Stereochem_Primer

Stereochem_Primer - 1 Visualization Skills 2 Chirality and...

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1. Visualization Skills 2. Chirality and Enantiomers 3. Diastereomers 4. Finding Meso 1. Visualization Skills Proper use of stereochemical concepts requires an ability to perceive 3-dimensional objects rendered on a two dimensional sheet of paper or a board. You need to be able to “see” how rotating a molecule in space changes the positions of groups around a stereocenter. You also need to “see” how reflecting a molecule in a mirror plane changes the relative positions of groups around a stereocenter . A stereocenter (also called a chiral center or stereogenic center) is an atom connected to four different groups. Rotate a molecule about an axis - changes the molecule’s orientation. Rotation about an axis does not change a molecule’s shape, conformation or any stereocenter’s configuration. CONVINCE YOURSELF THAT - ALL 3 ROTATED STRUCTURES ARE THE SAME AS THE OBJECT Reflection a molecule in a plane – not a physically achievable motion (try to convert your left hand into your right hand ). Reflection changes every stereocenter’s configuration. CONVINCE YOURSELF THAT - ALL 3 REFLECTED STRUCTURES ARE THE SAME - ALL 3 REFLECTED STRUCTURES ARE NOT SUPERIMPOSABLE ON THE OBJECT C D A B D C A B reflection in vertical plane perpendicular to the page object reflection C D A B object reflection reflection in plane defined by the page C D B A C D A B C D A B object reflection reflection in horizontal plane perpendicular to the page C D A B D C B A rotate about a vertical axis in the page object rotated C D A B object rotated rotate about a horizontal axis in the page C D B A D C A B C D A B object rotated rotate about an axis perpendicular to the page
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2. Chirality and Enantiomers A chiral entity is not superimposable on its mirror image . The above object and its reflection are chiral. Neither is superimposable on the other (its mirror image). The above molecule (object) and its reflection are one example of stereoisomers . Two molecules are stereoisomers if they have the same composition, constitution and connectivity but different 3-dimensional shapes that cannot be superimposed in any conformation without breaking bonds. There are two main classes of stereoisomers: enantiomers and diastereomers . Enantiomers: two molecules are enantiomers if they are stereoisomers and mirror images . (Alternate definition: two molecules are enantiomers if they have the same connectivity, are mirror images but are non-superimposable). object reflection 180 rotation rotated reflection Starting with the object [ (R)-3-methylheptane ], reflection in the plane generates a new structure (reflection). A 180 o rotation about a vertical axis generates the rotated reflection. Although the heptane chain of the object and the rotated reflection can be perfectly overlayed, the 3-CH 3 group of the object points toward the viewer whereas the 3-CH 3 group of the rotated reflection [ (S)-3-methylheptane ], points away from the viewer. The third carbon is a stereocenter.
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