Substitution Reactions

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Substitution Reactions

• The halogen atom on the alkyl halide is replaced
  with another group.
• Since the halogen is more electronegative than
  carbon, the C-X bond breaks heterolytically and
  X- leaves.
• The group replacing X- is a nucleophile. gt

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SN2 Mechanism

• Bimolecular nucleophilic substitution.
• Concerted reaction new bond forming and old bond
  breaking at same time.
• Rate is first order in each reactant.
• Walden inversion.
  gt

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Uses for SN2 Reactions

• Synthesis of other classes of compounds.
• Halogen exchange reaction.

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SN2 Nucleophilic Strength

• Stronger nucleophiles react faster.
• Strong bases are strong nucleophiles, but not all
  strong nucleophiles are basic.

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Trends in Nuc. Strength

• Of a conjugate acid-base pair, the base is
  stronger OH- gt H2O, NH2- gt NH3
• Decreases left to right on Periodic Table. More
  electronegative atoms less likely to form new
  bond OH- gt F-, NH3 gt H2O
• Increases down Periodic Table, as size and
  polarizability increase I- gt Br- gt Cl-
  
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Polarizability Effect
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Bulky Nucleophiles

• Sterically hindered for attack on carbon, so
  weaker nucleophiles.

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Solvent Effects (1)

• Polar protic solvents (O-H or N-H) reduce the
  strength of the nucleophile. Hydrogen bonds must
  be broken before nucleophile can attack the
  carbon.

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Solvent Effects (2)

• Polar aprotic solvents (no O-H or N-H) do not
  form hydrogen bonds with nucleophile
• Examples

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SN2 Reactivity of Substrate

• Carbon must be partially positive.
• Must have a good leaving group
• Carbon must not be sterically hindered.

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Leaving Group Ability

• Electron-withdrawing
• Stable once it has left (not a strong base)
• Polarizable to stabilize the transition state.

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Structure of Substrate

• Relative rates for SN2
  CH3X gt 1 gt 2 gtgt 3
• Tertiary halides do not react via the SN2
  mechanism, due to steric hindrance.
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