1559T_ch05_70-9810/22/0520:19Pa9e79 EQA Keys to the Chapter·79 1.Top stereocenter H 一CH Same as Structure CHCICH, B-CH →CH3 H →H Br Br Structure 3 Same as Structure4 ⊕ Two switches;therefore also identical. So the two structures shown arlier were identical to each other H -CH 一H Br are exactly the same molecule. It is useful to at this point that comparing pictorial formulas is justn of sever ways to deter and to determine the k or S co guration of ea Once you hay come com reochemical relationship is obvious.For two stereocenters.R.R and S.S are enantiomers of each other.R.S S.R are eacn other,and any other combination is a pair ays of drav ehredimre Wth prctice.hever comes eperienceand co need come exam time Keys to the Chapter • 79 1. Top stereocenter Two switches; therefore identical. 2. Bottom stereocenter Two switches; therefore also identical. So the two structures shown earlier were identical to each other. 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. Cl Br CH3 H H CH3 Br C C and H H CH3 CH3 Cl CHClCH3 H Br CH3 Structure 3 CHClCH3 Br H CH3 CHClCH3 Br CH3 H Same as Structure 4 Switch Br, H Switch H, CH3 CH3 CHBrCH3 Cl H Structure 3 Cl CHBrCH3 CH3 H Cl CHBrCH3 H CH3 Same as Structure 4 Switch Cl, CH3 Switch CH3, H 1559T_ch05_70-98 10/22/05 20:19 Page 79