Draw the Newman projections for the anti and gauche forms of 1,2-dibromoethane. The molecule has a dipole moment of 1.0 D.
a) Which form is responsible for the molecular dipole (gauche/anti)?
(b) Which conformation of 1,2-dibromoethane is present in the greatest amount at equilibrium (eclipsed/gauche/…
The Correct Answer and Explanation is :
a) Which form is responsible for the molecular dipole (gauche/anti)?
The gauche conformation of 1,2-dibromoethane is responsible for the molecular dipole. In this conformation, the bromine atoms are positioned at a 60° dihedral angle relative to each other, resulting in a net dipole moment because the individual bond dipoles do not cancel out. In contrast, the anti conformation, where the bromine atoms are 180° apart, has a net dipole moment of zero due to the cancellation of opposing dipoles. citeturn0search11
b) Which conformation of 1,2-dibromoethane is present in the greatest amount at equilibrium (eclipsed/gauche/anti)? Provide the correct answer and a 300-word explanation.
At equilibrium, the anti conformation of 1,2-dibromoethane predominates. This preference arises from a combination of steric and electronic factors:
- Steric Factors: In the anti conformation, the bulky bromine atoms are positioned opposite each other, minimizing steric repulsion. This arrangement reduces electron cloud overlap, leading to a more stable and lower-energy state.
- Electronic Factors: The gauche conformation places the bromine atoms closer together (at a 60° dihedral angle), resulting in increased electron-electron repulsion. This proximity raises the potential energy of the molecule, making the gauche form less stable.
Experimental measurements support this theoretical understanding. The observed dipole moment of 1,2-dibromoethane is approximately 1.0 D, indicative of a significant population of the gauche conformer. However, the anti conformation remains the most stable and is present in the greatest amount at equilibrium. citeturn0search11
In summary, while both anti and gauche conformations exist, the anti conformation is favored at equilibrium due to minimized steric and electronic repulsions, leading to greater stability.