Oxidation of Alcohols

Oxidation of Alcohols

General Overview

  • Alcohols can be oxidized to various products based on their structure:

    • Methyl Alcohol (Methanol):

      • Initially oxidizes to become an aldehyde.

      • Further oxidation leads to a carboxylic acid.

      • Complete oxidation results in CO2.

Types of Alcohols and Oxidation Products

  • Primary Alcohols:

    • Oxidize to aldehydes upon initial oxidation.

    • Can further oxidize to carboxylic acids under strong oxidizing agents.

    • Can fully oxidize to CO2 if conditions are strong (e.g., high heat and very strong oxidizing agents).

  • Secondary Alcohols:

    • Upon oxidation, they stop at the ketone level.

  • Tertiary Alcohols:

    • Generally resistant to oxidation (no reaction).

    • Lack alpha hydrogens, which are essential for oxidation reactions.

Key Reagents for Alcohol Oxidation

  • Pyridinium Chlorochromate (PCC):

    • Contains pyridine, HCl, and chromium oxide.

    • A mild oxidizing agent.

    • Converts primary alcohols to aldehydes, stops at that level.

  • Sodium Dichromate (Na2Cr2O7):

    • A strong oxidizing agent.

    • Oxidizes primary alcohols completely to carboxylic acids.

    • Converts methanol to CO2 under acidic conditions.

  • Sodium Hypochlorite with Acetic Acid:

    • Mild oxidizing agent.

    • Oxidizes primary alcohols to aldehydes.

  • TEMPO (2,2,6,6-Tetramethylpiperidinoxyl):

    • Works with sodium hypochlorite as a mild oxidizing agent.

  • Swern Oxidation:

    • Involves DMSO, oxalyl chloride, triethylamine, and dichloromethane.

    • Mildly oxidizes primary alcohols to aldehydes.

  • Jones Reagent:

    • Another strong oxidizing agent for oxidation to carboxylic acids.

Reaction Mechanisms

  • Starting with 2-Propanol and sodium hypochlorite in acetic acid:

    • The hypochlorite ion acts as a base, producing HOCl when reacting with acid.

    • Conversion into H2OCl+ forms a reactive intermediate.

  • The O-Cl bond:

    • Oxygen is more electronegative than chlorine, leading to a partial negative charge on oxygen and a partial positive on chlorine.

    • An SN2-like reaction occurs where oxygen attacks chlorine, resulting in the departure of water.

  • Elimination Reaction:

    • Requires the removal of an alpha proton (on the same carbon as the leaving group).

    • Acetate can act as a weak base to remove this proton.

    • Leads to the formation of a double bond and the generation of a ketone.

Special Cases: Phenol

  • Phenol typically does not undergo oxidation as tertiary alcohols due to lack of alpha hydrogens.

  • However, phenol can be oxidized to benzyl quinone because of the presence of the aromatic ring and free-flowing pi electrons.

  • Benzyl Quinone can be reduced back to hydroquinone (which has two hydroxyl groups).

Conclusion

  • Understand the differences between mild and strong oxidizing agents as they dictate the products formed during oxidation of alcohols.