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Unformatted text preview: 9/26/2011 1 • Stereochemistry : the study of spatial arrangements of atoms in molecules and the effects of these arrangements on the chemical and physical properties of substances. • Minor differences in 3D structure can result in vastly different properties. • Consider starch and cellulose, which are both composed of the same repeating unit--the connection is just oriented in a different way: C. Stereochemistry 1 • There are two major classes of isomers: constitutional isomers and stereoisomers . • Constitutional/structural isomers have different IUPAC names, the same or different functional groups, and different physical and chemical properties. • Stereoisomers are compounds with the same molecular and structural formulas , but differ in the spatial arrangements of the atoms. They have identical IUPAC names, except for an extra prefix such as cis vs . trans or R vs. S (not covered this semester ). 2 optical isomers = stereoisomers having non-superimposable mirror images • Although everything has a mirror image , mirror images may or may not be identical. • Some molecules are like hands. Left and right hands are mirror images, but they are not identical (i.e., not superimposable ). A molecule (or any object) that is not superimposable on its mirror image is said to be chiral . Optical Isomerism 3 9/26/2011 2 • Other molecules are like socks. Two socks from a pair are mirror images that are superimposable . A sock and its mirror image are identical. • A molecule or object that is superimposable on its mirror image is said to be achiral. 4 Chiral molecules possess no plane of symmetry ( asymmetric ) . plane of symmetry = a plane that divides an object into two parts, each a mirror image of the other. Plane of Symmetry No Plane of Symmetry C H Cl Br F C H Cl F F Note: 3-D models are best. 5 Q: Is there an easy way to identify chiral organic molecules? Ans: Yes, a chiral organic molecule will contain a carbon atom (termed a stereogenic center , a stereocenter , or a chiral center ) that is bonded to four different groups (approximate definition)....
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