An Undergraduate Organic Chemistry Laboratory Experiment: The Use of Thin Layer

Question

An Undergraduate Organic Chemistry Laboratory Experiment: The Use of Thin Layer Chromatography to Monitor the Hydrogenation of Unsaturated Esters

Another example of a reaction that cannot be run open to the atmosphere is the hydrogenation of alkenes when a balloon filled with hydrogen gas is the hydrogen source. It is quite common for organic chemists to use a hydrogen balloon as the source of hydrogen for small scale hydrogenations that occur at 1 atm. One of the questions that need to be addressed in this reaction is how does one use TLC to monitor a hydrogenation reaction?

Look at the apparatus that you have just assembled and state how you would use TLC to monitor the reaction. Remember, this is a closed system. Specifically, address how a sample for TLC will be taken.

Explanation / Answer

Withdraw a few drops of reaction mixture in to a microtest tube(by using the micropipette with long needle) and add 0.5 ml hexane. Fom this solvent portion. take a little to do the TLC.

Spot the starting material and crude reaction mixture and run the TLC using proper solvent system and observe under UV light. If starting material is totally consumed no UV active spot remain there in place of starting material spot . If starting material is there continue the reaction for some more time till it totally converts to product. A reaction that has gone to completion does not show a UV active spot or a yellow spot with the KMnO4 staining solution.

The sample of TLC can be taken by using an appropriate sized micropipette (30 µL) and 16G needle( from the arm closed with septum) .

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