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88 Chapter 5 STEREOISOMERS 5-8. Resolution of Enantiomers The most common laboratory method to separate enantiomers is presented. What is required is a pure enan- tiomer of a molecule that reacts with the molecules of your racemic mixture. The procedure outlined is a straightforward application of the ideas in Sections 5-2 (properties of chiral molecules) and 5-5 (molecules with more than one stereocenter). Solutions to Problems 30. Chiral: (b)*, (c; fan blades are always twisted!), (d), (e), (h). Achiral: (a), (f), (g), (i), (j), (k), (1). All the achiral objects contain a plane of symmetry: Spoon Knife x x yz symmetry plane xy symmetry plane *If you ignore the mounting hinges, then doors are achiral. The plane of a door is a symmetry plane. 31. (a) Enantiomers (b) Enantiomers (c) Diastereomers (d) Identical (if one pair is flipped over intact, it becomes superimposable with the other pair) 32. (a) Constitutional (structural) isomers (b) Identical (one is the other turned upside-down) (c) Constitutional isomers (d) Conformers (e) Constitutional (structural) isomers (f) Stereoisomers (enantiomers) (g) Conformers (h) Stereoisomers (enantiomers) If two conformational isomers are cooled to a temperature low enough to prevent their interconversion by bond rotation or other motion, then they can be described as stereoisomers: structures with the same connectivity but with different atomic arrangements in three dimensions. For example, the conformers in (d) are nonsuperimposable mirror images (enantiomers) if bond rotation is not allowed to occur (make a model!). Those in (g) are not enantiomers, but they are still stereoisomers. The temperatures required to "freeze out" conformational interconversion are very low: on the order of - 200°C for substituted ethanes and - 100°C for cyclohexanes. Conformers of these types are occasionally referred to as interconvertible stereoisomers. 33. The stereocenter has been labeled with an asterisk (*) in each chiral molecule. (a) Not chiral (achiral) (2 CH₃'s on the tertiary carbon!)