Chapter 7: Further Reactions of Haloalkanes

1
Chapter 7 Further Reactions of Haloalkanes

• E2 Elimination
• Competition between reaction types
• Lecture 6 Sections 7.7-7.9

2
E2 Bimolecular Elimination
3
E2 Deprotonation is First Step
4
Energy Diagram Similar to SN2
5
Reaction proceeds by Anti Elimination
6
SN2
SN1
E1
E2
7
Weakly Basic Nucleophiles Give Substitution
Weak Nucleophiles give some elimination product
8
Increased Substitution Favors Elimination
9
Secondary strongly favors elimination
10
Bulky Bases Favor Elimination
11
Substitution or Elimination?

• Factors to Consider
• How Basic is the Nucleophile?
• 2. Steric Hindrance at Reacting Carbon
• 3. Steric Hindrance at Nucleophile

12
Summary of Reactivity
13
Important Concepts

• Unimolecular Substitution in Polar Media
• Secondary haloalkanes slow
• Tertiary haloalkanes fast
• When the solvent is the nucleophile, the process
  is called solvolysis.
• Rate-Determining Step in Unimolecular
  Substitution
• Dissociation of the C-X bond to form a
  carbocation intermediate
• Added strong nucleophile changes the product, but
  not the reaction rate
• Carbocation Stabilization by Hyperconjugation
• Tertiary gt Secondary
• Primary and methyl unstable

14
Important Concepts

• Racemization Often occurs upon unimolecular
  substitution at a chiral carbon.
• Unimolecular Elimination Alkene formation
  accompanies substitution in secondary and
  tertiary system.
• Bimolecular Elimination May result from high
  concentrations of strong base. The elimination
  involves the anti conformational arrangement of
  the leaving group and the extracted hydrogen.
• Substitution Favored By unhindered substrates
  and small, less basic nucleophiles
• Elimination Favored By hindered substrates and
  bulky, more basic nucleophiles