Markovnikov vs Anti-Markovnikov in Alkene Addition Reactions

Introduction

• Focus on alkene addition reactions.
• Discuss Markovnikov and Anti-Markovnikov principles.

Symmetrical Alkenes

• Example: Hydrohalogenation with HBr.
• Symmetrical alkenes produce the same product regardless of where bromine is added.
• Product example: 2-bromobutane.

Asymmetrical Alkenes

• Analysis of asymmetrical alkenes (e.g., between tertiary and secondary carbons).
• Possible products:
  • H on carbon 2, Br on carbon 3 = 2-bromo-3-methylbutane (minor).
  • Br on carbon 2, H on carbon 3 = 2-bromo-2-methylbutane (major).
• Markovnikov’s Rule: Halogen adds to the more substituted carbon.

Markovnikov’s Rule Explained

• Halogen adds to the more substituted carbon in asymmetric alkenes.
• Key takeaway: Substituent favors the more substituted carbon.
• Importance of understanding over memorization.

Shortcuts to Remembering Markovnikov’s Rule

1. Nucleophile adds to the more substituted carbon.
2. Hydrogen adds to the less substituted carbon.
3. "The rich get richer":
   • Tertiary carbons are hydrogen-poor, secondary carbons are hydrogen-rich.

Understanding Mechanisms

• Importance of carbocation stability in determining products.
• Stability order: Tertiary > Secondary > Primary.
• The more stable intermediate forms faster, leading to the major product.

Examples of Markovnikov’s Rule Application

• Example reaction with H2SO4 and propene: OH adds to secondary carbon due to stability.
• Carbocation rearrangement:
  • Secondary carbocation near a tertiary carbon leads to hydration on the tertiary carbon.

Anti-Markovnikov Reactions

• No carbocation intermediate; examples include:
  • Hydroboration: Alcohol ends up on the less substituted carbon because of bulkiness of boron.
  • Radical Halogenation: Radicals prefer more substituted carbons for stability.

Halohydrin Formation

• Reaction with Cl2 and H2O follows Markovnikov's rule without a carbocation.
• Chloronium bridge mechanism leads to attachments on more substituted carbon due to electron density distribution.

Conclusion

• Review and practice alkene reactions.
• Refer to resources for further understanding:
  • alkene reaction series
  • Practice quizzes available on website.

Additional Notes

• Emphasize understanding over memorization for better application in complex scenarios.