Lecture 14 Preparation of Alkyl halides

1
Lecture 14 - Preparation of Alkyl halides

• Chapter 4
• Acid/base concepts - where an equilibrium will
  lie
• Mechanistic concepts, reaction profiles
• Preparation of alkyl halides from alcohols and
  HX
• Mechanism of the reaction
• Alcohol structure has an effect on rate of
  reaction
• Cation stability - Hyperconjugation
• Potential energy diagram for SN1 mechanism
• Primary alcohols - the SN2 mechanism
• Free radical halogenation of alkanes

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Acid-base reactions - which side of an
equilibrium is favoured?
The equilibrium will lie to the side of the
weaker conjugate base
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Energy diagram for concerted proton
transfer Figure 4.6
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Factors in Acid/Base Strength

• Electronegativity
• Acidity increases across periodic table as the
  atom attached to
• H gets more electronegative (HFgtH2OgtH2NgtCH4).
• Ion Size
• Acidity of HX increases (HIgtHBrgtHClgtHF down the
  periodic table
• as conjugate base (X- anion) size increases
  (bases strength of
• anion decreases).
• Resonance Stabilization in Anion
• Delocalization of charge in anion (resonance)
  makes the anion
• more stable and thus the conjugate acid more
  acidic
• (CH3CO2HgtCH3CH2OH).

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Alkyl Halides from Alcohols and Hydrogen Halides
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Mechanism - bond-breaking and bond-making events
e.g.
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Relative Rates of Reaction for Different
Alcohols with HX
Related to the stability of the intermediate
carbocation
Hyper-conjugation in ethyl cation Figure 4.10
sp2 - planar
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Energy diagram for formation of tert-butyl
chloride from tert-butyl alcohol and hydrogen
chloride Figure 4.13
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Activation energies for carbocation
formation Figure 4.14
Hammond postulate - states (e.g. T.S. and
intermediate) similar in energy will be similar
in structure
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Substitution Reaction Mechanism for 1o Alcohols -
SN2
Transition state

• Alternative pathway for alcohols that cannot
  form a good carbocation
• Rate determining step is bimolecular (therefore
  SN2)
• Reaction profile is a smooth, continuous curve
  (concerted)

Other Reagents
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Free Radical Halogenation of Alkanes
R-H X2 R-X H-X
Types of bond cleavage
heterolytic
homolytic
hn or heat
CH4 Cl2 CH3Cl HCl