Answer
Newman Projections of Alkanes
Concept Overview
Newman projections are a way to visualize the spatial arrangement of bonds around a particular carbon–carbon (C–C) bond in alkanes. This projection helps us understand conformational isomerism and torsional strain by looking down the C–C bond axis.
Problem Statement
We are given a dibromoalkane, and we are asked to draw Newman projections of the molecule by viewing it along the C1–C2 bond from two different directions:
- View (a): Looking from carbon
C1toC2 - View (b): Looking from carbon
C2toC1
Step-by-Step Explanation
Step 1: Analyze the Structure
The molecule contains two carbon atoms:
- C1 is bonded to Br, H, and CH₃
- C2 is bonded to CH₃, H, and Br
Each view gives a different perspective on the relative positions of the substituents.
Step 2: Newman Projection for View (a) – Looking from C1 to C2
- Front carbon (C1) is a dot.
- Back carbon (C2) is a circle.
- Use the wedge-dash structure to assign positions:
- Wedge (out of plane) → right
- Dash (into plane) → left
- Normal bond (in plane) → down
- Back carbon substituents are placed symmetrically around the circle.
Result:
- C1 (dot): CH₃ (down), H (left), Br (right)
- C2 (circle): CH₃ (up), H (left), Br (right)
Step 3: Newman Projection for View (b) – Looking from C2 to C1
- Front carbon (C2) is a dot.
- Back carbon (C1) is a circle.
- Orientations reverse due to the opposite direction of view:
- Wedge → right
- Dash → left
- Normal → down
Result:
- C2 (dot): CH₃ (down), H (left), Br (right)
- C1 (circle): CH₃ (up), H (left), Br (right)
Conclusion
The Newman projections for both viewing directions — C1 → C2 and C2 → C1 —
show different arrangements due to the perspective, although the molecular structure remains the same.
This demonstrates how Newman projections help visualize conformational changes and spatial orientations in organic molecules.