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Keys to the Chapter 83 Before leaving this section, note also that there is a bit of a trick to rule 3 as well. For priority determina- tion purposes, double and triple bonds are changed so that both atoms involved are doubled or tripled. For example, a carbon doubly bonded to another carbon is changed to a carbon singly bonded to two carbons. One is the carbon actually present, and the other is invented. So H H H H Atoms actually present C C H becomes C C H C C Carbons doubly bonded to another carbon These atoms are invented for priority evaluation purposes The procedure is similar for a carbon-oxygen double bond: The carbon bonds are changed and converted to single bonds to two oxygens (one real, one invented), and likewise the oxygen bonds are changed to single bonds to two carbons (one real, one invented). So CH₃ ( CH₃ Real atoms C becomes C C Invented atoms 5-4. Fischer Projections The purpose of Fischer projections is to simplify the on-paper drawing of asymmetric carbons by using a sim- ple convention to represent the three-dimensional structural details. The rules are straightforward, but again, following the text with a set of models handy will help you master this material more readily. 5-5 and 5-6. Molecules Incorporating Several Stereocenters: Diastereomers When a molecule has more than one stereocenter, as does, it will have more than two stereoisomers. In particular, n stereocenters give rise to as many as 2" stereoisomers. If we consider the case where n = 3, how are all the 2" = 8 stereoisomers related? Because an object has only one mirror image, if we pick any one of these (stereoisomer A), it may have no more than one enantiomer (stereoisomer B). What about the other six isomers? They are also stereoisomers of A, but they can't be its mirror images. The rela- tionship of any of these other six molecules with A is described by a new term: diastereomer. Diastereomers are stereoisomers that are not mirror images of each other. Because diastereomers are not mirror images of each other, they have different physical properties and can therefore be separated by standard laboratory techniques. This is very important. This feature distinguishes diastereomers from enantiomers, which cannot readily be separated from one another. Diastereomer is a very important term. as important as enantiomer, and one that you should make a point of understanding well before you leave this section. Notice that both "enantiomer" and "diastereomer" really describe relationships between structures. As described above, A is the enantiomer of B; A is also a