Lecture Notes: Alkene Addition Reactions

Overview

• Alkene addition reactions involve exchanging a pi bond for two sigma bonds.
• Will discuss 10-15 different reactions across alkenes.
• Key topics: reagents, regioselectivity, stereoselectivity, product prediction.

Anatomy of Alkene Addition

• Trading a pi bond for two sigma bonds (labeled as A and B).
• Reactions can add groups two different ways; prefer one over the other.
• The alkene acts as a nucleophile, usually attacking H⁺ or an electrophile.
• Electrophile typically ends up on the less substituted side.
• Formation of a carbocation intermediate, usually more stable secondary or tertiary.

Regioselectivity

• Markovnikov's Rule: Electrophile adds to less substituted carbon.
• Anti-Markovnikov: Rare, involves opposite regioselectivity.

Stereoselectivity

• Focuses on the relationship between the stereochemistry of added groups.
• Formation of new chiral centers is possible (sp3 hybridization).
• Syn addition: Groups add to the same side.
• Anti addition: Groups add to opposite sides.
• No stereoselectivity: All possible stereoisomers may form.

Example Reactions

Hydrohalogenation

• Reagents: HCl, HBr, HI.
• Markovnikov addition.
• Mechanism involves carbocation formation, potential rearrangements.
• No stereoselectivity unless chiral centers form.

Acid-Catalyzed Hydration

• Reagents: H₂SO₄ (dilute), H₂O.
• Adds H and OH across the alkene, Markovnikov.
• Carbocation intermediate.

Oxymercuration-Demercuration

• Reagents: Hg(OAc)₂, H₂O; NaBH₄.
• Markovnikov addition without rearrangement.

Hydroboration-Oxidation

• Reagents: BH₃, THF; H₂O₂, NaOH.
• Anti-Markovnikov, syn addition.

Halogenation

• Reagents: Cl₂ or Br₂.
• Anti addition.

Catalytic Hydrogenation

• Reagents: H₂ with a metal catalyst (Pd, Pt, Ni).
• Syn addition.

Halohydrin Formation

• Reagents: X₂ (Cl₂ or Br₂) and H₂O or an alcohol.
• Anti addition, Markovnikov orientation.

Epoxidation and Anti-Dihydroxylation

• Reagents for epoxidation: Peroxyacids (e.g., MCPBA).
• Acid-catalyzed ring opening leads to anti-dihydroxylation.

Syn-Dihydroxylation

• Reagents: OsO₄ or KMnO₄ (cold, dilute).
• Syn addition of OH groups.

Oxidative Cleavage

• Ozonolysis: O₃ followed by DMS or Zn (reducing) / H₂O₂ (oxidizing).
  • Cleaves double bonds, forms ketones and aldehydes (or carboxylic acids under oxidizing conditions).
• Permanganate Cleavage: KMnO₄ (hot, concentrated).
  • Similar to ozonolysis under oxidizing conditions.

Important Considerations

• Be cautious not to confuse 'anti-Markovnikov' (regioselectivity) with 'anti' (stereoselectivity).
• Pay attention to the number of chiral centers formed to determine stereochemistry and number of products.