Alkenes and Reactions Summary

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Alkenes

  • Cis vs. Trans Alkenes
    • Cis: Higher priority groups on the same side of the double bond.
    • Trans: Higher priority groups on opposite sides of the double bond.
    • Stability: Trans alkenes are more stable than cis alkenes.

Chapter 10 Reaction Schemes

  • Markovnikov's Rule:
    • Add to the most substituted sp² carbon (least number of hydrogens).
  • Anti-Markovnikov:
    • Add to the least substituted sp² carbon (most number of hydrogens).
  • Addition Types:
    • Syn-Addition: Add groups to the same face of the alkene (both wedged/dashed).
    • Anti-Addition: Add groups to opposite faces of the alkene (1 wedged; 1 dashed).
    • Note: Be cautious with stereochemistry.

Alkene Reactions Summary

  • Group 1: Rearrangements
  • Group 2: Anti-Markovnikov
  • Group 3: Anti-Additions
  • Group 4: Monsters
  • Group 5: Hydrogenations

Group 1: Rearrangements

  • Acids Introduced:
    • H+ from acids reduce alkenes forming carbocations (e.g. H-Br, H₂O+-H, H+).
  • Key Reactions:
    • Hydrogen Halides Addition:
    • Markovnikov: add halogen to the most substituted sp² carbon.
    • Carbocation rearrangement is possible.
    • Acid-Catalyzed Addition of Water:
    • Markovnikov: add OH to most substituted sp² carbon; carbocation rearrangement is possible.
    • Acid-Catalyzed Addition of Alcohol (ROH):
    • Markovnikov: adds ether to most substituted sp² carbon.

Group 2: Anti-Markovnikov

  • Radical Addition of HBr (Using Peroxides):
    • Peroxides (ROOR) direct addition of Br to the least substituted sp² carbon.
    • Only HBr is affected, not HCl or HI.
    • Mechanisms must use single-headed arrows.
    • Stability: 3° radical > 2° radical > 1° radical > methyl radical.
Hydroboration-Oxidation
  • Steps for Hydroboration:
    1. Use BH3 in THF.
    2. Follow with H2O2, NaOH, H₂O.
    • Anti-Markovnikov: add OH to least substituted sp² carbon.
    • Syn-Addition: add OH + hydrogen to the same face of the alkene.

Group 3: Anti-Additions

  • Addition of Halogens (e.g. Br₂):
    • Anti-addition: halogens are added to opposite faces of the alkene.
    • No carbocation rearrangement possible.
  • Halohydrin Formation:
    • Markovnikov addition of OH to the most substituted sp² carbon, with anti-addition of OH and halogen on opposite faces.

Group 4: Monsters (Oxymercuration-Reduction)

  • Reagents used:
    1. Hg(OAc)2, H₂O and THF.
    2. NaBH4, NaOH.
  • Markovnikov: add OH to most substituted sp² carbon.
  • No carbocation rearrangement possible.

Group 5: Hydrogenations

  • Hydrogenation of Alkenes:
    • Requires H2 + metal catalyst (Pd, Pt, Ni) to break H−H bond.
    • Syn-Addition: Hydrogens added to the same face of the alkene.
  • No rearrangement is possible under this process.
Summary of All Groups
  • Group 1:
    • HBr, H2O, H3O+: Markovnikov with rearrangement possible.
  • Group 2:
    • HBr with ROOR: Anti-Markovnikov without rearrangement.
  • Group 3:
    • Br2, CH2Cl2: Neither Markovnikov nor Anti-Markovnikov.
  • Group 4:
    • Oxymercuration-Reduction: Markovnikov without rearrangement.
  • Group 5:
    • Hydrogenation: Neither Markovnikov nor Anti-Markovnikov without rearrangement.