Cannizzaro Lab Lecture

Introduction

• Overview of the Cannizzaro reaction and lab procedures.
• Reaction involves transforming 4-chlorobenzaldehyde with potassium hydroxide.
• Focus on separating two products: 4-chlorobenzoic acid and 4-chlorobenzal alcohol.

Reaction Mechanism

• Hydroxide reacts with aldehyde forming a tetrahedral intermediate.
• The intermediate collapses allowing a hydride attack on another 4-chlorobenzaldehyde.
• Produces two products: deprotonated alcohol and acid.
• Note: Reaction yield is not efficient industrially (max 50% per product).

Separation Procedure

1. Initial Setup:
   • Begin with a conical vial containing reaction products (acid and alcohol).
   • Acid product deprotonated in aqueous solution (basic conditions).
   • Alcohol product precipitates out.
2. Creating a Bilayer:
   • Add methylene chloride to dissolve the alcohol product.
   • Methylene chloride (more dense than water) forms a bottom layer with the alcohol.
3. Extractions:
   • Separate alcohol and acid products.
   • Further methylene chloride extractions ensure complete separation.
   • Use sodium bicarbonate to remove any residual acid from the organic phase.
   • Dry the organic phase with sodium sulfate; evaporate solvent to obtain crude alcohol product.
4. Purification:
   • Recrystallize using LaGroin.
   • Characterize using melting point and IR spectroscopy.
5. Acid Product Recovery:
   • Add concentrated acid to protonate acid product; precipitates out.
   • Separate using vacuum filtration, recrystallize from ethanol.

Experimental Procedure

• Measure and mix reagents in a conical vial; reflux for an hour.
• Cool the solution; dilute and proceed with methylene chloride extractions.
• Troubleshoot dissolving challenges through stirring and additional extraction cycles.
• Prepare crude product for recrystallization in a Craig tube.
• Utilize hot and ice baths for optimal crystal formation.
• Centrifuge to separate and purify the alcohol product.
• Weigh and characterize the purified product.

Characterization Techniques

• Melting Point Determination
  • Determine purity using melting point range.
• IR Spectroscopy
  • Identify functional groups with absorption patterns.

NMR Analysis

• H-NMR Focus:
  • Confirm carboxylic acid via peaks.
  • Use D2O exchange for functional group identification.
  • Label protons according to resonance and shielding effects.

Practical Tips

• Consider electron withdrawing/donating groups in aromatic products.
• Utilize resonance structures for NMR peak analysis.
• Apply techniques learned to future aromatic compound analyses.

Conclusion

• This lab provides valuable experience in product separation and compound characterization.
• Skills developed include handling and purifying chemical compounds, utilizing spectroscopy techniques, and understanding product isolation in chemical reactions.