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86 Chapter 5 STEREOISOMERS 2. Then do the bottom stereocenter. Switch H. CH₃ H CH₃ CH₃ H Structure 1 Same as Structure 2 One switch made the bottom stereocenter of 1 identical to that of 2; therefore these carbons are opposite in configuration. Answer: The two structures are diastereomers (not completely identical and not mirror images). Let's now examine our problem of comparing a hashed-wedged line formula with a Fischer projection. The actual problem is how to interconvert hashed-wedged line and Fischer formulas. The key to this is recogniz- ing that Fischer projections are pictures of a molecule in an eclipsed conformation. Very important. So the first step in our comparison is to get the hashed-wedged line formula into an eclipsed conformation. Any 60° rotation will do, such as rotation of the left-hand methyl group up out of the page, toward you: Rotate CH₃ Br towards 60° Br C C H Becomes CH₃ you C C H H CH₃ H CH₃ Look at these two conformations of the same, identical molecule carefully-with the help of a model, if necessary- - to convince yourself that the pictures on the page are what I've said they are. We can now convert the eclipsed formula, above, into a Fischer projection. Imagine looking at it from a direction such that the carbon-carbon bond is vertical, and the groups horizontal to it point toward us; for example: CH₃ CH₃ Br C H H CH₃ H C C Br C CH₃ Br CH₃ H CH₃ H H If you look at it like this This is what you see Which is this Now you can compare the Fischer projection above with the one we wrote earlier: CH₃ CI H H CH₃ with Br CH₃ CH₃ H H Br Structure 3 Structure 4