Electrophilic addition reactions are key in understanding how alkenes react. These processes involve electrophiles and nucleophiles adding across carbon-carbon double bonds, forming new compounds. The mechanism typically follows a three-step process, with the formation of a carbocation intermediate being crucial.

Energy diagrams help visualize these reactions, showing the relative energies of reactants, products, and intermediates. Common electrophiles include hydrogen halides, water with acid catalysts, and halogens. The reaction mechanism and resulting stereochemistry are important factors in predicting and understanding the products formed.

Electrophilic Addition Reactions of Alkenes

Mechanism of electrophilic addition

• Involves addition of electrophile and nucleophile across carbon-carbon double bond
  • Electrophiles electron-poor species attracted to electron-rich alkenes (HX, H2O with acid catalyst, X2)
  • Nucleophiles electron-rich species donate electrons to form new bond (halide ions, water)
• General mechanism follows three steps
  1. Electrophile approaches and forms bond with one alkene carbon resulting in carbocation intermediate
     • More stable carbocation forms preferentially (tertiary > secondary > primary)
  2. Nucleophile attacks carbocation forming new bond and completing addition
  3. Proton transfer may occur to restore neutrality of molecule
• Regioselectivity determined by stability of carbocation intermediate
  • Markovnikov's rule more stable carbocation (usually more substituted) forms preferentially

Energy diagrams for addition reactions

• Show progress of reaction and relative energies of reactants, transition states, intermediates, and products
• Reactants and products represented as energy minima, transition states are energy maxima
• Carbocation intermediate represented as local energy minimum between two transition states
  • Stability of carbocation intermediate affects overall energy profile of reaction
• Rate-determining step formation of carbocation intermediate (Step 1)
  • Activation energy ($E_a$) difference in energy between reactants and first transition state
• Overall energy change of reaction ($ΔH$) difference in energy between reactants and products

Common electrophiles for alkenes

• Hydrogen halides (HX where X = F, Cl, Br, I)
  • H-X bond polarized hydrogen acting as electrophile halide as nucleophile
  • Reactivity HI > HBr > HCl > HF
• Water (H2O) in presence of acid catalyst
  • Acid protonates alkene creating carbocation intermediate
  • Water acts as nucleophile attacking carbocation forming alcohol
• Halogens (X2 where X = Cl, Br)
  • Halogen molecule acts as electrophile forming cyclic halonium ion intermediate
  • Nucleophile (usually halide ion) attacks halonium ion leading to vicinal dihalide product
• Other electrophiles
  • Sulfuric acid (H2SO4)
  • Mercury(II) salts (Hg(OAc)2)
  • Borane (BH3)

Reaction Mechanism and Stereochemistry

• Reaction mechanism describes step-by-step process of bond breaking and formation
• Transition state represents highest energy point along reaction coordinate
• Intermediate species (e.g., carbocations) form during the course of the reaction
• Stereochemistry of products influenced by reaction mechanism
  • Addition reactions often result in specific stereochemical outcomes