1. Consider the reaction shown below: W + X NaBH4 CH2Cl2 Y + Z After 15 minutes of reaction time, a sample of the reaction mixture was spotted on a TLC plate, eluted using 3:1 hexanes:ethyl acetate, and visualized as Plate-1. i. Perform an online search regarding the use of bromocresol green to visualize TLC plates. Which of the spot(s) from Plate-1 would be visualized and what would the spot(s) look like? ii. A second TLC plate is prepared, eluted and visualized (Plate-2). However, you forgot to record the identity of the mobile phase. The mobile phase could be either 5:1 hexanes:ethyl acetate, or 2:1 hexanes:ethyl acetate. Which of these options is likely to be the correct answer? Write a concise discussion. iii. You decide to isolate the least polar compound (W ? Z) using column chromatography. However, you may only one of the two mobile phases used before (Plate-1, Plate-2). Which of these two mobile phases would be more suitable for that objective and why? 2. This week, we monitored the compound elution from the column by observing colored bands moving through the stationary phase. However, most organic compounds are not colored. If the compounds you aimed to isolate were not colored, what technique could you use to indicate when your compounds are eluting from the column? Briefly discuss your answer.
Added by Katherine H.
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When 2-propanol was used as the developing solvent, two substances moved with the solvent front (Rf = 1) during TLC analysis on a silica gel plate. Can you conclude that they are identical? If not what additional experiments would you perform? The Rf value of compound A is 0.34 when the TLC plate is developed in hexane and 0.44 when the plate is developed in diethyl ether. Compound B has an Rf value of 0.42 in hexane and 0.60 in diethyl ether. Which solvent would be better for separating a mixture of compounds A and B? Explain. What would you expect to happen to your Rf values if you doubled the amount of acetic acid? Would you expect the solvent used to develop the TLC plates for the analgesics would work for beta-carotene? Why or why not? Predict what the TLC plate would look like. If you performed a TLC analysis of a reaction mixture using ethyl acetate and hexane in a 1:1 volume ratio (50% v/v) and you found the Rf of the mixture was zero (did not move from the origin), then what mobile phase would you use to run your next TLC analysis?
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Assigned Questions 1. What happens if the spots are made too large when preparing a TLC plate for development? 2. What are three uses of thin layer chromatography? 3. Why must the spots be above the level of the developing solvent in the developing chamber? 4. What would happen if the spotting line and positions were marked with a ball-point pen rather than a pencil? 5. A student spots an unknown sample on a TLC plate and develops it in ethanol. Only one spot appears, with an Rf of 0.97. Does this indicate that the unknown is a pure compound? What can be done to verify the purity of the sample? 6. Two students each develop a TLC plate and observe one spot with an Rf of 0.56, using the same brand of plate and the same solvent mixture. Were the two compounds necessarily the same? How could they prove if they were identical? 7. Which of the following compounds would have the higher Rf value, using a dichloromethane/ethyl acetate mixture? 8. Calculate the Rf of a spot that travels 3.75 cm, with a solvent front that travels 8.20 cm.
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