北京化工大学：《有机化学》课程教学资源（双语习题与答案）Chapter 04 Cycloalkanes.pdf_大学文库

numbers for substituent groups, using the same criteria for “lowest numbers” presented earlier. Second, rings have a “top” and a “bottom” face, relatively speaking. Therefore, substituents on different ring carbons may either be on the same face or on opposite faces, necessitating the cis or trans denotation in the name. All other principles of nomenclature follow unchanged. 4-2. Ring Strain and Structure Electron pairs repel each other and try to be as far apart as possible. Rings with only three or four atoms force the electron pairs of the COC bonds to be closer together than is normal for carbon atoms in molecules. The repulsion that results is the major cause of the high-energy nature of small ring compounds and is the physical cause of the ring strain referred to in the text. To examine the structural aspects of these molecules, you will find your set of models to be indispensable. Cyclopropane is the only flat cycloalkane ring. All larger cycloalkanes are nonplanar. Ring distortion away from a planar structure reduces eclipsing interactions between neighboring carbon–hydrogen bonds. 4-3 and 4-4. Cyclohexanes Before you do anything else, make a model of cyclohexane. Be sure to use the correct atoms and bonds from your kit. The completed model should not be too floppy and should be easily capable of holding the shape shown in Figure 4-5(B). This is the chair conformation, with three COH bonds pointing straight up and three COH bonds pointing straight down (the axial COH bonds). Starting from this point, you should be able to construct the other important cyclohexane conformations by moving an “end” carbon through the plane of the “middle” four carbons of the ring; that is, Learn to recognize axial and equatorial positions and their cis/trans interrelationships around the ring. Again, use your model in conjunction with the chapter text and illustrations. Note the congestion associated with large groups in axial positions, a result of 1,3-diaxial interactions, the main effect that causes differences in energy between the two possible chair conformations of a substituted cyclohexane. This is an example of a transannular (literally, “across the ring”) interaction, arising in this case from the ring structure forcing groups to adopt gauche conformational relationships. Be sure to use your models when trying to do the problems at the end of the chapter. 4-5 and 4-6. Larger Rings; Polycyclic Molecules The material in these sections is intended only to give a very brief introduction to areas of organic chemistry that are important in current research but are generally beyond the scope of a course at this level. Only a small number of selected molecules are mentioned with relevant points of structure and nomenclature presented where appropriate. Solutions to Problems 17. Start with the largest ring and systematically go through successively smaller rings: Cyclopentane cis-1, 2-Dimethylcyclopropane CH3 Methylcyclobutane 1, 1-Dimethylcyclopropane CH3 CH2CH3 CH3 CH3 CH3 trans-1, 2-Dimethylcyclopropane Ethylcyclopropane (Did you forget this one? Lots of students miss it.) CH3 CH3 Boat and boatlike conformations (rather floppy, too) Move up 56 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 56

cyclohexane in order to minimize eclipsing interactions of the true boat conformation (Section 4-3). (Make a model!) 31. Models may be helpful here. You should be able to construct structures similar to those pictured below. Notice how the ring-fusion hydrogens are trans to each other in trans-hexahydroindane and cis to each other in cis-hexahydroindane. In the drawings, the cyclohexane rings are chairs and the cyclopentane rings are envelopes. A slight twist around the cyclopentane bond away from the envelope “flap” would give rise to the cyclopentane half-chair conformation, which is similar in energy but harder to draw. 32. In trans-decalin each of the carbon–carbon bonds attached to the ring fusion occupy an equatorial position with respect to the ring on which they are attached. In the illustration below we have labeled the rings as A and B, and numbered the four relevant bonds 1–4: Bonds 1 and 2 (which are part of ring B) are equatorial substituents with respect to ring A. Bonds 3 and 4 (which are part of ring A) are equatorial substituents with respect to ring B. Both hydrogen atoms on the ring fusion carbons are axial with respect to both rings. In cis-decalin the situation is different. Look at the picture: Bond 1 (in ring B) is now axial with respect to ring A (rotate the page clockwise by 60° to see this more clearly). Bond 2 is still equatorial. Also, bond 3 (in ring A) is now axial with respect to ring B (bond 4 is still equatorial). So two of these four bonds are axial with respect to the ring on which they are substituents and, as a result, give rise to 1,3-diaxial interactions that raise the enthalpy of the compound. Notice that the ring-fusion hydrogens are now equatorial with respect to one ring and axial with respect to the other. If we were to assign an energy of about 1.75 kcal mol1 to each axial bond to a carbon in cis-decalin, we would conclude that the cis isomer is 3.5 kcal mol1 higher in energy (less stable) than the trans. This turns out to be an overestimate, in part because carbon atoms in rings cannot rotate freely and therefore do not generate as much steric interference as do simple alkyl groups, which can rotate a full 360°. An alternative way to estimate the energy difference is to search for butane structural fragments that possess gauche conformations. The cis isomer has three more than does the trans; assuming each gauche butane raises the energy content by about 0.9 kcal mol1 , one arrives at an energy difference of 2.7 kcal mol1 . H B A 2 3 4 1 H H A B 3 2 4 1 H H H H H trans-Hexahydroindane cis-Hexahydroindane H H C(CH3)3 (CH3)3C Solutions to Problems • 63 1559T_ch04_55-69 10/22/05 20:19 Page 63

33. You really do need to go to models to address this question. Start by examining a simpler compound, one in which a cyclopropane ring is fused to a single cyclohexane ring (below, left). Is the cyclohexane in a chair conformation at all? Actually, it is not. The geometry of the three-membered ring forces the cyclohexane ring bonds associated with it and four of the cyclohexane ring carbons to all be coplanar. Only the two cyclohexane ring carbons farthest from the ring fusion are movable. In tricyclo[5.4.01,3.01,7]undecane (above, right), the ring labeled ‘A’ is similarly constrained. Only two of its carbons (the one at the bottom and the one at the lower left) are capable of significant motion. This isn’t much more than a wiggling motion, one up and one down, relative to the plane of the remaining four carbons in that six-membered ring. As for ring ‘B’, it is capable of holding one reasonable approximation of a chair conformation, with the appearance shown below. However, if you attempt a chair-chair flip in ring ‘B’, you will encounter much more resistance from the model set. The flip can be forced, but the conformation that results is a distorted twist-boatlike shape. This is because the two carbons at the ring fusion cannot undergo the corresponding rotation that would finish the chair-chair interconversion. The rigidity of ring ‘A’ prevents that. 34. (a) The form is more stable because all of its substituent groups are equatorial. One of the OOH groups (the one on the ring carbon at the right of the picture) in the form is axial. (b) Keq 64/36 1.78. Using the equation G° 1.36 log10 Keq (Section 2-1) for an equilibrium constant at room temperature (25°C), we get G° 0.81 kcal mol1 . In Table 4-3 the axial/ equatorial free energy difference for an OOH substituent on cyclohexane is 0.94 kcal mol1 . The difference is small but real, and it derives largely from the fact that the ring in glucose is not a cyclohexane but an oxacyclohexane—a cyclic ether. Replacing a ring CH2 group with an oxygen has several effects, including removal of the steric interactions associated with the hydrogens on the carbon, and introduction of two polar COO bonds whose dipoles can give rise to either attractive or repulsive interactions with the COO bonds of nearby substituted ring carbons. 35. Count carbons in the molecule. If there are 10, the molecule is a monoterpene; if 15, a sesquiterpene; if 20, a diterpene. (a) 10 carbons, monoterpene (b), (c), and (d) 15 carbons, sesquiterpene (e) 11 carbons, but only 10 in the contiguous molecular “skeleton’’; monoterpene (f ) 15, sesquiterpene (g) 10, monoterpene (h) 20, diterpene 36. O O CH2 CH2 CH2 H3C H3C CH3 CH3 CH2OH CH3 OH CH3 CH3 Alkenes Alkenes Alcohol Ester (a) (b) (c) Alcohol H B A H A B H H 64 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 64