Chapter 5 - Download Now PowerPoint by Vikcyb7

VIEWS: 33 PAGES: 48

									                               Chapter 5
• Types of reactions.
• Reaction mechanism
     – Radical chlorination - homolytic cleavage, initiation,
         propagation, termination, disadvantage   .
     – Polar reaction - , reaction enhances polarity, polarizability,
         heterolytic cleavage, nucleophile, electrophile.

• Reaction profile - equilibria, kinetics, thermodynamic changes.

12/13/2011                                                                1
             Types of Organic Reactions

• Addition
• Elimination
• Substitution
• Rearrangement
12/13/2011                                2
                                     Addition
                  H                                           H H
          H                                          H
                      H          +     H2
                                                      H H
                                                                H
              H
                           sp2
                                                    sp3



      E           R                         E             P
                                      or
                           P                    R
                      RC                            RC
12/13/2011                                                          3
                           Elimination


               H H                       H
        H                      H                 +       O
                 OH                          H       H       H
         H H
                                   H
                     sp3
                                   sp2




12/13/2011                                                       4
                        Substitution



         H H                           H H
                    +   H OK                      +   rBK
         H     rB                  H         HO




12/13/2011                                                  5
             Rearrangement




12/13/2011                   6
                               Chapter 5
• Types of reactions.
• Reaction mechanism
     – Radical chlorination - homolytic cleavage, initiation,
         propagation, termination, disadvantage   .
     – Polar reaction - , reaction enhances polarity, polarizability,
         heterolytic cleavage, nucleophile, electrophile.

• Reaction profile - equilibria, kinetics, thermodynamic changes.

12/13/2011                                                                7
                       Mechanism


             A complete step-by-step description
             of which bonds break and which
             bonds form, in what order, in a
             chemical reaction.



12/13/2011                                         8
                               Chapter 5
• Types of reactions.
• Reaction mechanism
     – Radical chlorination - homolytic cleavage, initiation,
         propagation, termination, disadvantage   .
     – Polar reaction - , reaction enhances polarity, polarizability,
         heterolytic cleavage, nucleophile, electrophile.

• Reaction profile - equilibria, kinetics, thermodynamic changes.

12/13/2011                                                                9
             Chlorination of Methane



                     h or heat
                    
    CH 4  Cl 2    CH 3Cl  HCl

             Mechanism: radical reaction.




12/13/2011                                  10
    Radical reaction - chlorination of methane


• Initiation
• Propagation
• Termination
12/13/2011                                       11
                Mechanism - Initiation
        h or heat



              Cl — Cl           2Cl
               homolytic
                cleavage




12/13/2011                               12
             Mechanism - Propagation
                               Substitution

             Cl + H:CH3  HCl + CH3

                               Addition

                CH3 + Cl:Cl  CH3Cl + Cl

                               Chain Reactions

12/13/2011                                       13
             Mechanism - Termination


              Cl + CH3  CH3Cl

              Cl + Cl  Cl2

              CH3 + CH3  CH3CH3


12/13/2011                             14
        Chlorination of methane - disadvantage



H        H     Cl2        H       Cl     Cl2        H       Cl     Cl2
    C                         C                         C
H H          h or heat   H H          h or heat   H Cl         h or heat




12/13/2011                                                               15
                           Chlorination of alkanes
                                                   CH3

                                    CH3CH2CH2CHCH2Cl

                                                   CH3

                                    CH3CH2CH2CCH3

                                                   Cl

        CH3                                        CH3
                            h or
CH3CH2CH2CHCH3   +   Cl2
                             heat
                                    CH3CH2CHCHCH3        and others...
                                              Cl
                                                   CH3

                                    CH3CHCH2CHCH3

                                         Cl
                                                   CH3

                                    CH2CH2CH2CHCH3

                                    Cl

   12/13/2011                                                            16
                  Chlorination of alkanes
Which of the following compounds will give a single monochloro product?




    C2H6                     CH3CH2CH3




                                      CH3
     (CH3)2CCH2CH3
                                                      CH3C         CCH3



                 Ans.
12/13/2011                                                                17
               Free Radical Chlorination




     Write radical reaction mechanism of the chlorination
     of methane for the formation of all chlorinated methane
     - that is, CH3Cl, CH2Cl2, CHCl3, CCl4.




12/13/2011                                                     18
                    Free Radical Chlorination

        Considering only monochlorohydrocarbon. Explain
        why:


CH3CH2CH2CH2CH2CH3   +     Cl2   CH3CH2CH2CH2CH2CHCl     +   HCl   Low yield

                                          Cl
                +    Cl2                       +   HCl        High yield



   12/13/2011                                                          19
                               Chapter 5
• Types of reactions.
• Reaction mechanism
     – Radical chlorination - homolytic cleavage, initiation,
         propagation, termination, disadvantage   .
     – Polar reaction - , reaction enhances polarity, polarizability,
         heterolytic cleavage, nucleophile, electrophile.

• Reaction profile - equilibria, kinetics, thermodynamic changes.

12/13/2011                                                                20
                                  Polarity
             2.5     3.5              2.5       1.2            2.5   2.5
             C—O                      C — Mg                   C—I
             +       -               -       +              +   -


                             H                           H
                                                           +
                     H       O                  H       O
                           C H                        C H H
                           H                          H
                   Weakly polar C—O             Strongly polar C—O
                                      solvent

12/13/2011                                                                 21
             A generalized polar reaction


        A+          +     :B-             AB
   electrophile         nucleophile     heterolytic cleavage

      neutral or         neutral or
      positively         negatively
      charged            charged
                                        Arrow shows the
                O                       flow of electrons.
  H+,   CH3X,           NH3, OH-, Br-


12/13/2011                                                 22
                  Flow of Electrons
              H                              D
                  +       D        Cl        H          +   Cl
              H                             +H
              D                         D
              H       +       Cl            +       H       Cl
             +H                         H



              H                             D
                  +       D        Cl           +   H       Cl
              H                             H



12/13/2011                                                       23
                              Flow of Electrons

                                             nucleophile

                                                   Cl
             H                             D                 D
                 +   D       Cl            H                     +   H    Cl
          H                               +H                 H


nucleophile          electrophile    carbocation
                                                           substitution




12/13/2011                                                                24
                 Predict Products



                           O
                   H       C
       HO    +         C       CH3
                    H H




12/13/2011                           25
                               Chapter 5
• Types of reactions.
• Reaction mechanism
     – Radical chlorination - homolytic cleavage, initiation,
         propagation, termination, disadvantage   .
     – Polar reaction - , reaction enhances polarity, polarizability,
         heterolytic cleavage, nucleophile, electrophile.

• Reaction profile - equilibria, kinetics, thermodynamic changes.

12/13/2011                                                                26
                        Equilibria
                       A B  C  D
                                [C][ D]
                       K eq   
                                [ A][ B]

               CH 4  Cl 2  CH 3Cl  HCl
                      [CH 3Cl ][ HCl ]
             K eq                      1.11019
                       [CH 4 ][Cl 2 ]
12/13/2011                                          27
                 Equilibrium                    Thermodynamics
                    Keq                              G


                G  G(products)  G(reactan ts)
                                           G o / RT
                               K eq  e
             @ 25 o C, RT  2.48 kJ / mol  0.592 kcal / mol



        For CH4 Cl2  CH3Cl  HCl , Keq  1.11019
               G o  108kJ/ mol  25.9kcal/ mol
12/13/2011                                                       28
For CH 4  Cl 2  CH3Cl  HCl ,
K eq  1.11019
R=        8.31E-03 kJ/(mol*K)
                                  K eq  exp( G o / RT )
T=        298 K
RT=       2.48 kJ/mol                                   100
                                  %C _ to _ P 
                                                1  exp( G o / RT )
Go            Keq   %C_to_P
kJ/mol
100       2.96E-18   2.96E-16                        120




                                   % conversion to
80        9.48E-15   9.48E-13                        100




                                      products
60        3.04E-11   3.04E-09                         80
40        9.74E-08   9.74E-06                         60
                                                      40
20        3.12E-04   3.12E-02                         20
0         1.00E+00   5.00E+01                          0
-20       3.20E+03   1.00E+02                           -150   -100    -50    0     50      100        150
-40       1.03E+07   1.00E+02                                         Free Energy(kJ/mol)
-60       3.29E+10   1.00E+02
-80       1.05E+14   1.00E+02
-100      3.38E+17   1.00E+02
-120      1.08E+21   1.00E+02
  12/13/2011                                                                                      29
                        Thermodynamics

                         G o  Ho  TSo

              Go determines the spontaneity of a reaction.
               Ho & So are driving forces of a reaction.

                Ho - Exothermic(-) or Endothermic(+).
                So - Randomness or Freedom of Motion

             For most (not all) organic reactions, Ho > So
                            Thus, Go  Ho

12/13/2011                                                     30
       Enthalpy Change in Chlorination of Methane


                     CH4 + Cl2  CH3Cl + HCl
                    Bonds Broken(+)   Bonds Formed(-)
        Table 5.3
        page 172    CH3—H +438         CH3—Cl -351
                    Cl—Cl +243         H—Cl     -432

                            +681                -783

             Ho (or Go) = +681 + (-783) = -102 kJ/mol

12/13/2011                                                31
       Enthalpy Change in Chlorination of Methane


                 CH4 + Cl2

             E
                      Ho (or Go) = -102 kJ/mol


                                   CH3Cl + HCl




12/13/2011
                    Reaction Coordinate             32
       Enthalpy Change in Bromination of Methane


                     CH4 + Br2  CH3Br + HBr
                    Bonds Broken(+)   Bonds Formed(-)
        Table 5.3
        page 172    CH3—H +438         CH3—Br -293
                    Br—Br +193         H—Br     -366

                            +631                -659

             H (or Go) = +631 + (-659) = -28 kJ/mol

12/13/2011                                              33
       Enthalpy Change in Bromination of Methane


                 CH4 + Br2

             E
                      H (or Go) = -28 kJ/mol


                                   CH3Br + HBr




12/13/2011
                    Reaction Coordinate            34
         Chlorination and Bromination of Methane


                 CH4 + Cl2          CH4 + Br2

                                      -28 kJ/mol
             E
                  -102 kJ/mol            CH3Br + HBr




                      CH3Cl + HCl



12/13/2011
                         Reaction Coordinate           35
               H of Chlorination of Alkane
                                                            1o             Ho
      CH3                                        CH3
               +   Cl2
                         h or heat   CH3CH2CHCH2Cl              +   HCl   -107
CH3CH2CHCH3

                                      1o              CH3

                                      ClCH2CH2CHCH3              +   HCl   -107

                                       2o        CH3
                                                                 +   HCl   -127
                                      CH3CHCHCH3

                                            Cl
                                                 CH3

                                       CH3CHCCH3                 +   HCl   -129

                                                 Cl
                                                        3o

  12/13/2011                                                                  36
                        Kinetics

                    A+BC+D
                     Rate  k[ A]a [B]b

                     k: rate constant
                a: order with respect to A.
                b: order with respect to B.
             a+b: overall order of the reaction.

12/13/2011                                         37
                             Rate Law

             CH3Br   +     OH-           CH3OH   +   Br

                         rate  k[CH3 ][OH ]




         (CH3)3CBr   +       OH-          (CH3)3OH   +   Br

                         rate  k[(CH3 ) 3 Br]


12/13/2011                                                    38
             Rate Law Determination

• Reactions that appear to be similar do not
  necessarily have the same rate law (or rate
  equation). That is, in general, rate law can
  not be predicted by the stoichiometry of the
  reaction.
• Rate law can only be determined
  experimentally.

12/13/2011                                   39
                           Example
        Given:
               CH3Cl + -CN  CH3C N + Cl-
        When [CH3Cl] is doubled, the observed rate doubled.
        When [-CN] tripled, the observed rate tripled.


        Questions:
        What is the order with respect to CH3Cl?
        What is the order with respect to -CN?
        What is the rate law of this reaction?

12/13/2011                                                    40
                               Example
                                                          H
        Given:   H         H
                                                     H H C
                     C C       +   H2                  C   H
                 H         H                         H H

  When [H2] is fixed and [ethylene] is doubled, no change in observed rate.
  When [ethylene] is fixed and [H2] tripled, no change in observed rate.


        Questions:
        What is the order with respect to ethylene?
        What is the order with respect to H2?
        What is the rate law of this reaction?

12/13/2011                                                              41
             Temperature Dependence of Rate


                    A+BC+D
                    Rate = k[A]a[B]b

                     k = exp(-G‡/RT)

                    G‡: activation energy


12/13/2011                                    42
                       Activation Energy

             Page 175
             ―…many organic reactions have activation energies
             in the range 40-150 kJ/mol … activation energies
             less than 80 kJ/mol take place at or below room
             temperature, … ‖




12/13/2011                                                       43
             Graphic Description of Reaction Profile
                               Single Step Reaction
                                                                   transition state
                                [ CH3----H----Cl ]‡


             E
                                          G‡


                                                H    •CH3 + HCl

                   CH4 + Cl•



                               Reaction Coordinate
12/13/2011                                                                     44
                     Multistep Reactions

   Propagation of Methane:

                                       Ho or Go     G‡
                                        kJ/mol      kJ/mol

         Cl + CH4  HCl + CH3            4         17


            CH3 + Cl2  CH3Cl + Cl     -109         4



12/13/2011                                                   45
             Graphic Description of Reaction Profile
                         Multistep Reaction




             E




                         Reaction Coordinate
12/13/2011                                             46
                       H of Chlorination of Alkane
                                                                    1o             Ho
      CH3                                                CH3
                   +    Cl2
                                 h or heat   CH3CH2CHCH2Cl              +   HCl   -107
CH3CH2CHCH3

                                              1o              CH3

                                              ClCH2CH2CHCH3              +   HCl   -107
       Reactants
 E                                             2o        CH3
                                                                         +   HCl   -127
                                 1o           CH3CHCHCH3
                            1o
                                                    Cl
                            2o                           CH3
                            3o                 CH3CHCCH3                 +   HCl   -129

                       RC                                Cl
                                                                3o

  12/13/2011                                                                          47
                               Chapter 5
• Types of reactions.
• Reaction mechanism
     – Radical chlorination - homolytic cleavage, initiation,
         propagation, termination, disadvantage   .
     – Polar reaction - , reaction enhances polarity, polarizability,
         heterolytic cleavage, nucleophile, electrophile.

• Reaction profile - equilibria, kinetics, thermodynamic changes.

12/13/2011                                                                48

								
To top