Chapter 2 - Instructor Supplemental Solutions to Problems...

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Instructor Supplemental Solutions to Problems © 2010 Roberts and Company Publishers Chapter 2 Alkanes Solutions to In-Text Problems 2.1 (b) No; all alkanes—indeed, all hydrocarbons—must have an even number of hydrogens. Thus, if 2 n + 2 = 23, then n would have to be 10.5—an impossible number of carbons. 2.3 (a) The staggered conformations of isopentane are A, C, and E; the eclipsed conformations are B, D, and F. (b) The curve of potential energy versus angle of rotation is shown in Fig. IS2.1. The staggered conformations B and D have the highest energy because they have eclipsed methyl groups. (c) Conformations A and E have the lowest energy because they have one less interaction in which methyl groups are “close together”—that is, the C—CH 3 bonds have a 60° dihedral angle—than conformation C ; hence, either of these conformations is present in greater concentration than conformation C . 2.4 (a) The projected bond is the carbon–carbon bond at the end of the butane molecule: There are three identical staggered conformations and three identical eclipsed conformations. Figure IS2.1. A diagram of potential energy versus angle of internal rotation in isopentane to accompany the solution to Problem 2.3b. The conformations are shown the solution to part (a). Each conformation differs from the adjacent ones by a rotational angle of ±60°.
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 2 2 (b) The curve of potential energy versus angle of internal rotation is essentially identical to the curve for ethane (Fig. 2.3, text p. 52), except that the energy difference between staggered and eclipsed forms is slightly greater. That is, the energies of all staggered forms lie at equal minima, and the energies of all eclipsed forms lie at equal maxima. (c) All staggered conformations are present in equal amount because they are identical. There is a subtlety here. Problem 2.4 has considered the conformation about the C1–C2 bond in isolation. Any staggered conformation about the C1–C2 bond contains a mixture of gauche and anti conformations about the C2–C3 bond. 2.5 (b) 2,4-Dimethylhexane (d) 2,5,5-Trimethylheptane. Rule 8 rules out 3,3,6-trimethylheptane. 2.6 (b) Following the systematic approach used in part (a) (see the Study Guide and Solutions Manual), we first draw the structure of hexane itself. CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 hexane Next, draw the structures with a principal chain of five carbons and one methyl branch: Finally, we draw the structures with two methyl branches. 2.7
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Chapter 2 - Instructor Supplemental Solutions to Problems...

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