Alkene Addition Reactions

Overview

• Pi bond exchanged for two sigma bonds
• Focus on 10-15 different reactions
• Discuss reagents, added groups, regioselectivity, stereoselectivity
  • Regioselectivity: Preference for one regioisomer over another
  • Stereoselectivity: Relationship in the stereochemistry of the added groups
• Predicting products for alkene addition reactions

Anatomy of Alkene Addition

• Swapping a pi bond for two new sigma bonds
• Potential for two addition pathways
  • Groups A and B can add in different configurations
  • Reactions typically favor one configuration

Mechanism of Alkene Addition

• Nucleophile: Alkene attacks an H+ ion/electrophile
  • Often ends up on the less substituted side
  • More stable carbocation (secondary over primary)
• Nucleophile attack: Subsequent nucleophile attacks carbocation
  • Markovnikov Addition: H+/electrophile on less-substituted, nucleophile on more-substituted side
  • Anti-Markovnikov: Opposite of Markovnikov
  • Regioselectivity: Preference for Markovnikov in most reactions; Anti-Markovnikov is rare

Stereoselectivity

• Formation of potential chiral centers
  • 0 chiral centers: Single achiral product
  • 1 chiral center: Two enantiomers (R/S configuration)
  • 2 chiral centers: Up to four stereoisomers; syn and anti additions considered

Important Reactions

Hydrohalogenation

• Reagents: HCl, HBr, HI
• Product: H and halogen across alkene
• Mechanism: Alkene as nucleophile, attacks electrophile, carbocation formation
• Regioselectivity: Markovnikov, consider carbocation rearrangements
• Example: H add to less-substituted, Br to more-substituted

Acid Catalyzed Hydration

• Reagents: H2SO4 (dilute) or H3O+
• Product: Addition of water (H and OH)
• Regioselectivity: Markovnikov
• Mechanism: Similar to hydrohalogenation with proton transfer
• Stereochemistry: No chiral centers often

Oxymercuration-Demercuration

• Reagents: Hg(OAc)2, NaBH4
• Product: H and OH (hydration)
• No Rearrangements: No carbocations
• Mechanism: Forms mercurinium ion, nucleophile attacks more substituted site

Hydroboration-Oxidation

• Reagents: BH3, THF; H2O2, NaOH
• Product: Anti-Markovnikov addition of H and OH
• Stereochemistry: Syn addition

Halogenation

• Reagents: Cl2 or Br2
• Product: Halogen on each side of alkene
• Mechanism: Bromonium/chloronium ion intermediate
• Stereochemistry: Anti-addition

Halohydrin Formation

• Reagents: Halogen with water/alcohol
• Product: Halogen and OH/OR
• Mechanism: Similar to halogenation but with nucleophilic solvent
• Regioselectivity: Markovnikov

Epoxidation and Anti-Dihydroxylation

• Reagents: MCPBA (peroxy acid) for epoxidation
• Product: Epoxide formation, then dihydroxylation with H3O+
• Stereochemistry: Anti-addition

Syn-Dihydroxylation

• Reagents: OsO4 or KMnO4 (cold, dilute)
• Product: Syn addition of OH groups
• Mechanism: Formation of cyclic ester intermediates

Catalytic Hydrogenation

• Reagents: H2 with metal catalyst (Pd, Pt, Ni)
• Product: Reduction to alkane
• Stereochemistry: Syn addition

Oxidative Cleavage

Ozonolysis

• Reagents: Ozone (O3), followed by reducing (DMS, Zn) or oxidizing agents (H2O2)
• Product: Cleavage of alkene to form ketones or aldehydes/carboxylic acids
• Mechanism: Formation of ozonide, cleavage to carbonyl compounds

Permanganate Cleavage

• Reagents: KMnO4 (hot, concentrated)
• Product: Similar to oxidative ozonolysis

Examples

• Predicted products and stereochemistry for various reactions
• Discussion of chiral center formation and stereoisomers

Use these notes to study the mechanisms, predict products, and understand the concepts of regioselectivity and stereoselectivity in alkene addition reactions.