not abh品aaetndeergmehorc0e8aghvhetacd ocoa08reaecepreftoeyoeneghernemeeta erformulbthem the of exchanges,v Fisher formula in cl,e, 5-5 Molecules Incorporating Several Stereocenters:Diastereomers h8et858n26amotaeitosomer H CH.CCH.CH hoabr二2R7aoemar8n8cweeaaee but c A&e58ga8eetntioienmed CH.CRCCH Br H The possible combinations are RR,RS,SR,and SS. 品6 c2an2n地goeR The center under scrutiny is 5. 55 Rotating a Fischer projection may or may not change the absolute configuration. Rotating a Fisher projection formula by 90o converts the structure into that of the enantiomer of the molecule originally represented. Rotating a Fisher projection formula by 180o keeps the same enantiomer. Exchanging substituents in a Fischer projection also changes the absolute configuration. To compare a Fischer projection to another in a different orientation in order to see if they represent the same enantiomer: •Exchange any two substituents. This turns the molecule into its mirror image. •Exchange another two substituents. This then turns the molecule back into the original enantiomer. •Using a series of exchanges, convert one Fisher formula into the other. •If an odd number of exchanges are required, the two projection formulas represent different enantiomers. •If an even number of exchanges are required, the two projection formulas represent the same enantiomer. Fischer projections tell us the absolute configuration. •Draw any correct Fischer projection formula of a chiral center. •Assign priorities to all of the substituents. •Using two consecutive substituent exchanges (to preserve the chirality of the Fischer formula), place group d (lowest priority) on the top. •If the a,b,c groups are now arranged in a clockwise order, the enantiomer is R: if in a counterclockwise order, the enantiomer is S. Molecules Incorporating Several Stereocenters: Diastereomers 5-5 Two stereocenters can give four stereoisomers: chlorination of 2-bromobutane at C3. Consider the chlorination of 2- bromobutane. Several products are formed, but consider only the 2- bromo-3-chlorobutane. A second stereocenter is formed by the addition of the chlorine atom. The possible combinations are RR, RS, SR, and SS. Because all of the horizontal bonds in a Fischer projection formula point towards the viewer and all vertical bonds away from the viewer, a Fischer projection formula represents the molecule in its eclipsed conformation. In order to convert a Newman or dashed-wedged representation into a Fischer representation, first rotate the molecule to form an eclipsed rotomer. Treat each stereocenter separately and regard the group containing the other stereocenter as a simple substituent