Lecture Notes: Marconico's Rule and Anti-Marconico's Rule

Key Concepts

Pi Electrons

• Pi electrons are part of double bonds and are loosely bonded.
• Electrophiles like positive hydrogen ions can easily attack these electrons.

Symmetrical vs. Unsymmetrical Alkenes

• Symmetrical alkenes: Both sides of the double bond have the same groups.
• Unsymmetrical alkenes: Different groups on each side of the double bond.

Stability of Carbocations

• 1° (Primary) Carbocation: Positive carbon attached to one carbon.
• 2° (Secondary) Carbocation: Positive carbon attached to two carbons.
• 3° (Tertiary) Carbocation: Positive carbon attached to three carbons.
• Stability order: 3° > 2° > 1° due to positive inductive effect.

Hydrogen Halides

• Reactivity order: HI > HBr > HCl > HF.
• Hydrogen iodide is most reactive; Hydrogen fluoride is least reactive.

Marconico's Rule

• Applies to unsymmetrical alkenes.
• When adding HBr to an unsymmetrical alkene:
  • The negative bromine attaches to the carbon with fewer hydrogen atoms.
  • The positive hydrogen attaches to the carbon with more hydrogen atoms.

Mechanism (3 Steps)

1. Bond Breaking: HBr bond breaks, producing H⁺ and Br⁻.
2. Carbocation Formation:
   • Electrons shift, forming a carbocation at the more substituted carbon.
   • Stability of carbocations determines the major product.
3. Product Formation:
   • Br⁻ attacks the more stable carbocation.
   • Major product: 2-bromo propane.

Anti-Marconico's Rule (Peroxide Effect)

• Observed with HBr in the presence of peroxides.
• Reversal of Marconico's rule:
  • Br⁻ attaches to the carbon with more hydrogen atoms.
  • Produces 1-bromo propane as the major product.

Mechanism (4 Steps)

1. Formation of Free Radicals:
   • Benzoyl peroxide decomposes via homolytic cleavage, forming free radicals.
2. Interaction with HBr:
   • Free radicals interact with HBr, forming bromine free radicals.
3. Radical Attack:
   • Bromine radicals attack the more stable secondary free radical.
4. Final Product Formation:
   • Result: 1-bromo propane is formed as the major product.

Summary

• Marconico's Rule: Negative part of the halogen acid attaches to the less hydrogenated carbon.
• Anti-Marconico's Rule: Occurs only with HBr in the presence of peroxides, where the negative halogen attaches to the more hydrogenated carbon.

Remember these mechanisms and rules for understanding reactions involving alkenes with hydrogen halides. They are crucial for predicting and explaining product formation in organic chemistry.