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Keys to the Chapter 85 immediately visualize the mirror plane between them, and recognize them as enantiomers. Similarly, CH₃ CH₃ H H and Br H Br H CH₃ CH₃ clearly lack a mirror-image relationship and must be diastereomers. The hard part is determining, for example, the relationship between Cl CH₃ H CH₃ C C H and Br CH₃ H H CH₃ Br without getting all messed up (or taking forever to do it, and then getting all messed up). In short, you need to be able to move quickly and accurately among Newman, hashed-wedged line, and Fischer structures. This capability takes practice at visualizing three-dimensional structures from flat drawings and requires application of a couple of specific techniques. The text covers comparison and interconversion of Fischer projections very thoroughly. The one important precaution here is never operate on more than one stereocenter at a time when interconverting Fischer projections. Example: What is the relationship between the following structures: identical, enantiomeric, or diastereomeric? Br Cl Br a CH₃ Br CH₃ and Cl H Cl CH₃ Ci CH₃ H Structure 1 Structure 2 Procedure: 1. Operate on the top stereocenter. Cl Switch CH₃ Br Switch CH₃ CH₃, Br Br Br CH₃ CH₃ CHCICH₃ CHCICH₃ CHCICH₃ Same as Structure 2 Structure 1 Two switches made the top stereocenter of 1 identical to that of 2; therefore these carbons are identical in configuration.