140A Chapter 4 Notes

140A Chapter 4 Notes - Chemistry 140A Winter 2011 (K....

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Chemistry 140A Winter 2011 (K. Albizati) 1 Chapter 4 – Cycloalkanes Because of the strength of the C-C bond, carbon chains can readily form ring-like structures or cycles. Such cyclic structures (when composed of only carbon atoms) are called cycloalkanes. Rings are very important substructures in organic and biochemistry. Practically all pharmaceuticals contain at least one ring. The general formula for a cycloalkane in C n H 2n
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Chemistry 140A Winter 2011 (K. Albizati) 2 Nomenclature Look for the longest carbon chain and size of any rings. Whichever is larger becomes the parent: Note that when there is only 1 substituent, there is no need to designate its position with a number. What if there are 2 substituents? To designate positions of substituents, number substituents in alphabetical priority using the lowest possible numbers. Number around the ring accordingly. 1-Ethyl-2-methylcyclopentane F 1-cyclopropyl-2-fluoro-4-propylcyclohexane
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Chemistry 140A Winter 2011 (K. Albizati) 3 A Digression into Isomerism We have seen constitutional isomers already. Stereoisomers have the same bond connectivity but differ in the arrangement of atoms in 3- dimensional space. Disubstituted and higher substituted rings can exist as stereoisomers. Consider cyclopropane: Each carbon bears 2 hydrogens, one of which comes out at us from the plane of the board and one which goes back into the plane of the board. We can use dashed and wedged bond notation. H H H H H H The individual H’s do not interchange positions. Isomers same mol formula different structures Stereoisomers Same atom connectivity but different arrangement of atoms in 3D space Constitutional Isomers differ in the order of attachment of the atoms
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Chemistry 140A Winter 2011 (K. Albizati) 4 Suppose the ring bears 2 methyls on different ring carbons. Two stereoisomers are possible: A slightly clearer picture… Substituents on the same face of the ring – cis
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140A Chapter 4 Notes - Chemistry 140A Winter 2011 (K....

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