Carey Chapter 3 Alkane onformations Oglethorpe University卡蕾3章烷烃构象奥格尔索普大学.pptVIP

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Carey Chapter 3 Alkane onformations Oglethorpe University卡蕾3章烷烃构象奥格尔索普大学.ppt

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Carey Chapter 3 Alkane onformations Oglethorpe University卡蕾3章烷烃构象奥格尔索普大学

Dr. Wolfs CHM 201 202 most stable conformation is chair substituent is more stable when equatorial Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial. Axial methyl group is more crowded than an equatorial one. Source of crowding is close approach to axial hydrogens on same side of ring. Crowding is called a 1,3-diaxial repulsion and is a type of van der Waals strain. Crowding is less pronounced with a small substituent such as fluorine. Size of substituent is related to its branching. Crowding is more pronounced with a bulky substituent such as tert-butyl. tert-Butyl is highly branched. 3.11Disubstituted Cycloalkanes:Stereoisomers Stereoisomers are isomers that have same constitution but different arrangement of atoms in space 1,2-Dimethylcyclopropane There are two stereoisomers of 1,2-dimethylcyclopropane. They differ in spatial arrangement of atoms. 1,2-Dimethylcyclopropane cis-1,2-Dimethylcyclopropane has methyl groupson same side of ring. trans-1,2-Dimethylcyclopropane has methyl groupson opposite sides. Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other. Analogous to 1,4 in that trans is more stablethan cis. Unlike 1,2 and 1,4; cis-1,3 is more stable than trans. Compound Orientation -DH° cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-eq 5217* cis-1,3-dimethyl eq-eq 5212*trans-1,3-dimethyl ax-eq 5219 cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-eq 5212* More complicated than cyclohexane. Common for several conformations to be of similar energy. Principles are the same, however.Minimize total strain. Contain more than one ring….. bicyclic, tricyclic, tetracyclic, etc. equals minimum number of bond disconnectionsrequired to give a noncyclic species requires one bond disconnection requires two bond disconnections requires two bond disconnections spirocyclic fused ring bridged ring one atom common to two rings adjacent atoms