CHEM281 Carran Chapter 8 2023 def

Chapter Overview: Alkenes and Alkynes II - Addition Reactions

1. Introduction to Addition Reactions of Alkenes and Alkynes

• Addition reactions involve adding elements across the double or triple bonds of alkenes and alkynes.

• General Reaction Formula: C=C + E-Nu → C-C

  • E = Electrophile, Nu = Nucleophile

2. Understanding Additions to Alkenes

• Addition Reaction: Electrophile (E) and nucleophile (Nu) are added across a double bond.

  • Bonds broken: pi bond (∏-bond)

  • Bonds formed: two sigma bonds (σ-bonds)

• Double Bond Characteristics:

  • Pi bonds are formed by overlapping p orbitals, with electron clouds positioned above and below the double bond plane.

3. Electrophiles in Addition Reactions

• Electrophilic Nature:

  • Electrophiles are electron-seeking species.

  • Common electrophiles include:

    • H⁺

    • X⁺ (where X = Cl, Br, I)

    • Hg²⁺

  • Alkenes (C=C) and alkynes (C≡C) are susceptible to electrophilic attack.

4. Mechanism of Electrophilic Addition

• Mechanism of Electrophilic Addition:

  1. Electrophile attacks and forms a carbocation.

  2. A nucleophile attacks the carbocation to form a final product.

  • Charged Centers: Formation of positive charge on the adjacent carbon after breaking the π-bond.

5. Markovnikov’s Rule

• Markovnikov’s Rule Statement:

  • In the addition of HX to an alkene, the hydrogen atom will attach to the carbon with more hydrogen atoms already attached.

• Regioselectivity:

  • For unsymmetrical alkenes, regioisomers are formed based on stability of the resulting carbocations.

  • More stable carbocations (tertiary > secondary > primary) are preferred.

6. Addition of Hydrogen Halides

• Addition of HCl, HBr, or HI to alkenes follows Markovnikov's rule, typically yielding more substituted and stable alkyl halides.

7. Exception to Markovnikov’s Rule

• Radical Mechanism:

  • In some cases, especially with peroxides, the addition can proceed via an anti-Markovnikov mechanism.

8. Stereochemistry of Ionic Addition to Alkenes

• Formation of Chiral Centers:

  • Processes may result in racemic mixtures or specific stereochemistry, depending on whether the addition is syn or anti.

9. Hydroboration-Oxidation: Anti-Markovnikov Addition

• Reagents Involved:

  • BH3 followed by oxidation with H2O2 gives anti-Markovnikov addition of H and OH across the double bond.

10. Summary of Methods for Converting Alkenes to Alcohols

• Comparison of hydration methods (acid-catalyzed, oxymercuration-demercuration, hydroboration-oxidation) in terms of regioselectivity, stereochemistry, and occurrence of rearrangements.

11. Electrophilic Addition of Halogens

• Mechanism of halogen addition (Br2 or Cl2) results in vicinal dihalides.

• Anti addition is typical, leading to racemic mixtures of products.

12. Electrophilic Addition of Alkynes

• Alkynes undergo similar electrophilic additions to form alkenes or halides, and regioselectivity observations apply as with alkenes.

• Synthesis Considerations: The chapter concludes with guidance on synthesizing compounds through addition reactions, focusing on stereochemical considerations and retrosynthetic analysis.