Stereoisomer nomenclature - CIP rules to determine R/S both 1-bromo-1-chloroethane isomers rank atoms attached to chiral center according to atomic # (only first atom attached) rotate the molecule such that the lowest priority atom (4), which is usually H, is in the back (dashed line). trace the path of 1, to 2, to 3 R= clockwise, S= counterclockwise for alkenes only: prioritize 2 groups on same carbon H₂C=CHBr H₂C=CHCl Br Same side Z (cis) E (trans) look at relative orientation of top priority groups: trans/E= across, cis/Z= same side in the case that there is a tie, the immediate atoms are ranked by each of their top priority atoms, ex: if lowest priority is not already ex: oriented to the back, 1) switch 2 highest priority groups but know it is the opposite hand now being evaluated (if you find it's R, the original is S) or 2) prioritize with lowest priority wedge but notice it's being viewed backwards (seems like R but is S or vice versa). Compounds w/ more than one stereocenter: 2 asym. carbons = 4 stereoisomers to get enantiomer, change handedness of both carbons (switch 2 groups) to get diastereomer, change handedness of one (not all) carbon, ex: [A] RS, RR or SS, [B] RS, RR or SS molecule with two chiral centers and its stereoisomers asym. carbons but internally mirrored achiral, no enantiomer for every asym. c meso compounds (identical "enantiomer") loc-internal sym (even w/ asym. c) meso compound with internal mirror plane diast. Відкриється в новому вікні chem.libretexts.org diastereomer of the meso compound enant molecule with three chiral centers Cannot be meso even if add # of stereocenters; even if other left alone are mirror images, other left alone front view of ring -front and back C connected look down cyclopentane ring dimethylcyclopropane A & D have diff connectivity dimethylcyclopropane molecule with different orientations of the methyl groups B & C only have one diff center's groups switch to become the other E has 2 asym. C but also an internal mirror plane (same w/ C) meso CH₂OH meso compound with a plane of symmetry H OH CH₂OH H CH₃ H CH₃ methyls on same C in H axial cis axial same as J but flat finding symmetry in molecules 6-fold rotation axis indistinguishable due to 2-fold axis turns one into another every rotation turns one H into another each H is homotopic to one another ⇒ C and H homotopic to one another