Lecture Notes: Oxidation of Aldehydes using Tollen's Reagent

Introduction to Oxidation of Aldehydes

• Tollen's Reagent:
  • Created by Bernhard Tollens, a German chemist.
  • Used for oxidizing aldehydes.

Composition of Tollen's Reagent

• Components Needed:
  • Silver nitrate (source of silver ions)
  • Sodium hydroxide
  • Ammonia
• Formation: Leads to the formation of diamine silver cation.

Reaction Process

• Oxidation of Aldehyde:
  • Aldehyde is oxidized to a carboxylate anion.
  • Oxidation occurs when the carbon-to-hydrogen bond is replaced with a carbon-to-oxygen bond.
  • Assigning oxidation states:
    • Aldehyde carbon: +1
    • Carboxylate anion carbon: +3
    • Indicates oxidation (increase in oxidation state).
• Reduction of Silver Ions:
  • Silver cation (Ag+) reduced to solid silver (Ag) with an oxidation state of 0.
  • Results in the formation of a silver mirror, indicating the presence of an aldehyde.

Diagnostic Test

• Selectivity: Tollen's reagent selectively oxidizes aldehydes, not ketones.
• Result: Formation of a silver mirror distinguishes aldehydes from ketones.
• Post-reaction: Protonation of carboxylate anion converts it to carboxylic acid.

Additional Reactions

• Using Sodium Dichromate:
  • Oxidizes both aldehydes to carboxylic acids and secondary alcohols to ketones.
• Using Tollen's Reagent:
  • Selectively oxidizes only aldehydes to carboxylic acids, leaving alcohols untouched.

Tollen's Reagent in Experiments

• Example with Glucose:
  • Utilizes open-chain form of glucose which contains an aldehyde group.
  • Oxidizing the aldehyde produces a carboxylate anion and reduces silver.
  • Glucose is water-soluble, aiding in the reaction.

Practical Applications

• Silver Mirrors:
  • Can be created using a glass ornament for decoration.
  • A microscope slide can be used to produce flat silver mirrors.
  • Process perfected by Justus von Liebig in the 1830s.
  • Encouraged as a fun experiment in undergraduate organic chemistry labs.

Conclusion

• Tollen's reagent provides a visually appealing and selective method to oxidize aldehydes, resulting in the creation of silver mirrors and serving as a diagnostic test to distinguish between aldehydes and ketones.