Chpt.8 Alkyl Halides

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Chpt.8 Alkyl Halides Radical Rxs
Structure Nomenclature Physical
Properties Halogenation of Alkanes Mechanism of
Halogenation Allylic Halogenation
Note the Chapter Summary and Key Rxs
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8.1 Structure of R-X
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8.2 Nomenclature
IUPAC - halides (X) are substituents Substituent
names halo fluoro, chloro, bromo, iodo
-haloalkane
-halocycloalkane
(R)-4-bromo-1-chloro-4-fluoro-1-cyclopentene stru
cture ?
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Common Names (alkyl halide) or (special names)
(isopropyl bromide) vs 2-bromopropane
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8.1 Structure of R-X
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8.3 Physical Properties
polar covalent bond - dipole - mismatch of
electronegativity -size
C
H
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8.4 Halogenation of Alkanes
Substitution of X for H
hv ultraviolet light, ? heat
X2 Cl2, Br2 seldom F2 (too reactive -
exothermic) or I2 (endothermic, unreactive)
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Substitution, products and by-products
other R-Xs
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Generally halogenation not useful -
mixtures (separate)
A few rxs are useful, e.g.
Others - allylic benzylic
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Substitution, products and by-products
monobromination
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initiation
propagation
terminations
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Regioselective for 3o gt 2o gt 1o C-H
(57) (43)
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radical stability like carbocation -
electron deficient
gt etc.
R. is electron deficient (not charged)
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Order of stability of R()
E
write condensed
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Order of stability of R()
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Selectivity 3o gt 2o gt 1o, but Cl and Br are
different
major mono-X product
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Hammonds Postulate
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Hammonds Postulate Cl vs Br

• Hammonds Postulate the structure of the
  transition state
• for an exothermic reaction looks more like the
  reactants of that step

- for an endothermic reaction looks more like the
products of that step
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Hammonds Postulate

• In halogenation of an alkane, the rate-limiting
  step is hydrogen abstraction
• this step is endothermic for bromination

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Hammonds Postulate
For chlorination (hydrogen abstraction is
exothermic)

• transition state resembles the alkane and
  chlorine atom
• little radical character on carbon in t.s.
• regioselectivity only slightly influenced by
  radical stability

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Hammond
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Halogenation (free radical substitution)
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Allyl Radical - resonance
H
H
H
H
H
C
C
H
C
C
C
C
H
H
H
H
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NBS for Br2
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NBS for Br2
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Free Radical Stability
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Free Radical Stability
H
H
C
H
gt
gt
gt
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Radical Addition of HBr to Alkenes

• Addition of HBr to alkenes gives either
  Markovnikov addition or non-Markovnikov addition
  depending on reaction conditions
• Markovnikov addition occurs when radicals are
  absent
• non-Markovnikov addition occurs when peroxides or
  other sources of radicals are present

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Radical Addition of HBr to Alkenes

• addition of HCl and HI gives only Markovnikov
  products
• to account for the the non-Markovnikov addition
  of HBr in the presence of peroxides, chemists
  proposed a radical chain mechanism
• Chain initiation

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Radical Addition of HBr to Alkenes

• Chain propagation

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Radical Addition of HBr to Alkenes

• Chain termination
• This pair of addition reactions illustrates how
  the products of a reaction can often be changed
  by a change in experimental conditions
• polar addition of HBr is regioselective, with Br
  adding to the more substituted carbon
• radical addition of HBr is also regioselective,
  with Br adding to the less substituted carbon