TLC PLATE ANALYSIS HELP How does the TLC plate help monitor the reaction took pl
ID: 705067 • Letter: T
Question
TLC PLATE ANALYSIS HELP
How does the TLC plate help monitor the reaction took place and the the product was present?
Reduction of a Ketone: Preparation of Fluorenol from Fluorenone This semester we have focused on the reactions of carbonyl containing compounds. The reduction of the carbonyl group using sodium borohydride can be used to functionalize ketones and aldehydes to the corresponding alcohol. In this lab, students will prepare fluorenol from the reduction of fluorenone using sodium borohydride. The progress of the reaction is monitored by TLC. The products purity is analyzed by melting point determination and characterized using infrared spectroscopy. Reaction scheme and mechanism. The OH Peah NaBH4 CH3OH xO ou ??Explanation / Answer
The question can best be answered quantitatively. TLC analysis can be used to predict the progress of a reaction. In this reaction, Fluorenone is converted to Fluorenol by NaBH4 reduction. Initially, we have Fluorenone only (no Fluorenol and NaBH4, t = 0 where the 0 indicates the beginning of the experiment). Note that, we obtain (after developing), a single spot on the TLC plate at Rf = 0.78. This Rf corresponds to pure Fluorenone and indicates that no reaction has taken place.
NaBH4 is added to Fluorenone and the progress of the reaction (if any) is monitored every 15 seconds. Note that at 15 seconds, we have two spots, one at Rf = 0.81 and the second at Rf = 0.40. This indicates that a reaction has taken place and we have unreacted Fluorenone (corresponding to Rf = 0.81) and a product (this product is most definitely Fluorenol, since we assume no side reactions take place). The product appears at Rf = 0.40.
We carry out this practice of TLC measurement every 15 seconds for a total time of 75 seconds and note that we obtain two sets of spots, the first set corresponding to Rf around 0.80 and the second set corresponding to Rf around 0.50. The first set of spots correspond to the unreacted reactant (Fluorenone) and the second set corresponds to the product (Fluorenol).
Since the spot at Rf around 0.80 doesn’t disappear after 75 seconds, we can assume that all the Fluorenone is not converted to Fluorenol, i.e, the reaction doesn’t go to 100% completion.
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