Posted by Sheryl on Sunday, October 15, 2006 at 5:04pm.
Question: A colorless unknown substance is spotted on a TLC plate and developed in the correct solvent. The spots do not appear when visualization with a UV lamp or iodine vapors is attempted. What could you do to visualize the spots if the compound is the following:
My answer: Note that most organic functional groups can be made visible if they are charred by sulfuric acid.
A. An alkyl halide
My answer: Alkyl halides can be visualized if a dilute solution of silver nitrate is sprayed on the plates.. When exposed to light, dark spots will form on the plate.
B. A ketone
My answer: Colorless ketones can be colored by by making derivatives before spotting them on the plate. A derivative example can be preparation of 2,4-dinitrophenylhydrazone from ketones. You can also spray this reagent on the plate after the ketones have separated. Red and yellow spots will form where the compounds are located.
Please check these. Two more to come.
Sheryl
You need to get more opionions than mine but here are mine for what they are worth.
Why do you mention charring with H2SO4? Won't many organic chemicals char with H2SO4? And if it doesn't char what has it told you? And does that statement answer either of the two questions posed?
For the first one, I don't know if your procedure will work or not. Most alkyl halides do NOT have enough chloride ion (available as the ion) to react with AgNO3 and give a AgCl ppt. I assume that is what you are suggesting and the AgCl then does its thing and breaks down to Ag and Cl2 and the Ag is black. The other thing I am unsure about: AgNO3 will turn black, anyway, if left in the light long enough. All it takes is something to reduce enough silver ion to silver metal and the black is there. For the second one, ok EXCEPT that the question is what you can do to visualize the spot, not what you can do to make it separate into different spots. Therefore, spraying 2,4 DNPH is the answer you want. I think making the derivative BEFORE is superfluous to the question although it may be a perfectly good procedure to use.
I am beginning to see spots before my eyes.
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