Part III. Dipole Moments and A Gedanken Synthesis 1. (1 point) Compare the dipole moment magnitudes of propane and 2-chloropropane. Why do they differ so drastically? 2. (1 point) Draw Newman projections of gauche-1,2-dichloroethane and anti-1,2-dichloroethane. Draw the individual bond-dipole arrows for each polar bond within each molecule. 3. (1 point) Compare the dipole moment magnitudes for gauche-1,2-dichloroethane and anti-1,2-dichloroethane. Why does one have a dipole moment, whereas the other does not? Refer to the orientation of the polar bonds within each conformer. 4. (1 point) Recall that gauche-1,2-dichloroethane and anti-1,2-dichloroethane are different conformations of the same molecule. Does the interconversion of these two occur readily at room temperature? 5. (1 point) Draw cis- and trans-1,2-dichloroethene. Draw the individual bond-dipole arrows for each polar bond within each molecule ( 2 bonds for each molecule).
The Correct Answer and Explanation is:
1. Dipole Moment Comparison: Propane vs. 2-Chloropropane
- Propane has a dipole moment of ~0 D because it is a symmetrical, nonpolar molecule with only C–C and C–H bonds, which have nearly equal electronegativities.
- 2-Chloropropane has a significantly larger dipole moment (~2.05 D) because the C–Cl bond is highly polar (Cl is much more electronegative than C or H), creating an asymmetrical electron distribution.
The large difference arises because 2-chloropropane introduces a strongly electronegative chlorine atom that draws electron density toward itself, resulting in a permanent dipole. Propane lacks any such electronegative substituent.
2. Newman Projections and Dipole Arrows
Gauche-1,2-dichloroethane:
Cl H
\ /
C—C
/ \
H Cl
- Newman view: Front C – Cl and H; Back C – Cl and H
- Cl atoms are 60° apart (gauche).
Anti-1,2-dichloroethane:
Cl H
\ /
C—C
/ \
Cl H
- Newman view: Front C – Cl; Back C – Cl
- Cl atoms are 180° apart (anti).
Draw dipole arrows (→) from C to Cl for each Cl bond.
3. Dipole Moment: Gauche vs. Anti
- Gauche: The Cl–C dipoles are not opposite and don’t cancel, so the molecule has a net dipole moment.
- Anti: The Cl–C dipoles are equal and opposite, so they cancel out, resulting in no net dipole moment.
Dipole cancellation depends on vector addition; in anti, the vectors are 180° apart, canceling each other.
4. Interconversion at Room Temperature
Yes, gauche and anti conformers interconvert readily at room temperature via rotation around the C–C sigma bond. This rotation requires very little energy and happens frequently at ambient conditions.
5. Cis- and Trans-1,2-dichloroethene Structures & Dipole Arrows
Cis-1,2-dichloroethene:
Cl H
\ /
C = C
/ \
H Cl
- Dipole arrows from each C to Cl both point upward → net dipole adds up.
Trans-1,2-dichloroethene:
Cl H
\ /
C = C
/ \
Cl H
- Dipole arrows from each C to Cl point in opposite directions → dipoles cancel → no net dipole.
