What about fluorination and iodination?
Alkanes undergo chlorination and bromination, but not iodination
Fluorination is too violent a reaction to be useful:
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Addition of Radicals to an Alkene
Peroxide is used to generate Br radical in trace quantities
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An alkyl peroxide is a radical initiator
The electrophile adds to the sp2 carbon that is bonded to
the greater number of hydrogens
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Radicals do not rearrange as readily as carbocations:
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Why is the peroxide effect observed only for the addition
of HBr to alkene?
Because both of the propagation steps in the HBr addition
reaction are exothermic.
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Stereochemistry of Radical Substitution
Reactions
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Stereochemistry of Radical Addition
Reactions
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Why are both enantiomers formed?
Consider the first propagation step:
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Radical Substitution of Benzylic and
Allylic Hydrogens
The more stable radicals form faster:
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The percentage of substitution at the benzylic or allylic
carbon is greater for bromination. Note the reactivity–
selectivity principle:
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Because of the presence of an alkene functional
group, N-bromosuccinimide is used as a
brominating agent for allylic compounds:
No Br2 addition
to the alkene
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Mechanism of NBS Bromination
Azobisisobutylnitrile
radical initiator
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Advantage: The low concentrations of Br 2 and HBr
present during NBS bromations result in no alkene
addition reactions:
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Designing a Multistep Synthesis
Example 1
Synthesis:
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Example 2
Racemic
Synthesis:
Racemic
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Example 3
Retosynthetic analysis:
Synthesis:
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