Day_Lecture_V

Day_Lecture_V - ORGANIC CHEMISTRY 307 LECTURE V...

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1 ORGANIC CHEMISTRY 307 LECTURE V Stereoisomerism : Already encountered. Two compounds are stereoisomers when they differ only in the spatial relationship of their parts. The composition is the same, the connectivity is the same. It’s only when we consider three dimensions that the difference manifests itself. Define stereoisomerism in general and contrast with constitutional isomerism. Define conformation and configuration. Emphasize difference, conformations are interconvertible by free rotation about single bonds, configurations are not. A. Restriction of rotation can lead to isomerism. 1.This kind of isomerism is generally called cis-trans or geometric isomerism. We have already seen examples in the cycloakanes. We will see more examples when we get to alkenes. 2. How do we recognize it? Identify a plane, which leads to a top and a bottom. Then there must be two (or more) substituents on at least two different atoms. Then same side = cis and opposite side = trans. 3. What can restrict rotation at RT? a. Rings, tying up the two ends b. π bonds, which have a stereoelectronic requirement. If you rotate then the pi overlap weakens because the p-orbitals are no longer parallel.
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2 In general it is hard to break bonds at RT, so isomers that could interconvert by bond breaking and then free rotation don’t. B. We have seen that there can be two ways to arrange atoms in 3D (keeping connections the same) when there is a plane. As we know from experience, there are other situations that lead to more than one way to arrange objects in 3D. 1. The classic is your hands. Discuss. a. How can we tell they are really different? Notice they are both 5 fingers attached to a palm etc. b. The superimposability test . Discuss and mention models. c. But hands are also related. How? Mirror images, but nonsuperimposable. 2. Whenever an object is different from its mirror image, the object is said to be chiral . The difference can be described as “handedness” by analogy to left and right hands. 3. How can we tell when an object is chiral? Symmetry. If it is not symmetrical, it is chiral. If it is symmetrical then it is not chiral (achiral). How can we tell if an object is symmetrical? Look for a symmetry element. If you can find one then the object is symmetrical and superimposable on (identical with) its mirror image. If there is none then it is chiral and nonsuperimposable on its mirror image.
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3 a. Mirror plane. Define: slices object imagine it is a mirror Examples: cube, many, beaker one, any flat molecule has at least one, example trans-1,2-dichloroethene; cis-1,2- dichlorocyclobutane, has one, (extend to methyls) which spin but trans-1,2-dichlorocyclobutane has none. [Figure 5.4 for more examples] b. Center of symmetry (not in book) define point in center, extend any line, it meets the same thing at the same distance in either direction. Example cube, trans-1,3-dibromo- trans-2,4-dichlorocyclobutane 4. How can we tell when a molecule is achiral without
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This note was uploaded on 04/04/2008 for the course CHEM 307 taught by Professor Several during the Spring '08 term at Rutgers.

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Day_Lecture_V - ORGANIC CHEMISTRY 307 LECTURE V...

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