Reactions of Alkanes: Chapter 12

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					Reactions of Alkanes:
      Chapter 12




       Radicals




         Chapter 12     1
Contents of Chapter 12
   Reactivity Considerations
   Chlorination and Bromination of Alkanes
   Reactivity–Selectivity Principle
   Radical Substitution of Benzylic and Allylic
    Hydrogens
   Stereochemistry of Radical Substitution
   No Radical Reactions in Biological Systems

                      Chapter 12                   2
        Radicals
   Sources include:
       Hydrogen peroxide
       Alkyl peroxides
       Light causes homolysis of the weak O-O
        bond




                         Chapter 12              3
     Heterolysis & Homolysis
Heterolytic bond
cleavage or heterolysis

               H     Br                        H+      +   Br
Homolytic bond
cleavage or homolysis

               H     Br                        H    +      Br

Homolysis produces radicals, which are very reactive
species
                           Chapter 12                       4
    Radical Chain Reactions

   Initiation turns stable species into
    radicals by breaking a bond.




                       Chapter 12          5
Radical Chain Reactions
   Propagation causes products to form
    without resulting in net consumption of
    radicals. (production = consumption)




                   Chapter 12             6
    Radical Chain Reactions
   Termination results in net radical destruction.
   Generally occurs when reactants are used up.




                        Chapter 12                    7
Chlorination and Bromination
of Alkanes




            Chapter 12         8
Product Distribution




            Chapter 12   9
Chlorination Rates




            Chapter 12   10
Chlorination Product
Distribution




            Chapter 12   11
Bromination Rates




           Chapter 12   12
      Reactivity–Selectivity Principle
   The very reactive chlorine atom will have
    lower selectivity and attack pretty much
    any hydrogen available on an alkane
   The less reactive bromine atom will be
    more selective and tends to react
    preferentially with the easy targets, i.e.
    benzylic/allylic > 3° > 2° > 1°

                       Chapter 12                13
  Radicals
Stability of alkyl radicals is similar to
stability of carbocations




                      Chapter 12            14
     Crude Radical Stability Index




   Add 1 for each attached carbon.
   Add 3 for adjacent double bond or phenyl ring.
   Subtract 1 if C+ on double bond with or without R on
    other end.
   Profile similar to carbocations but resonance contributes
    more to radical stability than to C+ stability

                            Chapter 12                     15
      Addition of Radicals


          Markovnikov Addition



         Anti-Markovnikov Addition
Source of confusion regarding addition of HBr for
years; can’t do ionic product analysis with radicals!!!
                          Chapter 12                 16
  Radical Addition is Unique to
  Hydrogen Bromide




HBr and peroxide goes anti-Markovnikov!
                   Chapter 12             17
  Hydrogen Halide Addition in
  the presence of Peroxide
Hydrogen peroxide has same effect on hydrogen
bromide addition to an alkyne as to an alkene
(reversed regioselectivity).




                    Chapter 12              18
    Benzylic/Allylic Radicals
   Benzylic and allylic radicals are even more
    stable than tertiary alkyl radicals
   It should be easy for a halogen radical to
    abstract a benzylic or allylic hydrogen




                       Chapter 12                 19
 Benzylic/Allylic Radicals
Problem in making allylic radical is the
greater likelihood that HBr or Br2 will
add electrophilically to the double bond
rather than making the allylic radical.
          Br2                                Br
                               Br
                                    +
          HBr
                                        Br



                  Chapter 12                      20
    Benzylic/Allylic Radicals
   Problem solved with N-bromosuccinimide
    (NBS) is a good reagent for supplying low
    concentrations of bromine radical
   NBS doesn’t produce much Br2 or HBr so
    it only replaces benzylic/allylic H’s




                      Chapter 12                21
    Benzylic and Allylic Radicals
   Allylic or benzylic radical stabilized by resonance.
   Only H which leads to most stable set of resonance
    structures is removed (like C+).
   Unlike C+ case no thermo or kinetic product exists; Br
    found on all radical-bearing carbons




                          Chapter 12                    22
Stereochemistry of Radical
Substitution




 Radicals give both enantiomers when
 they make products like carbocations
                 Chapter 12             23

				
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