Alcohol Oxidation Mechanisms

Overview

This lecture focuses on the chemical mechanisms for the oxidation of alcohols using three oxidizing agents:

• Chromic Acid
• Pyridinium Chlorochromate (PCC)
• Potassium Permanganate (KMnO₄)

Chromic Acid Oxidation

• Components: Chromium with oxygen
• Preparation: From chromium trioxide in sulfuric acid and acetone, or sodium dichromate in sulfuric acid and water.
• Structure: Central chromium atom with two double-bonded oxygens and two hydroxy groups.
• Mechanism:
  • Acid dissociates in solution, forming a free acidic proton which can react with chromic acid.
  • Oxidation of alcohol (e.g., propanol) involves a two-step process:
    1. Formation of an aldehyde.
    2. Further oxidation to a carboxylic acid.
  • Protonation of chromic acid increases the positive charge on chromium, facilitating electron transfer and oxidation.
  • Critical steps include proton transfers and formation of a chromate ester, which resembles an ester structure.
  • Alpha hydrogen involvement is crucial for breaking away the oxygen from chromium.

Pyridinium Chlorochromate (PCC) Oxidation

• Components: Pyridine with chlorochromate
• Mechanism:
  • Similar initial attack on chromium by alcohol oxygen.
  • Formation of chromate ester intermediate.
  • Key differences: No sulfuric acid, weaker acidic environment allows for the presence of negative oxygen.
  • Stops at aldehyde stage due to lack of water (cannot hydrate the aldehyde for further oxidation).
  • Final product: Aldehyde for primary alcohols, ketone for secondary alcohols.

Potassium Permanganate (KMnO₄) Oxidation

• Components: Ionic dissociation into K⁺ and MnO₄⁻.
• Mechanism:
  • Initial attack by alcohol oxygen on manganese.
  • Cyclic electron flow facilitates oxidation.
  • Suitable for basic conditions due to the structure of permanganate.
  • Final product: Ketone for secondary alcohols, carboxylic acid for primary alcohols.

General Concepts

• Reactivity and Conditions: Differences in reactivity and conditions (acidic vs. basic) affect the oxidation outcomes.
• Proton Transfers: Key to stabilizing intermediates and driving oxidation forward.
• Final Products: Depend on the type of alcohol (primary vs. secondary) and the oxidizing agent used.

Conclusion

• The lecture covers the step-by-step oxidation mechanisms for alcohols using different reagents, highlighting the importance of understanding reaction conditions and intermediate stability.

Next Steps

• Future topics will include reduction mechanisms using hydrides, such as sodium borohydride and lithium aluminum hydride.
• Additional resources and practice materials are available at Leia's website.