cis and trans isomers are cyclic diastercomers ooRROgR5egtereocntes,fourtrog Consider 1-bromo-2-chlorocyclobutane: sareabomrorimegesoteachctheran CenRrSe8gaatmermegesofesd Sgermolsomennoctreatedasojetandmirormageaealeg melting points,boiling points,densities,and A cis isomer and a trans isomer are diastereomers of each othe ren two stereocenters means still more 5-6 Meso Compounds ocenters give ris 2-rombuae HB SRR I RSS CH.CCH.CH,.Hc -CH, RRS ISSR ectega0EceS发rtegnhggregcwemoteapeG 合Sg品eo ormati ruleen =农 H.C 式击无法戏 208ee6a68eaoawaar Examples of meso unds having multiple chiral center 头戏 66 When a chiral molecule has two stereocenters, four stereoisomers are possible: RR, RS, SR, and SS. The RR and SS isomers are mirror images of each other and are therefore enantiomers. The RS and SR isomers are also mirror images of each other and are enantiomers. The RR and RS stereoisomers are not mirror images of each other; nor are the SS and SR stereoisomers. Stereoisomers not related as object and mirror image are called diastereomers. Diastereomers are distinct molecules with different physical and chemical properties. They can be separated by fractional distillation, crystallization, or chromatography. They have different melting points, boiling points, densities, and specific rotations. Cis and trans isomers are cyclic diastereomers. Consider 1-bromo-2-chlorocyclobutane: There are 4 stereoisomers: RR, SS, RS, and SR. The two cis isomers, SR and RS, are enantiomers and the two trans isomers, RR and SS are enantiomers. A cis isomer and a trans isomer are diastereomers of each other. More than two stereocenters means still more stereoisomers. A compound containing 3 stereocenters will exist as 8 stereoisomers which can be grouped into 4 pairs of enantiomers: RRR | SSS SRR | RSS RSR | SRS RRS | SSR In general, a compound having n stereocenters can have a maximum of 2n stereoisomers. 5-6 Meso Compounds Two identically substituted stereocenters give rise to only three stereoisomers. Consider the radical bromination of 2-bromobutane: Since there are 2 chiral centers in the product, we might expect 4 distinct stereoisomers: RR, RS, SR, and SS. These could then be organized into enantiomeric pairs: RR | SS and RS | SR. A closer look at the RS and SR pair of molecules, however, shows that they are superimposable molecules and are therefore identical. A compound containing 2 or more stereocenters that is superimposable with its mirror image is called a meso compound. Meso compounds contain an internal mirror plane which divides the two halves of the molecule which are mirror images of each other. The presence of a mirror plane in any energetically accessible conformation of a molecule is sufficient to make it achiral. 2,3-Dibromobutane exists as three stereoisomers only: a pair of enantiomers and an achiral meso diastereomer. Examples of meso compounds having multiple chiral centers: