CHAPTER SEVEN Stereochemistry 7. 3 SYMMETRY IN ACHIRAL STRUCTURES Certain structural features can sometimes help us determine by inspection whether a mol ecule is chiral or achiral. For example, a molecule that has a plane of symmetry or a cen ter of symmetry is superposable on its mirror image and is achiral a plane of symmetry bisects a molecule so that one half of the molecule is the mirror image of the other half. The achiral molecule chlorodifluoromethane, for exam- ple, has the plane of symmetry shown in Figure 7.3 A point in a molecule is a center of symmetry if any line drawn from it to som element of the structure will, when extended an equal distance in the opposite direction encounter an identical element. The cyclobutane derivative in Figure 7. 4 lacks a plane of symmetry, yet is achiral because it possesses a center of symmetry. PROBLEM 7. 3 Locate any planes of symmetry or centers of symmetry in each of the following compounds. Which of the compounds are chiral? Which are achiral? (a)(Er-1, 2-Dichloroethen (c)cis-1, 2-Dichlorocyclopropane (b)(Z)-1, 2, Dichloroethene (d) trans-1, 2-Dichlorocyclopropane SAMPLE SOLUTION (a)(E)-1, 2-Dichloroethene is planar. The molecular plane is plane of syi Furthermore, (E)-1, 2-dichloroethene has a center of symmetry located at the mid point of the carbon-carbon double bond. It is achira FIGURE 7. 3 A plane f symmetry defined atoms H-C-Cl chlorodifluoromethane into Br Br FIGURE 7. 4(a)Struc tural formulas a and b are drawn as mirror images. (b) The two mirror images are superposable by rotating form b 180 about an axis passing through the center B≡A f the molecule. the center of the molecule is a center of (a) (b) Back Forward Main MenuToc Study Guide ToC Student o MHHE Website7.3 SYMMETRY IN ACHIRAL STRUCTURES Certain structural features can sometimes help us determine by inspection whether a mol￾ecule is chiral or achiral. For example, a molecule that has a plane of symmetry or a cen￾ter of symmetry is superposable on its mirror image and is achiral. A plane of symmetry bisects a molecule so that one half of the molecule is the mirror image of the other half. The achiral molecule chlorodifluoromethane, for exam￾ple, has the plane of symmetry shown in Figure 7.3. A point in a molecule is a center of symmetry if any line drawn from it to some element of the structure will, when extended an equal distance in the opposite direction, encounter an identical element. The cyclobutane derivative in Figure 7.4 lacks a plane of symmetry, yet is achiral because it possesses a center of symmetry. PROBLEM 7.3 Locate any planes of symmetry or centers of symmetry in each of the following compounds. Which of the compounds are chiral? Which are achiral? (a) (E)-1,2-Dichloroethene (c) cis-1,2-Dichlorocyclopropane (b) (Z)-1,2,Dichloroethene (d) trans-1,2-Dichlorocyclopropane SAMPLE SOLUTION (a) (E)-1,2-Dichloroethene is planar. The molecular plane is a plane of symmetry. Furthermore, (E)-1,2-dichloroethene has a center of symmetry located at the mid￾point of the carbon–carbon double bond. It is achiral. 264 CHAPTER SEVEN Stereochemistry F F Cl H Br Br Cl Cl Br Br Cl Cl A B (a) Br Br Cl Cl B (b) Br Br Cl Cl BPA FIGURE 7.4 (a) Struc￾tural formulas A and B are drawn as mirror images. (b) The two mirror images are superposable by rotating form B 180° about an axis passing through the center of the molecule. The center of the molecule is a center of symmetry. FIGURE 7.3 A plane of symmetry defined by the atoms H±C±Cl divides chlorodifluoromethane into two mirror-image halves. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website