Question

TLC is often used to monitor the progress of chemical reactions.

For example for the reaction A + B -----> C, how could you determine when the reaction is done? Explain thoroughly.

Answer 1

The reaction can be monitored easily using the TLC and can be understood from the image shown below:

The reactants can either be fully consumed or not completely consumed in the reaction as can be seen from the above image.

After spotting the reactants A, B and the reaction mixture or the product C in the respective positions in the baseline of the TLC plate, the TLC is run in eluents with increasing polarity (e.g. starting from pure hexane the TLC can be run with an increasing percentage of ethyl acetate or dichloromethane). Now each component of the reactants or products has each unique Rf value depending on the polarity of the eluent used. At a certain polarity, when the spots are significantly separated, from the Rf value one can check if any new spot has appeared or not in the C lane.

For this one has to run TLC from time to time. Now, two cases can happen-

(a) The A and B spot have completely disappeared, with the generation of a new spot C. Then the reaction is complete and should be quenched.

(b) The A and B spots are there along with another spot(s) in the C lane, and even after waiting for a long time or applying more drastic conditions, the spot intensities do not change i.e. the spots corresponding to A and B do not fade further. Then the reaction is finished and should be stopped.