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Solutions to Problems 61 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 bonds to be closer together than is normal. 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 C-H bonds pointing straight up and three C-H bonds pointing straight down (the axial C-H 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, Move up Boat and boatlike conformations (rather floppy, too) 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 trans- annular (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 21. Start with the largest ring and systematically go through successively smaller rings: CH₃ CH₃ CH₃ Cyclopentane Methylcyclobutane 1, 1-Dimethylcyclopropane CH₂CH₃ CH₃ CH₃ CH₃ CH₃ cis-1, 2-Dimethyl- trans-1, 2-Dimethyl- Ethylcyclopropane cyclopropane cyclopropane (Did you forget this one? Lots of students miss it.)