Please help with questions 2-5 please and maybe bonus if you can. A chemist perf
ID: 494723 • Letter: P
Question
Please help with questions 2-5 please and maybe bonus if you can.
A chemist performed the reaction shown below. He then wanted to separate the mixture using column chromatography. In his protocol he used silica as the stationary phase and a mixture of hexanes/ethyl acetate (75:25).
2. What would be the consequence of changing the mobile phase to a 50/50 mixture of hexanes/ethyl acetate rather than 75:25? Would the order of elution change (the order in which the compounds come out of the column)? Explain your answer.(4 pts)
3. In our column chromatography of spinach most students obtain three or four bands. However, a TLC of the same spinach extract can yield up to 7 spots on the TLC. Explain why one can not isolate the same number of bands from the column as there are on the TLC? (6 pts)
4. What was the effect of changing the mobile phase’s polarity on the column chromatography in this week’s lab? Could you tell from the TLC the effect on the separation?(2 pts)
5. Suppose you wanted to separate a mixture of ibuprofen and aspirin using column chromatography using dichloromethane and ethyl acetate mixture as the mobile phase and silica as the stationary phase. Which compound would you expect to elute the column first? Explain why? How would changing the mobile phase composition to dichloromethane and acetic acid effect the separation of these two compounds in column chromatography? Explain your answer.
Ibuprofen
Bonus: Ibuprofen has a chiral center and is generally sold as a racemic mixture. Would you be able to use column chromatography as described above (silica and dichloromethane/ethyl acetate) to separate the two enantiomers? Yes or no? Explain your answer.
NO2 HNO3 NO2 NO2 1. Assuming the reaction did not go to completion and starting material is present inExplanation / Answer
Answer to Que-2: Basic principle for column chromatography is adsorption and desorption. More polar molecules adsorps on silica more strongly and les polar molecules adheres less strongly to silica which is a stationary phase. hence during the course of elution with mobile phase, more polar molecules travels slow compare to less polar molecules. hence by changing the mobile phase to 50/50 mixture of hexanes/ethyl acetate rather than 75:25, mobile phase will become more polar and seperation of the mixture may be hamperred.
Since compound is a mixture of Benzene, Nitrobenzene, 1,3-dinitro benzene and 1,3,5-trinitro benzene order of polarity is 1,3,5-trinitro benzene > 1,3-dinitro benzene > Nitrobenzene > Benzene. Thus benzene will be eluted first followed by Nitrobenzene, 1,3-dinitro benzene and finnaly 1,3,5-trinitro benzene. By changing mobile phase to 50/50 from 75:25 may end up with elution of mixture of compound.
Answer to Que-3:column chromatography of spinach
On TLC spinach extract can yield up to 7 spots but most students obtain able to isolate three or four bands column chromatography, this may be due to following factors which affect the seperation in column chromatography:
1. Lenght of stationary phase
2. Mobile phase (Polarity)
3. Flow of the column
4. Band width of silica adsorbed crude extract.
Due to this factors students might have isolted few spots as a mixture of 2 or more spots.
Answer to Que-4:
As we discussed above polar compounds travels fast in polar mobile phase and nonpolar compounds travels fast in nonpolar mobile phase. Thus change in the polarity of mobile phase will decide the seperation of mixture.
Answer to Que-5:
Separation of ibuprofen and aspirin using column chromatography using dichloromethane and ethyl acetate mixture as the mobile phase and silica as the stationary phase, ibuprofen being nonpolar compared to aspirin, will be eluted first.