Question

For 1,2-dichloroethane: (a) Draw Newman projections for all eclipsed conformations formed by rotation from \(0^{\circ}\) to \(360^{\circ}\) about the carbon-carbon single bond. (b) Which eclipsed conformation(s) has (have) the lowest energy? Which has (have) the highest energy? (c) Which, if any, of these eclipsed conformations are related by reflection?

Short answer

Answer: The energy levels of eclipsed conformations in 1,2-dichloroethane are affected by steric hindrance and repulsion between atoms, especially between larger atoms like chlorine. The more stable a conformation, the lower its energy.

Step-by-step solution

Draw Newman projections for all eclipsed conformations

To draw the Newman projections of 1,2-dichloroethane, start by looking down the carbon-carbon single bond \((C_1-C_2)\). The compound has a total of 3 eclipsed conformations formed by rotation from \(0^{\circ}\) to \(360^{\circ}\). Draw each Newman projection by placing the two chlorine atoms and two hydrogen atoms on the front and back carbons.

Identify the lowest and highest energy conformations

To determine the energy of each conformation, consider the interaction between the atoms. The more stable a conformation, the lower its energy. Steric hindrance and repulsion between atoms, especially between larger atoms like chlorine, will contribute to higher energy levels. Compare the energy levels of each eclipsed conformation and identify which one has the lowest energy and which one has the highest energy.

Determine the conformations related by reflection

To identify which eclipsed conformations are related by reflection, look for conformations that are mirror images of each other. If the positions of chlorine and hydrogen atoms on each carbon match when reflected, the two conformations are related by reflection.