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					Conformational Analysis of
Substituted Cyclohexanes
             Methylcyclohexane

              CH3


                                           CH3

      5%                         95%
Chair chair interconversion occurs, but at any
instant 95% of the molecules have their methyl
group equatorial.
Axial methyl group is more crowded than an
equatorial one.
              Methylcyclohexane




       5%                         95%
Source of crowding is close approach to axial
hydrogens.
Crowding is called a "1,3-diaxial interaction"
and is a type of van der Waals strain.
             Fluorocyclohexane
              F


                                             F

     40%                           60%

Crowding is less pronounced with a "small"
substituent such as fluorine.
              tert-Butylcyclohexane

             C(CH3)3


                                         C(CH3)3

Less than 0.01%            Greater than 99.99%


  Crowding is more pronounced with a "bulky"
  substituent such as tert-butyl.
tert-Butylcyclohexane
            1,2-Dimethylcyclopropane




                H 3C    CH3


CH3            H           CH3         CH3

H              CH3         H           H
      cis                      trans
                 Isomers




Constitutional isomers     Stereoisomers
             1,2-Dimethylcyclopropane




There are two stereoisomers of
1,2-dimethylcyclopropane.
They differ in spatial arrangement of atoms.
1,2-Dimethylcyclohexane Stereoisomers




                CH3                CH3
            H                      H
H                      H3C
          CH3                  H

    cis                    trans
                                                  CH3
 Conformational analysis of
cis-1,2-dimethylcyclohexane                   H
                                    H    CH3

               CH3
                                        CH3
                  H
                CH3                                CH3
                                         H
           H                                  H
  Two equivalent conformations; each has one axial
  methyl group and one equatorial methyl group
                                                    CH3
   Conformational analysis of
trans-1,2-dimethylcyclohexane                   H
                                  H3C       H

              CH3
                                        H
                      H
                  H                                  CH3
                                        H3C
            CH3                                 H
  Two conformations are not equivalent; most stable
  conformation has both methyl groups equatorial.
 1,2-Dimethylcyclohexane Stereoisomers



                 CH3                     CH3
             H                           H
 H                        H3C
           CH3                       H
     cis                     trans
5223 kJ/mol                5217 kJ/mol
less stable                more stable
      1,3-Dimethylcyclohexane Stereoisomers




               CH3                          CH3

H              H          H3C               H
H3C                             H
        cis                         trans
                                               CH3
 Conformational analysis of
cis-1,3-dimethylcyclohexane     H             H
                                H3C

              CH3
        CH3
                    H
                              H3C                 CH3

    H                               H        H

  Two conformations are not equivalent; most stable
  conformation has both methyl groups equatorial.
                                              CH3
   Conformational analysis of
trans-1,3-dimethylcyclohexane H3C            H
                                    H

             CH3
       H
                   H
                              H                  CH3

 H3C
                                  H3C        H

  Two equivalent conformations; each has one axial
  and one equatorial methyl group.
     1,3-Dimethylcyclohexane Stereoisomers



                  CH3                        CH3

H             H          H3C               H
H3C                            H
        cis                        trans
    5212 kJ/mol                5219 kJ/mol
    more stable                less stable
      1,4-Dimethylcyclohexane Stereoisomers




H3C                CH3   H3C                  H

  H            H               H           CH3
        cis                        trans
 Conformational analysis of H3C                   CH3
cis-1,4-dimethylcyclohexane
                               H              H

                  CH3           CH3

                        H   H
H3C                                                CH3

      H                                      H

  Two equivalent conformations; each has one axial
  methyl group and one equatorial methyl group
   Conformational analysis of  H                     CH3
trans-1,4-dimethylcyclohexane
                              H3C                H

                 CH3           H

                       H H3C
H                                                    CH3

    H3C                                      H

 Two conformations are not equivalent; most stable
 conformation has both methyl groups equatorial.
       1,4-Dimethylcyclohexane Stereoisomers


                        CH3
H3C                           H3C                     H

  H                 H               H             CH3
          cis                             trans

      5219 kJ/mol                       5212 kJ/mol
      less stable                       more stable
       Table 3.2 Heats of Combustion of
        Isomeric Dimethylcyclohexanes


Compound               Orientation -H°
cis-1,2-dimethyl       ax-eq        5223
trans-1,2-dimethyl     eq-eq        5217*
cis-1,3-dimethyl       eq-eq        5212*
trans-1,3-dimethyl     ax-eq        5219
cis-1,4-dimethyl       ax-eq        5219
trans-1,4-dimethyl     eq-eq        5212*

*more stable stereoisomer of pair

				
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