CHEM 2211 1st Edition Lecture 11 Outline of Last Lecture I II III IV Isomers are Optically Active Measuring Rotation Enantiomeric Excess Compounds with more than One Asymmetric Center Outline of Current Lecture I II Stereoisomers of cyclic compounds Meso Compounds are optically inactive III How to Name Isomers with more than one Asymmetric Center Lecture Notes I Stereoisomers of Cyclic Compounds A Some cyclic molecule like 1 bromo 2 methylcyclopentane have two asymmetric centers and four stereo isomers Cis isomer of 1 bromo 2 methylcyclopentane Trans isomer of 1 bromo 2 methylcyclopentane B However other cyclic molecules like 1 bromo 3 methylcyclobutane do not have any asymmetric centers i Two groups are identical because substituents are equidistance from each other Cis 1 bromo 3 methylcyclobutane Trans 1 bromo 3 methylcyclobutane C Another example is 1 bromo 4 methylcyclohexane The molecule on the left is the cis isomer and the molecule on the right is the trans isomer II Meso Compounds are optically inactive A Some asymmetric compounds like 2 3 dibromobutane only have three stereoisomers Staggered Conformer of 2 3 dibromobutane Eclipsed Conformer of 2 3 dibromobutane B Meso compounds have asymmetric centers but are achiral because they are superimposable on their mirror images Enantiomers Meso compound i They have a plane of symmetry that runs through the middle so that one side is the mirror image of the other Plane of symmetry for 2 3 dibromobutane C For cyclic molecules the cis isomer will be the meso compound and the trans isomer will be the pair of enantiomers Meso Compound Enantiomers i Hexane has a plane of symmetry only in the boat conformation Chair Conformer of Hexane Boat Conformer of Hexane III How to Name Isomers with more than one Asymmetric Center 3 bromo 2 butanol A Determine the priority of the constituents attached to C2 and assign it as either an R or an S Carbon has an S configuration B Repeat the first step with C3 Carbon is an R configuration C The final compound is named 2S 3R 3 bromo 2 butanol 2S 3R 3 bromo 2 butanol
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