During your final practical exam you were asked to run a TLC. There were suppose
ID: 1073144 • Letter: D
Question
During your final practical exam you were asked to run a TLC. There were supposed to be three spots: 1 = 2, 4 dinitrophenylhydrazine 2 = crude product 3 = recrystallized product Let's assume the ketone you were assigned was 2-pentanone. Is there any specific reason why we didn't ask you to spot the ketone as well? Please explain. While examining several TLC plates Anna saw one TLC plate that looked like this: The crude and the final product spots were streaking. The student made the product solutions too concentrated. Anna took a point of for the technique. Anna is not a mean person, however she took an extra point off for a different reason. Can you explain why?Explanation / Answer
In the formation of 2,4-DNP derrivative with a ketone, here 2-pentanone, the ketone is not spotted on the tlc plate.
The ketone which is an aliphatic linear chain carbonyl compound does not show any UV absorbance and thus does not appear on the tlc plate when spotted. On the other hand, the product and the recrystallized product both have strong chromophore and absorbs strongly.
Thus the ketone is not spotteed on the plate. the ketone is also more polar and would not rise on the plate and stick at to the bottom when we run the system.
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