Organic Chemistry

Organic Chemistry - Ch 11 556 Daley & Daleystereoisomers. They are not enantiomers, however, because they arenot mirror images of each other. C2 has an (S) configuration in bothstructures, and C3 has an (R) configuration in the structure on the leftand an (S) configuration on the right. Carbon C3 has a mirror imagerelationship in the molecules, but C2 does not. Because these twomolecules are stereoisomers, but they do not have a mirror-imagerelationship, they are diastereomers.Both diastereomers are stereogenic, and each has anenantiomer giving a total of four stereoisomeric 3-chloro-2-butanols.To help visualize this, make molecular models of the two mirror-imagemolecules.CH 3CH 3HHCH 3OHCH 3ClHOClCH 3HHHClCH 3OHCH 3HHOHHCH 3ClWith the 2n rule youcan calculate themaximum possiblenumber ofstereoisomers with nstereogenic centers.You now have two pairs of enantiomers, and either member of onepair of enantiomers is a diastereomer of both members of the otherpair. Because each stereocenter can have the atoms in an (R) or an (S)configuration arrangement, a molecule with n stereocenters has 2 npossible stereoisomers. This rule, called the 2 n rule, allows you tocalculate the maximum number of stereoisomers possible for amolecule.However, not all molecules have this maximum number ofstereoisomers. 2,3-Butanediol, for example, does not. It has twostereocenters, C2 and C3 and, according to the 2 n rule, it should havefour stereoisomers.CH 3CH 3HHCH 3OHCH 3OHHOHOCH 3HHHHOCH 3OHCH 3HHOHCH 3HOHHowever, if you draw all four of the isomers, or make molecularmodels of them, you will find that the apparent enantiomeric pair onthe left above is actually different representations of the samemolecule. (Remember that rotating a Fischer projection 180 o in theplane of the page is acceptable.)www.ochem4free.com 5 July 2005