CH₃ (back) = lowest energy - **Gauche:** CH₃ (front) 60° from CH₂CH₃ (back) = higher energy - **Eclipsed:** CH₃ in front of CH₂CH₃ = highest energy **Draw the sinusoidal graph with higher maxima due to larger group interactions.** --- ## **c) 2,3-Dimethylbutane down C2–C3** ### **Structure:** CH₃–CH(CH₃)–CH(CH₃)–CH₃ Looking down C2–C3: - **Front (C2):** CH₃, H, CH₃ - **Back (C3):** CH₃, H, CH₃ ### **Angles:** Draw Newman projections at 60° intervals. #### **Lowest Energy (Anti)** - Large groups (CH₃–CH₃) are opposite #### **Highest Energy (Eclipsed)** - Large groups are eclipsed **Graph:** Peaks are even higher due to CH₃–CH₃ eclipsing. --- ## **Summary Table** | Angle (°) | Conformation | Strain Energy | |:---------:|:------------:|:---------------:| | | Eclipsed | Maximum | | 60 | Gauche | Intermediate | | 120 | Eclipsed | Maximum | | 180 | Anti | Minimum | | 240 | Eclipsed | Maximum | | 300 | Gauche | Intermediate | | 360 | Eclipsed | Maximum | --- ## **Final Notes** - **Most stable:** anti (largest groups opposite) - **Least stable:** fully eclipsed (largest groups eclipsed) - **Graph:** Sinusoidal, with higher peaks for larger substituents. --- ## **Sketch Example for Propane** ``` ° (Eclipsed) 60° (Staggered) H H H \ / \ C2--C3 C2--C3 / \ / CH₃ H H--CH₃ (Back hidden) ``` **Graph:** - X-axis: Rotation angle (° to 360°) - Y-axis: Strain energy (arbitrary units) - Peaks at °, 120°, 240°, 360° (eclipsed) - Valleys at 60°, 180°, 300° (staggered/anti) --- **If you need explicit drawings for each Newman projection, let me know which molecule and angle you want illustrated, or I can provide one as an example.**