Stereochemistry 1 Stereochemistry 1

Document Sample
Stereochemistry 1 Stereochemistry 1 Powered By Docstoc
					                                                                                                                 1
Stereochemistry
1) Constitutional/Structural Isomer VS Stereoisomer




                Constitutional isomer                                   Stereoisomer
      -different IUPAC names                    -identical IUPAC names (except for a prefix like cis or trans)
      -same or different functional groups      -same functional group(s)
      -different physical & chemical properties -different only in orientation in space (3D; configuration)



2) Stereochemistry




3) Chiral VS Achiral Molecule
3.1 Chiral Molecule: Molecule or object that is not superimposible on its mirror image




                             Chiral


                                                                                          Achiral
                           2
Chiral Molecule:
One stereogenic center




No plane of symmetry




3.2 Drawing Tetrahedron




4) Enantiomers
                                                                                                                       3
4.1) Labeling Chiral Centers with R or S (Cahn-Ingold-Prelog system)
Assign a priority (1,2,3,4) to each group bonded to chiral center
Use priority to label R or S isomer

Priority Rule
Rule 1: assign priorities (1,…,4) to the atoms directly bonded to chiral center in order of decreasing atomic number
                                                4     H
Atomic number: Br> Cl > F >H                3       F C Br          1
                                                2      Cl
Rule 2: If two atoms on chiral center are identical, assign priority based on atomic number of the atoms bonded to
these atoms. One atom of higher atomic number determines a higher priority




Rule 3: If two isotope are bonded to the chiral center,assign priorities in order of decreasing mass number.

           4                    3 heavier isotope
                    H
       1       Br   C    D

                    Cl

               2


Rule 4: To assign a priority to an atom that is part of a multiple bond, treat a multiple bonded atom as an equivalent
number of single bon atoms




Labeling R or S to Chiral Center
Step 1: assign priorities from 1 to 4
                                          2-butanol
                                  1                         1
                                                            OH
                                  OH
                               4                                4       Priority: -OH, -CH 2CH3, -CH 3, -H
                               H C    2                      C H
                                                        2
                              H 3C   CH 2CH3        H 3CH 2C  CH3
                             3
                                   A                       B   3
                                                                                                                                                          4
Step 2: orient the molecules with lowest priority (4) back and visual the remaining priorities (1-3)
                                                                                               eye
                                                                       1
                                                                                                                                         1
                                                                       OH
                                      Enantiomer A                4                                                                      OH
                                                                  H C
                                                               3 H 3C            CH 2CH 3                                       H 3C         CH 2CH3
                                                                                 2                                               3             2


                                                                                             1                                                1
                                                                                              OH                                              OH
                                                                                            4
                                      Enantiomer B                               2     C H                                      H 3CH 2C           CH 3
                                                                             H 3 CH 2C  CH 3 3                                       2              3



Step 3: Trace a circle from priority group 123
                                                 1

                                                 OH
                                                                      clockwise directiion                                  R-isomer
                                          H 3C       CH 2CH 3 2
                                      3

                                          Enantiomer A                                                                                   1
                                                                                                                                         OH

                                                 counterclockwise directiion                       S-isomer             2 H 3 CH 2C           CH 3 3


                                                                                                                               Enantiomer B


5) Diastereomers
             H H
H 3CH2 C   * *       CH 3      chiral center, n = 2
                              max. stereoisomers = 4
             Br Br
 2,3-dibromopentane




             Alkene                                                                    Cycloalkane

                                                           CH3                    H                  CH3                       CH3
 H3C     H            H3C     CH 3

   H     CH 3           H     H                            H                      CH3                H                         H

t rans-2-Butene        cis-2-Butene              t rans-1,3-dimethylcyclopentane                   cis-1,3-dimethylcyclopentane


6) Meso Compounds


                                                                        H 3C          CH 3                 H 3C         CH 3
                                                                        Br             Br                  Br            Br
                                                                             H       H                          H       H
                                                                                 C                                  D


                                                                                               identical
                                                                                                                                                          5
                                                              Isomers
                                                   diff erent compounds with the
                                                      same molecular formula




                            Constitutional isomers                            Stereoisomers
                             isomers having atoms bonded to               isomers with a difference in 3D
                                     diff erent atoms                           arrangement only




                                                        Enantiomers                                 Diastereomers
                                                          mirror images                               not mirror images




                                                                            Congifurational                                 Cis-Trans
                                                                            diastereomers                                 diastereomers



7) Optical Activity of Stereoisomer




7.1 Rotation of Polarized Light




            Achiral Molecule >> Optically Inactive                                                                  Chiral Molecule >> Optically Active




7.2 Specific Rotation
                                                                                    6
7.3 Racemic Mixture and Enantiomeric Excess (ee)
: A mixture containing equal amount of two enantiomers. Denoted by the symbol ()

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:58
posted:11/26/2011
language:English
pages:6