1.The stockroom ran out of dichloromethane CH 2 Cl 2 , so you have to substitute
ID: 510589 • Letter: 1
Question
1.The stockroom ran out of dichloromethane CH2Cl2, so you have to substitute another solvent for the solvent extraction step. The only solvents available are ethanol (CH3CH2OH), chloroform (CHCl3) and diethyl ether ([CH3CH2]2O).
a. What properties does dichloromethane (CH2Cl2) possess that need to be considered?
b. Which solvent would be best? Explain based on your answer to (a) above. You may need to look up pertinent properties.
2. You run a TLC plate to check the purity of the isolated caffeine compare to a pure sample you have as reference.
a. If your Rf values for the caffeine sample and your isolated caffeine spots will be different from lane to lane, what could be one cause? (Think carefully: can you assume that you isolated the caffeine and not an impurity?)
b. Suppose you found contaminating substances after TLC (other spots in a lane in addition to the one for caffeine). How could you tell if they are more polar or less polar than caffeine?
Pls I need the asnwer asap!
Explanation / Answer
1a) Dichloromethane is good solvent for caffeine because it has solubility of 37 g/ 100 mL. Besides, it is immiscible with water and has low boiling point.
1b) Chloroform is the best solvent, among ethanol, chloroform and diethyl ether. We could decide based on their solubility parameters given below along with their miscibility with water and boiling point.
Based on the above information, we understand that diethylether and ethanol have poor solubility parameters when compared to chloroform. Hence, only chloroform can replace dichloromethane.
2a) Since, Rf values are specific, if the compounds don’t have the same Rf then they are different. Hence, the isolated compound must be impurity and not caffeine.
2b) In general, if the Rf values is lower than that of reference sample then it is polar when compared to reference sample and if the Rf values is higher than that of reference sample then it is non-polar when compared to reference sample.
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