Thin layer chromatography (TLC), is often used to monitor the progress of chemical reactions. Paper chromatography and silica TLC may be used.
First a solvent system which leads to a good separation of the starting materials and products for the reaction must be found, along with a technique for visualising the compounds on the TLC ‘plate’.
In the examples below, aspirin and salicylic acid are shown on silica TLC plates run in a solvent system consisting of a mixture of petroleum ether (boiling point range 60 to 80oC) and acetone.
Two mixtures of these solvents were compared; pet.ether : acetone 85:15 and pet.ether : acetone 7:3. The ratios indicate the relative volumes of each solvent in the mixture.
Both TLC plates were labelled at the top in pencil to indicate the samples spotted. S = salicylic acid and A = aspirin. The aspirin and salicylic acid were dissolved in a mixture of ethanol and dichloromethane 50:50 for ‘spotting’ on the plates, although the aspirin did not dissolve completely.
The spots were visualised under short wavelength u.v. light (around 254 nm). The TLC plates can be seen to ‘glow’ with a characteristic yellow/green colour due to a u.v. fluorescent indicator that is incorporated into the silica. Organic molecules such as salicylic acid and aspirin block the u.v. light from reaching the fluorescent indicator and appear as dark purple spots.
One must wear gloves and goggles which absorb the harmful u.v. light when carrying out such experiments.
A hydrolysis experiment using aspirin
Approximately 0.1g of aspirin was dissolved in 1cm3 ethanol in a small glass vial. Then 1cm3 of 2M sodium hydroxide was added to the vial and a sample spotted onto a TLC plate and labelled t=0 (for time = zero at the start of the experiment). The mixture was left to react overnight at room temperature and then acidified (to pH 1) using 6M HCl. Another sample of the reaction mixture was then spotted onto the TLC plate and labelled t=18h (for reaction time = 18 hours). Reference samples of aspirin and salicylic acid were also spotted. All samples were spotted using small glass capillary tubes drawn out to a fine point in a Bunsen burner flame.
A few points to note:
Samples t=18h and SA are overloaded in size, meaning too much of these two samples were spotted on the plate.
The sample at t=0 was underloaded and nothing much could be seen apart from the material remaining on the baseline.
The sample at t=18h does show a small spot with approximately the same Rf value as the large salicylic acid reference spot. But then again another small spot at this Rf value is also shown by the aspirin sample spot, although this is much weaker.
Does the TLC plate show evidence of a base hydrolysis reaction having occurred?
We think it does, but the evidence is not conclusive and more tests would need to be carried out. What the TLC plate does show is that no starting material was detected in the reaction mixture after 18 hours indicating that some reaction had occurred.