A Newman projection of 1,1,2-trichloroethane along the C-C bond shows two chlorines on the front carbon and one chlorine on the back carbon. What is the most likely energy barrier for rotation about this bond compared to ethane?
- Approximately 12 kJ/mol, similar to ethane, because the number of eclipsing interactions remains constant during rotation
- Approximately 25-30 kJ/mol, significantly higher than ethane, due to severe steric interactions between chlorine atoms in eclipsed conformations (correct answer)
- Approximately 8 kJ/mol, lower than ethane, because the large chlorine atoms prefer to remain in staggered positions
- Approximately 18-20 kJ/mol, moderately higher than ethane, due to increased van der Waals repulsion and dipolar interactions in the eclipsed transition state
Explanation: 1,1,2-trichloroethane has multiple large chlorine substituents that create severe steric hindrance when eclipsed during rotation. The eclipsed conformations involve Cl-Cl eclipsing interactions, which are much more destabilizing than H-H eclipsing in ethane (12 kJ/mol). The barrier is typically 25-30 kJ/mol or higher. Choice A underestimates the effect of large substituents. Choice C incorrectly suggests the barrier is lower than ethane. Choice D underestimates the severity of Cl-Cl steric interactions in the eclipsed forms.