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4 Cycloalkanes It is convenient to cover ordinary alkanes and cyclic ones in separate chapters of organic chemistry textbooks. In most cases, however, the presence or absence of a ring in a molecule makes little difference to its physical properties or its chemical behavior. What you have learned in Chapters 2 and 3 can be applied virtually with- out change to the molecules presented in Chapter 4. Cyclic alkanes are nonpolar, lacking in any functional groups, and therefore are relatively unreactive, like acyclic alkanes. For most of them, the only important re- actions are radical reactions. The major topics of concern are those dealing with the shapes (conformations) of the types of ring systems, and the effects of these shapes on the bonding and stability of each size ring. Some new points of nomenclature are presented. On the whole, however, the chapter contains only one new topic that is not a direct extrapolation of what has gone before: the concept of bond angle strain in compounds con- taining small rings. Outline of the Chapter 4-1 Nomenclature and Physical Properties Basic material. 4-2 Ring Strain and Structure The bonding consequences of closing a chain of atoms into a ring of three, four, or five carbons. 4-3 Cyclohexane The most common and most important ring size (six carbons). Its shapes, and their consequences. 4-4 Substituted Cyclohexanes More of the same. 4-5 Larger Cycloalkanes Very brief overview. 4-6 Polycyclic Alkanes Ditto. 4-7 Carbocyclic Natural Products Common ring-containing molecules of biological importance. Keys to the Chapter 4-1. Nomenclature and Physical Properties The naming First, because of ring compounds requires two new procedures in addition to those associated with acyclic sys- tems. rings have no "ends," numbering starts at that carbon around the ring giving the lowest 60