Lecture Notes: Alcohol Oxidation and Halide Formation

Alcohol Oxidation

Types of Alcohols

• Primary Alcohol: Carbon with O is attached to one other carbon.
• Secondary Alcohol: Carbon with O is attached to two other carbons.
• Tertiary Alcohol: Carbon with O is attached to three other carbons; cannot be oxidized further.

Oxidation Pathways

• Primary Alcohols:
  • Oxidized to aldehydes.
  • Aldehydes can be further oxidized to carboxylic acids.
• Secondary Alcohols:
  • Oxidized to ketones; only one oxidation step possible.
• Tertiary Alcohols:
  • No oxidation occurs.

Oxidizing Reagents

• Weak Oxidizing Reagents:
  • Pyridinium chlorochromate (PCC)
  • Swern oxidation
• Strong Oxidizing Reagents:
  • Chromic acid
  • Potassium permanganate

Mechanism of Oxidation

• PCC Mechanism:
  • Alcohol attacks chromium, breaks pi bond, kicks out Cl.
  • Proton transfer stabilizes intermediate.
  • Oxygen takes on additional proton, forming aldehyde.
• Chromic Acid Mechanism:
  • Water attacks aldehyde to form a hydrate.
  • Hydrate attacks chromium, leading to carboxylic acid formation.

Key Points

• Weak oxidizers stop at aldehyde stage.
• Strong oxidizers proceed to carboxylic acids.
• Water presence is crucial in chromic acid reactions but detrimental in PCC.

Halide Formation

Alcohol to Halide Conversion

• Primary and Secondary Alcohols:
  • Use Thionyl chloride (SOCl2) or Phosphorus tribromide (PBr3).
  • Reactions proceed via SN2 mechanism.
• Tertiary Alcohols:
  • Use HCl or HBr.
  • Reactions proceed via SN1 mechanism.

Mechanisms

• Thionyl Chloride Mechanism:
  • Alcohol attacks sulfur, Cl leaves, forming SO2 gas.
  • Cl attacks from backside, completing inversion via SN2.
• Phosphorus Tribromide Mechanism:
  • Alcohol attacks phosphorus, Br leaves.
  • Br attacks resulting in SN2 substitution.
• Tertiary Alcohols with HCl/HBr:
  • Alcohol is protonated, water leaves.
  • Carbocation is formed, attacked by Cl or Br.

Summary

• Primary/Secondary: Use SOCl2 or PBr3 for SN2.
• Tertiary: Use strong acids like HCl/HBr for SN1.

Miscellaneous

• Attend office hours or discussion sections for further clarification.
• Mechanisms emphasize electron movement and formation of intermediates.
• Understanding reagent strength and reaction conditions is crucial for predicting product formation.