Overview
This lecture covers the reduction of 4-nitrobenzaldehyde and the use of thin layer chromatography (TLC) to monitor reaction progress.
TLC Plate Preparation
- Draw a start line 1 cm from the bottom and a finish line 2 cm from the top of the TLC plate using a pencil.
- Fill the TLC chamber with eluent solvent (eluate); the solvent level must be below the start line.
- Place a half-circle filter paper in the chamber to saturate the atmosphere with solvent vapor.
Sample Preparation and Spotting
- Mix 4-nitrobenzaldehyde with methanol and spot it on the TLC plate, labeling as SM (starting material).
- Blow air on the spot three times to dry.
- Add sodium borohydride to the reaction mixture and stir for three minutes.
Reaction Monitoring with TLC
- After three minutes, spot the reaction mixture on the TLC plate and label as CP1 (crude product 1).
- Stir for seven more minutes and spot again as CP2 (crude product 2).
- Ensure each spot is 0.5 cm apart on the plate.
Developing and Visualizing the TLC Plate
- Place the TLC plate in the chamber, making sure the solvent does not cover the starting line.
- Monitor the plate as the mobile phase rises until it reaches the finish line.
- Remove the plate, dry it, and use UV light to observe and circle the spots.
Interpretation of Results
- CP1 spot will show both starting material and product since the reaction is incomplete at three minutes.
- CP2 spot will show mainly the product, indicating reaction completion after ten minutes.
Key Terms & Definitions
- Thin Layer Chromatography (TLC) — A technique for monitoring reaction progress and analyzing compounds.
- Eluate/Eluent — The solvent used to develop the TLC plate.
- Starting Material (SM) — The original compound before reaction.
- Crude Product (CP) — Reaction mixture sampled at various times to check for product formation.
Action Items / Next Steps
- Practice preparing and running a TLC plate to monitor your own reaction progress.
- Review how to interpret TLC results under UV light.