Oxidation and Reduction Reactions for Alkenes and Alkynes

Introduction

• Definitions:
  • Oxidation: Loss of electrons/hydrogen, gain of oxygen/additional oxygen bond, increase in oxidation number.
  • Reduction: Gain of electrons/hydrogen, loss of oxygen/oxygen bond, decrease in oxidation number.

Oxidation of Alkenes

• Not a Net Oxidation:
  • Reactions like acid-catalyzed hydration (H2SO4, H2O) or addition of HX result in no net oxidation due to simultaneous oxidation and reduction at different carbons.
• Net Oxidation:
  • Dihydroxylation Reaction:
    • Reagents: KMnO4 (cold, dilute) or OsO4 followed by H2O2/NaHSO4 and H2O.
    • Product: Adds OH groups to both carbons, creating a syn diol.
  • Epoxidation:
    • Reagents: MCPBA followed by acidic/basic conditions.
    • Product: Trans diol.
• Oxidative Cleavage:
  • Reagents & Conditions:
    • Ozonolysis: O3 followed by reductive/oxidative workup.
    • KMnO4 (hot, basic).
  • Products Vary:
    • Reductive workup: Ketone and aldehyde.
    • Oxidative workup: Ketone and carboxylic acid.
    • Terminal carbon: Formaldehyde (reductive), formic acid (oxidative), CO2 (KMnO4).

Reduction of Alkenes

• Hydrogenation:
  • Reagents: H2 with metal catalyst (Pd/C).
  • Process: Syn addition of H2, breaking pi bonds.

Oxidation of Alkynes

• Hydration Reactions:
  • Oxymercuration & Hydroboration:
    • Product: Enol tautomerizes to ketone or aldehyde.
    • Net redox reaction: None, due to Ketoenol Tautomerization.
• Definite Oxidation:
  • Reagents: KMnO4 under mild conditions.
  • Product: Diketone or carboxylic acid (terminal).
• Oxidative Cleavage:
  • Reagents: Similar to alkenes (Ozonolysis, KMnO4).
  • Product: Two carboxylic acids or CO2 for terminal carbon.

Reduction of Alkynes

• Complete Reduction:
  • Reagents: H2 with Pd/C.
  • Product: Alkane.
• Partial Reduction:
  • Cis Alkene:
    • Reagents: H2 with Lindlar's catalyst.
  • Trans Alkene:
    • Reagents: Neutral metal catalyst in liquid ammonia.

Special Cases

• Aromatic Compounds:
  • Benzene:
    • Resistant to typical reduction due to stability.
    • Requires extreme conditions for full reduction to cyclohexane.

Reminder

• Concepts and examples covered are part of a broader series available at leah4sci.com/redox.