Chapter 7 Alkenes and Alkynes I Properties and Synthesis - PDF

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					ORGANIC CHEMISTRY                                 Chapter 7




     7.11 The Acidity of Terminal Alkynes




                         Department of Chemistry, Xiamen University
 ORGANIC CHEMISTRY                                           Chapter 7

                   sp             sp2         sp3
                CH CH       CH2   CH2     CH3 CH3
          pKa        25       44               50

Relative Basicity         CH C < CH2 CH < CH3 CH2

 Relative Acidity

    H2O > ROH > RC CH > NH3 > CH2 CH2 > CH3 CH3
pKa 15.7 16-17   25      38      44        50

Relative Basicity

   OH < RO < RC C < NH2 < CH2 CH < CH3 CH2

                                    Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                              Chapter 7



   Acetylenic hydrogens can be deprotonated with
   relatively strong bases (sodium amide is typical)

                    −40
2Na + NH3(liq.)                  2NaNH2 + H2

                     NH3(l)                    NaNH2
CH CH + NaNH2                     CH CNa                        NaC CNa
                     −33

                           NH3(l)
CH3C CH + NaNH2                      CH3C CNa
                           −33

             The products are called alkynides

                                      Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                  Chapter 7




       RC CH + HOBr      RC CBr + H2O



     CH CH + Ag(NH3)+      AgC CAg white
      CH CH + Cu(NH3)+     CuC CCu              red




                          Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                Chapter 7




      7.12 Replacement of the Acetylenic
      Hydrogen Atom of Terminal Alkynes




                        Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                            Chapter 7

                    CH3CH2CH2Br
    CH CNa                        CH CCH2CH2CH3
    (1) NaNH2
                    CH3C CCH2CH2CH3
    (2) CH3I
   Only primary alkyl halides can be used or else elimination
   reactions predominate




                                    Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                   Chapter 7




           7.13 Hydrogenation of Alkenes




                           Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                            Chapter 7




                          Pd, Pt or Ni
    CH2 CH2 + H2                               CH3 CH3
                             25°C

                          Pd, Pt or Ni
  CH3CH CH2 + H2                               CH3CH2 CH3
                             25°C

                    Catalytic hydrogenation


An unsaturated compound                    A saturated compound




                                    Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                 Chapter 7




      7.14 Hydrogenation: The Function of
                 the Catalyst




                         Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                                    Chapter 7


  Hydrogenation of an alkene is an exothermic reaction.
                                   Pt
 R CH CH R + H2                             R CH2 CH R + heat

  Both hydrogen atoms usually add from the same side
  of the molecule — syn addition.
                             Pt
                    C C                     C C
                     +                  H             H
                    H H
                                                                                         Y
C C                       C C                 C C                            C C
 +                                             +
               X              Y               X Y                      X
X Y
  A syn addition (˳ʽ¼ÓÉ      )                An anti addition (·´Ê½¼Ó³É               )

                                            Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                           Chapter 7


  In heterogeneous catalysis the hydrogen and alkene
  adsorb to the catalyst surface and then a step-wise
  formation of C-H bonds occurs

                                                              C C
                    H2   H H   H H       C C             H H            H H

 Surface of
metal catalyst

                C
           H        C
         H          HH         H          H
                                                  +          C C
                                                         H               H

                                   Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                   Chapter 7




           7.15 Hydrogenation of Alkynes




                           Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                   Chapter 7




                    H2                                   H2
   CH3C CCH3             CH3CH CHCH3
                    Pt                                   Pt

        CH3CH2 CH2CH3




                           Department of Chemistry, Xiamen University
 ORGANIC CHEMISTRY                                             Chapter 7




          7.15A Syn Addition of Hydrogen:
              Synthesis of cis-Alkenes
 P-2 catalyst                           Lindlar's catalyst
               NaBH4                       Pd/CaCO3-quinoline
   (CH3CO2)2Ni               Ni2B
               C2H5OH
                             P-2             Pd/BaSO4-quinoline


                      H2 / Ni2B(P-2)         CH3CH2                    CH2CH3
CH3CH2C CCH2CH3                                            C      C
                     or Lindlar's catalyst            H                H
                        Syn addition
                                     Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                                          Chapter 7


       7.15B Anti Addition of Hydrogen:
           Synthesis of trans-Alkenes
 A dissolving metal reaction which uses lithium or
 sodium metal in low temperature ammonia or amine
 solvent produces trans-alkenes
                                              Li/ C2H5NH2, −78°C
 CH3(CH2)2C C(CH2)2CH3
                                                or Na / NH3(l)


                CH3(CH2)2                     H
                                   C     C                           anti addition
                              H               (CH2)2CH3
    Dissolving metal reduction (ÈÜ»¯½ðÊô¹Ô-                     )
                                                  Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                                     Chapter 7

The mechanism is a free radical reaction with two electron transfer reactions from
the metal
The vinylic anion prefers to be trans and this determines the trans stereochemistry
of the product




                                             Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                Chapter 7




         7.16 Molecular Formulas of
     Hydrocarbons: The Index of Hydrogen
                 Deficiency




                        Department of Chemistry, Xiamen University
ORGANIC CHEMISTRY                                           Chapter 7


   General formula
                    Alkanes                    CnH2n+2
           Alkenes and cycloalkanes            CnH2n
            Alkynes and cycloalkenes            CnH2n-2

Index of hydrogen deficiency — the number of pairs of hydrogen
atoms that must be subtracted from the molecular formula of the
corresponding alkane to give the molecular formula of the
compound under consideration.

                         2n+2 − X1 + N3
                    Ω=
                                  2

                                   Department of Chemistry, Xiamen University