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Keys to the Chapter 87 1. Top stereocenter CH₃ Switch Switch CH₃ CH₃, H CI H CH₃ H H CH₃ CHBrCH₃ CHBrCH₃ CHBrCH₃ Structure 3 Same as Structure 4 Two switches; therefore identical. 2. Bottom stereocenter CHCICH₃ Switch CHCICH₃ Switch CHCICH₃ Br, H H, CH₃ Br CH₃ H CH₃ CH₃ H H Br Br Structure 3 Same as Structure 4 Two switches; therefore also identical. So the two structures shown earlier were identical to each other. CH₃ H CH₃ Br C C H and CH₃ H H CH₃ Br are exactly the same molecule. It is useful to recognize at this point that comparing pictorial formulas is just one of several ways to deter- mine the relationship between possible stereoisomeric structures. If you have the time, making models is al- ways useful, especially for visualization purposes. An even better way is to apply the rules of nomenclature to each structure and to determine the R or S configuration of each stereocenter. Once you have become com- fortable with this technique as it applies to the different types of structural pictures, you may find that this is the quickest method of all: Once R-S assignments have been made to structures under consideration, their stereochemical relationship is obvious. For two stereocenters, R,R and S,S are enantiomers of each other, R,S and S,R are enantiomers of each other, and any other combination is a pair of diastereomers. Notice that I never said this was easy. It takes practice to do and to extend to other common ways of draw- ing molecules that imply three-dimensional structure. With practice, however, comes experience and confi- dence, just what you will need come exam time. 5-7. Stereochemistry in Chemical Reactions The apparent complexity of this section is due to the number of situations that are explained in detail. The ma- terial actually develops very naturally, with no special pitfalls.