TLC Reaction Monitoring
Thin-layer chromatography, or TLC, is a quick way to check whether a reaction mixture still contains starting material, whether product is forming and whether obvious by-products are present.
What TLC answers
Section titled “What TLC answers”TLC is useful for three routine questions:
- Has the starting material disappeared?
- Has a new product spot appeared?
- Is the reaction mixture getting cleaner or more complex?
It does not prove full structure by itself. It is a monitoring tool, not final characterisation.
Basic workflow
Section titled “Basic workflow”- Spot the starting material reference.
- Spot the reaction mixture.
- Co-spot starting material and reaction mixture if needed.
- Develop the plate in an appropriate solvent system.
- Visualise spots under UV or with a stain.
- Compare spot positions and intensities.
Rf value
Section titled “Rf value”The retention factor is:
Rf = distance travelled by compound / distance travelled by solvent front
Rf values are only comparable when the same stationary phase, solvent system and plate conditions are used.
Choosing a solvent system
Section titled “Choosing a solvent system”The aim is to separate spots clearly.
If all spots stay near the baseline, the solvent system is too non-polar for the compounds.
If all spots run with the solvent front, the solvent system is too polar.
For many undergraduate organic reactions, a hexane/ethyl acetate mixture is a common starting point.
Reading a reaction plate
Section titled “Reading a reaction plate”| Observation | Likely meaning |
|---|---|
| Starting material spot remains strong | Reaction may be incomplete |
| Starting material spot disappears | Reaction may be complete |
| New spot appears | Product or by-product forming |
| Many new spots appear | Side reactions or decomposition possible |
| Streaking | Sample overloaded, too polar, or impure |
Common mistakes
Section titled “Common mistakes”- Forgetting to mark the solvent front before it evaporates.
- Spotting too much sample and causing streaking.
- Letting the solvent level sit above the baseline.
- Comparing Rf values from different solvent systems as if they are absolute.
- Assuming TLC alone proves product identity.
Documentation template
Section titled “Documentation template”Record:
- plate type
- solvent system and ratio
- lane labels
- visualisation method
- observed Rf values
- interpretation and next action
Related pages
Section titled “Related pages”Quick summary
Section titled “Quick summary”TLC is a fast reaction monitoring method.
Use it to decide whether to continue, quench, purify or change conditions, then confirm product identity with stronger characterisation data.