1559r_ch05.70-9810/22/0520:19Page78 78.Chapter 5 STEREOISOMERS 2.Then do the bottom stereocenter. CCIBrCH CI--H -CH; CH3- Structure Answer:The two structures are diastereomers(not completely identical and not mirror images). Let's now examine our problem of comparing a dashed-wedged line formula with a Fischer projection.The obemshowterone-wedged The res of a mo rotation will do,such as rotation of the left-hand methyl group up out of the page.toward you: cH 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. CH CH C CI-C-H CI- H H Br-C-CHs Br- H H H fyou look at it like this This is what you sce Which is this Now you can compare the Fischer projection above with the one we wrote earlier -H H with Br- CHH Br Structure3 Structure42. Then do the bottom stereocenter. 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 dashed-wedged line formula with a Fischer projection. The actual problem is how to interconvert dashed-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 dashed-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: 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: Now you can compare the Fischer projection above with the one we wrote earlier: CH3 H Cl H CH3 Structure 3 Structure 4 with Br Cl Br CH3 H H CH3 If you look at it like this This is what you see Which is this Br C C H H CH3 Rotate CH3 Cl towards you 60 Becomes CH3 C C H H CH3 Br Cl CClBrCH3 CH3 Cl H Structure 1 Switch H, CH3 CClBrCH3 H Cl CH3 Same as Structure 2 78 • Chapter 5 STEREOISOMERS 1559T_ch05_70-98 10/22/05 20:19 Page 78