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CHEM2213 Topic 2c: Organic Structure #3: Chirality Notes.docx

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Chemistry 1027A/B
Mark Workentin

Topic 2c Organic Structure #3: Chirality  Mirror Image: reflection of an object in a mirror o Cannot superpose one on the other What Are Stereoisomers?  Stereoisomers: isomers that have the same molecular formula and same connectivity but different orientations of their atoms in space o Ex: cis-trans isomers in cycloalkanes and alkenes What Are Enantiomers?  Enantiomers: stereoisomers that are nonsuperposable mirror images, refers to a relationship between pairs of objects  Most common cause of enantiomerism among organic molecules is a C bonded to 4 different groups  If the mirror image and original represent the same molecule then the two are superposable  If the mirror image does not fit exactly on the original with every detail matching, then the two are nonsuperposable (different molecules) o If they are nonsuperposable, then they are enantiomers (always occur in pairs)  Chiral: objects that are not superposable on their mirror images  Achiral: object that lacks chirality, no handedness and is superposable on its mirror image o Has at least one plane of symmetry o Plane Of Symmetry: imaginary plane passing through an object and dividing it such that one half is the mirror image of the other half  Chiral Centre: atom, such as C, with 4 different groups bonded to it  Stereocenter: atom at which the interchange of two atoms / groups bonded to it produces a different stereoisomer o Chiral centres are a type of stereocenters How Do We Designate The Configuration Of A Stereocenter?  R,S System: set of rules for specifying the configuration about a Stereocenter o R (right) shows the order of priority of groups on a stereocenter is clockwise o S (left) shows the order of priority of groups on a stereocenter is counterclockwise  Locate stereocenter and identify the 4 substituents and assign priority  Orient molecule in space so group of lowest priority is directed away  Read 3 groups projecting toward you in order, from 1 to 3  If it proceeds clockwise, configuration is R / if it is counterclockwise, configuration is S n What Is The 2 Rule?  For a molecule with n stereoenters, the maximum number of stereoisomers possible is 2 n  Diastereomers: stereoisomers that are not mirror images of each other o If A and B are enantiomers and C and D are enantiomers, A and C are diastereomers o All configurations the same = identical compounds o All configurations opposite = enantiomers o Any other scenario = diastereomers  Certain molecules containing two ore more stereocenters have special symmetry properties that reduce the number of stereoisomers to fewer n than the maximum number predicted by the 2 rule  If it has a plane of symmetry that bisects the molecule in such a way that the top half is the reflection of the bottom half then the molecule is achiral  Meso Compound: achiral compound possessing two ore more stereocenters How Do We Describe The Chirality Of Cyclic Molecules With Two
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