Chapter 5. Alkenes and Alkynes II: Reactions
Elimination Reactions
Addition Reactions
Addition of Halogens
Examples
Mechanism
Halohydrin Formation
Mechanism
Orientation of the OH toward the more stable cabocation (follows Markovnikov addition)
Hydrogenation Addition of Hydrogen
Addition of H-H across C=C Reduction in general is addition of H 2 or its equivalent Requires Pt, Pd, or Ni as powders on carbon and H 2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous Reduction of Alkenes
Mechanism
Addition of Water to Alkenes Acid-Catalyzed Hydration Oxymercuration-Demercuration Hydroboration-Oxydation
Acid-Catalyzed Hydration
Mechanism
Markovnikov Addition
Oxymercuration-Demercuration Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation via mercurinium ion
Mechanism
Hydroboration-Oxidation Herbert Brown invented hydroboration (received the 1979 Nobel Chemistry Prize.) Borane (BH3) is electron deficient is a Lewis acid Borane adds to an alkene to give an organoborane
Hydroboration-Oxidation
BH 3 Is a Lewis Acid Six electrons in outer shell Coordinates to oxygen electron pairs in ethers
Addition of H-BH 2 (from BH 3 -THF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene Hydroboration-Oxidation Forms an Alcohol from an Alkene
Orientation in Hydration via Hydroboration Regiochemistry is opposite to Markovnikov orientation (Anti-Markovnikov) –OH is added to carbon with most H’s H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)
Anti-Markovnikov Addition
Syn Addition
Mechanism of Hydroboration Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of BH 3 are across C=C