Reactions of Alkenes

Introduction to Alkene Reactions

• Review of carbocations and their formation through alkene reactions.
• Double Bond as Nucleophile: Attacks hydrogen ion, forming carbocations.
  • Tertiary carbocation more stable than secondary due to hyperconjugation and inductive effects.

Mechanism of Alkene Reactions

• Electrophilic Addition: Double bond attracts hydrogen ion to pi orbitals.
  • Partial positive charge forms.
  • Hydrogen's s orbital interacts with pi orbital.
• Carbocation Formation: Decision on hydrogen and electron movement determines secondary vs. tertiary carbocations.
• Nucleophilic Attack: Nucleophile attacks sp2 hybridized carbocations.

Selectivity in Alkene Reactions

• Regioselectivity: Specific area of molecule undergoes chemical change.
• Facial Selectivity: Attack can occur on different faces of carbocation.
  • Non-selective if both faces can be attacked equally.

Key Alkene Reactions

1. Hydrohalogenation

• Mechanism: Alkene attacks HX, forming carbocations.
  • Tertiary carbocations form more frequently.
• Products: Attack results in stereochemistry based on different groups.
  • 50-50 mixture for carbons with different groups.

2. Hydration

• Similar mechanism to hydrohalogenation.
• Reaction with H⁺ and H₂O: Forms tertiary and secondary alcohols.
  • Addition of water attacks carbocation and forms intermediates.

3. Diatomic Halogenation

• Reaction with X₂ (e.g., Cl₂, Br₂), adds across double bond in a trans manner.
• Mechanism: Temporary dipole allows alkene to attack halogen.
  • Forms cyclic halonium ions, attacked by nucleophile opposite to halogen.

4. Halohydrin Formation

• Variant of diatomic halogenation with water or alcohol.
• Mechanism: Water or alcohol acts as nucleophile, forming halohydrins.
  • Requires hydrogen for proton transfer to stabilize.

5. Hydroboration-Oxidation

• Reagents: BH₃ with H₂O₂/OH⁻.
• Mechanism: Syn addition of boron and hydrogen across double bond.
  • Subsequent oxidation replaces boron with OH.
  • Opposite regioselectivity to hydration due to concerted process.

6. Hydrogenation

• Reduction of double bond using H₂ gas with Pd/Pt catalyst.
• Mechanism: Metal holds H₂; double bond attacks hydrogen, forming alkane.

Key Concepts

• Carbocation Stability: Determines regioselectivity and major products.
• Stereochemistry and Selectivity: Important in product distribution.
• Variations in Mechanisms: Subtle changes drastically affect outcomes.

Conclusion

• Practice and understanding of why reactions occur as they do is crucial.
• Revisit concepts and mechanisms to solidify understanding.