Day_Lecture_IV - ORGANIC CHEMISTRY 307 LECTURE NOTES IV R...

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1 ORGANIC CHEMISTRY 307 LECTURE NOTES IV R. Boikess CYCLOALKANES Consider an imaginary process in which a homolytic cleavage of a C-H bond at each end of a chain occurs and then the two ends come together to form a new C-C bond. Such a compound has tw o fewer H’s (because there are no ends), C n H 2n . Its key feature is of course a ring. When there are no functional groups the compound is called a cycloalkane and the chemistry is very like that of alkanes with a very few exceptions that we will explain. A. Representing Cycloalkanes. 1. Skeletal structures are almost universally used to show rings, very important, we understand that there are enough H atoms to make every C tetravalent. Regular polygons. Show examples and identify the missing H’s. Except f or the three carbon ring, none of the cycloalkanes have the actual structure of the regular polygon. We will see why soon. B. Nomenclature 1. The presence of a ring is signaled by the prefix cyclo. The number of atoms in the ring is designated in the usual way starting (no ring with 2 atoms) with 3 = prop, 4 = but etc. The absence of any functional groups is signaled by the suffix ane. 2. A substituted cycloalkane can be named in one of two ways. (a) By identifying and locating the substituents on the ring. Same basic method as for acyclic (define) alkanes, but if there is only one substituent no number is needed because all positions are equivalent. Example, methylcycloheptane. If there is more than one substituent, then the
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2 first one gets position 1and we follow the same rules as for chains. Consider 1, 2-diethylcyclopentane and number in the right direction (b) By naming the ring as a cycloalkyl radical and treating it like any other alkyl radical. Consider 3-cyclopropylpentane. If the ring has additional substituents the point of attachment is always 1. Consider 3-(3-methylcyclobutyl)hexane. (c) Which way do we do it? (a) or (b)? Use common sense in situations where one name is much simpler than the other. Two rules in order i. maximum number of substitutions on a single unit of structure [1-ethyl-2-methylcyclohexane, not (2-methylcyclohexyl) ethane. And dicyclobutylmethane, not (cyclobutylmethyl)cyclobutane ii. smaller unit as a substituent on a larger [3- cyclopropylpentane (3 on 5), not (1,1-diethylmethyl) cyclopropane (5 on 3)] 3. Free rotation around C-C bonds in cyclic structures is not possible. (Because the ends are tied together). This restriction has a number of consequences. One is that a ring has a top and bottom. Of course this makes no difference (because you can just turn the ring over in space) except when there are two or more substituents on two or more carbons of the ring. Consider 1,2-dimethylcyclobutane. We could put both methyls on the same side of the ring (the top or the bottom) or we could put the methyls on opposite sides of the ring. Because there is no free rotation around the C-C bonds of the ring, the only way these two arrangements can be interconverted is by breaking a C-C bond, which does not occur anywhere near room temperature.
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