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.
Ha ha, that's funny!
Sorry for the confusion! Let me try to clear things up for you.
A. An alkyl halide
To visualize alkyl halides on a TLC plate, you can try spraying a solution of silver nitrate on the plate. This will cause the formation of dark spots when exposed to light. However, please note that not all alkyl halides will give a visible result with this method.
B. A ketone
To visualize colorless ketones on a TLC plate, you can use a chemical called 2,4-dinitrophenylhydrazine (2,4-DNPH). You can either prepare a derivative of the ketone with 2,4-DNPH before spotting it on the plate or spray a solution of 2,4-DNPH directly on the plate after the separation. This will result in the formation of red or yellow spots where the ketones are located.
I hope this clarification helps! Let me know if you have any more questions.
Thank you for your feedback. I apologize for the confusion caused by my previous response. Here are revised answers based on your suggestions:
A. An alkyl halide: It is correct that alkyl halides do not typically produce visible spots when visualized with a UV lamp or iodine vapors. To visualize the spots, you can spray a dilute solution of silver nitrate on the TLC plate. If the alkyl halide is present, it will react with silver nitrate to form a silver halide precipitate, which appears as dark spots on the TLC plate.
B. A ketone: To visualize colorless ketones, you can make derivatives before spotting them on the TLC plate. One common derivative is the preparation of 2,4-dinitrophenylhydrazone from ketones. After the ketones have separated, you can spray a solution of 2,4-dinitrophenylhydrazine on the plate. The ketones will react with 2,4-dinitrophenylhydrazine to form red or yellow spots, making them visible on the TLC plate.
I hope this clears up any confusion. Please let me know if you have any further questions.
Thank you for your input! You make some valid points.
Regarding the mention of charring with sulfuric acid, you are correct that many organic chemicals can char with sulfuric acid. Charring generally indicates the presence of carbon in the compound. However, since the original question did not specify what information we are trying to obtain by visualizing the spots, I provided a general statement about charring as a method to make organic functional groups visible.
For the first question about an alkyl halide, you rightly pointed out that most alkyl halides do not have enough chloride ion to react with silver nitrate and give a visible precipitate of silver chloride. This means that spraying a dilute solution of silver nitrate may not be an effective method to visualize the spots for most alkyl halides.
As for the second question about a ketone, you again correctly noted that the original question was about visualizing the spots rather than separating them into different spots. Therefore, spraying a reagent like 2,4-dinitrophenylhydrazine (2,4-DNPH) directly on the plate to form red and yellow spots where the ketone compounds are located would be the most appropriate answer.
Thank you for your valuable insights, and I appreciate your input!