CHAPTER 1: Question 5

2D Explanations

a) The only difference between this case and ethane is that all of the hydrogen atoms in ethane have been replaced by chlorines. A chlorine atom is much bigger than a hydrogen atom, so the chlorine atoms will sterically repel one another more than the hydrogen atoms in ethane do. This will be especially pronounced in the eclipsed conformations since this is when the chlorine atoms are closest together. Thus the graph will have the same shape as Figure 1.8, but the energy difference between the staggered and eclipsed conformations will be greater (10.8 kcal / mol; 45.2 kJ / mol) as shown in the diagram below.

b) The only difference between this case and ethane is that one of the hydrogen atoms in ethane has been replaced by an ethyl group. The ethyl group is bigger than a hydrogen atom, so there will be a larger repulsion between the ethyl group and the hydrogens of the CH3 group than occurs in ethane. This will be especially pronounced in the eclipsed conformations since this is when the ethyl group is closest to a hydrogen. Thus the graph will have the same shape as Figure 1.8, but the energy difference between the staggered and eclipsed conformations will be greater (3.4 kcal / mol; 14.2 kJ / mol) as shown in the diagram below. Note however, that the energy difference between the staggered and eclipsed conformations is not as great as in part a, since only one of the hydrogen atoms has been replaced by a larger group.

c) An explanation of the answer to this question is given in Chapter 8, section 8.2 of the book.

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