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Chapter 2: Problem 8

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

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c) Are any of the eclipsed conformations related by reflection? #Answer#: a) There are three eclipsed conformations: 1) Cl and H atoms directly eclipsed: Cl | C H - C - H | Cl 2) Cl atoms eclipsed with H atoms: H | C Cl - C - Cl | H 3) H atoms eclipsed with Cl atoms: Cl | C H - C - Cl | H b) The first conformation has the highest energy, while the second and third conformations have the lowest energy. c) The second and third eclipsed conformations are related by reflection.

Step by step solution

01

Understanding the structure of 1,2-dichloroethane and Newman projections

1,2-dichloroethane has the following structure: H-C(Cl)-C(Cl)-H. In a Newman projection, we view the molecule down the carbon-carbon bond axis (front atom partially hiding the rear atom). Circles represent carbon atoms, with the front carbon in the center and the rear carbon at the perimeter.
02

Identifying and drawing all eclipsed conformations

There are three different eclipsed conformers of 1,2-dichloroethane: 1) Chlorines and hydrogens are directly eclipsed (\(0^{\circ}\) rotation): Newman Projection: Cl | C H - C - H | Cl 2) Chlorines are eclipsed with hydrogens (\(120^{\circ}\) rotation): Newman Projection: H | C Cl - C - Cl | H 3) Hydrogens are eclipsed with chlorines (\(240^{\circ}\) rotation): Newman Projection: Cl | C H - C - Cl | H
03

Identifying the eclipsed conformations with the lowest and highest energy

The energy of a conformation depends on the steric interactions between atoms. When atoms are closer together, there is increased electron repulsion, leading to higher energy conformations. 1) In the first conformation, both chlorines and hydrogens are directly eclipsed, leading to increased electron repulsion. Hence, the first conformation has the highest energy. 2) The second and third conformations have similar energy levels, as both have a chlorine atom eclipsed with a hydrogen. These conformations have lower energy compared to the first one because there is less electron repulsion. Therefore, the second and third conformations have the lowest energy.
04

Identifying eclipsed conformations related by reflection

Among the eclipsed conformations, we can notice that the second and third conformations are related by reflection. Specifically, we see that the positions of the chlorines and hydrogens in the second conformation can be obtained by reflecting those in the third conformation across the vertical axis passing through the center carbon atom.

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Most popular questions from this chapter

Write structural formulas for the following alkanes and cycloalkanes. (a) \(2,2,4\)-Trimethylhexane (b) 2,2-Dimethylpropane (c) S-Ethyl-2,4,5-trimethyloctane (d) 5-Butyl-2,2-dimethylnonane (e) 4 -(1-Methylethyl) octane (f) 3,3-Dimethylpentane (g) trans-1,3-Dimethylcyclopentane (h) cis-1,2-Diethylcyclobutane

Draw structural formulas for the cis and trans isomers of hydrindane. Show each ring in its most stable conformation. Which of these isomers is the more stable?

\(1,2,3,4,5,6-\) Hexachlorocyclohexane shows cis,trans isomerism. At one time a crude mixture of these isomers was sold as an insecticide. The insecticidal properties of the mixture arise from one isomer, known as lindane, which is cis- \(1,2,4,5\)-trans-3,6hexachlorocyclohexane. (a) Draw a structural formula for \(1,2,3,4,5,6\)-hexachlorocyclohexane disregarding, for the moment, the existence of cis,trans isomerism. What is the molecular formula of this compound? (b) Using a planar hexagon representation for the cyclohexane ring, draw a structural Formula for lindane. (c) Draw a chair conformation for lindane, and label which chlorine atoms are axial and which are equatorial. (d) Draw the alternative chair conformation of lindane, and again label which chlorine atoms are axial and which are equatorial. (e) Which of the alternative chair conformations of lindane is more stable? Explain.

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