Question

In a 25 mL round bottom flask and stir bar, dissolve approximately 2 mmol of indene in approximately 10 mL of 85:15 isopropanol/water. Using a capillary pipette, spot a tiny amount of the resulting solution onto several TLC plates as described above. Add 4 mmol of NBS and attach a distillation condenser on to the flask but don’t bother to hook up to water. Stir to get some into solution (it may not all dissolve at this point). Stop stirring. Add 1 drop of 10% HCl. You should soon begin to observe a brown “cloud” forming in the bottom of the flask. Stir again.-The color should go clear and the flask should get warm. Assay the reaction progress following the disappearance of the starting material using the TLC plates you had set aside earlier (develop with 10% ethyl acetate/hexane). To do so, get four TLC plates; label them as “5 min”, “10 min”, "15 min", and “20 min”. Spot samples of the starting stilbene solution on the left side and middle of each plate, but don’t develop the plates. Consequently, if the indene spot is smaller than the new spot(s) produced in the reaction, the reaction is going well. If the reaction is not yet complete, try gently heating for a few minutes. When the reaction appears mostly complete, cool the reaction mixture (if necessary) and dilute 10 mL of hexane. Transfer the mixture to a separatory funnel and wash with 5 mL of 10% sodium bisulfite and afterward with 5 mL of water. Dry the organic layer with a small amount of anhydrous Na₂SO₄. Filter and wash the residual solids with a few mL of ethyl acetate. Concentrate the filtrate on the rotary evaporator.  

Q: (1) In this experiment, you "followed the course of the reaction by TLC." (i) Think of another reasonable way to follow the course of a reaction, and (ii) explain how you might use it in this experiment.

(2) Having now purified a compound by column chromatography, what could you have done differently to improve the process?

Answer 1

Q: (1) This is a very good researcch based question. In most of the labs, the monitoring of any reactions is carried out by TLC, since they are cheap and readily available. But you want to follow another pathway to study the course of reaction. Another way to monitor reaction is a) using real time IR spectroscopic analysis. This study is gaining momentum recently in those cases where some changes in functional group are observed in reactant and product. In this experiment too, this technique can be applied. b) Real time UV-vis spectroscopic analysis. This technique is valid for those molecules where some change in functional group or electron density is observed. Any change in signal of UV-vis spectroscopy is directly related to synthesis of new product. Both these analysis can be done after fixed time duration to study the changes.

Q: (2) The most common form of purification is column chromatography but another method can be distillation if the two reactants are liquid in nature or even crystallization can be applied for separation.

The improvement of the above process cannot be predicted theoretically. You have to do some research practically in lab by following proecdures performed by other scientist.