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Isomer overview Whitney High School

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Isomer overview Whitney High School Powered By Docstoc
					      Is there a double bond?
YES                             NO
            ALKENES
cis and trans
                  E and Z
cis   AND   trans   ISOMERS
      CIS / TRANS ISOMERS
Because there is no rotation about a
carbon-carbon bond, isomers are possible.


  R                 R    R               H

      C         C            C       C

  H                 H    H               R
          cis                trans
 substituents on        substituents on
 the same side of       opposite sides of
 main chain             main chain
     TWO IDENTICAL SUBSTITUENTS

If an alkene has two identical substituents on one of
the double bond carbons, cis / trans ( or E / Z )
isomers are not possible.


 H      CH3             H3C     H       H    CH2CH3


  H     CH2CH3      CH2CH3     H       H     CH3



         all of these compounds are identical
                 no cis / trans isomers
 NAMING cis / trans ISOMERS OF ALKENES
    main chain stays             main chain crosses
    on same side of              to other side of
    double bond = cis            double bond = trans


CH3CH2           CH2CH3      CH3CH2            H

     H         H                    H         CH2CH3

     cis-3-hexene                trans-3-hexene


             notice that these
             prefixes are in
             italics
E / Z SYSTEM OF NOMENCLATURE
 E/Z SYSTEM OF NOMENCLATURE
To avoid the confusion between what the main
chain is doing and the relationship of two similar
groups ….. the IUPAC invented the E/Z system.



                  Cl       F      cis ?

                   I      H       trans ?


    This system also allows alkenes like the
    one above to be classified …..
         an impossibility with cis / trans.
                  E / Z NOMENCLATURE

In this system the two groups attached to each carbon
are assigned a priority ( 1 or 2 ).

If priority 1 groups are both on same side of double bond:
    Z isomer = zusammen = together (in German)

       same
       side                                opposite
              1       1       1        2   sides

              2       2       2        1
                  Z               E

If priority 1 groups on opposite sides of double bond:
    E isomer = entgegen = opposite (in German)
        ASSIGNING PRIORITIES
 1. Look at the atoms attached to each carbon of
    the double bond.
 2. The atom of higher atomic number has higher (1)
     priority.


   example
                     1               1
                         F       I
         F>H                                I > Br
                        H      Br
                    2                2

Since the 1’s are on the same side, this compound is Z

       (Z)-1-bromo-2-fluoro-1-iodoethene
                notice use of parentheses
3. If you can’t decide using the first atoms attached,
   go out to the next atoms attached. If there are
   non-equivalent paths, always follow the path with
   atoms of higher atomic number.

  Once you find a difference, you can stop.

                                          path goes to
     1                     1     H        F not to H
                                 C   F
         CH3     CH2F
                                 H       comparison
          H     CH2CH3           H       stops here

     2                     2     C C
                                 H        path goes to
                                          C not to H


     This molecule has Z configuration.
4. The atoms in double bonds are “replicated” at either
    end of the double bond.


                                C   C     2

            CH CH2              CH CH2

                                 O C
             C O                C O
             H                  H        1


    Then, when comparing groups, follow the path
    of highest priority as before.

				
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posted:5/10/2011
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