Question

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).

Benzene -----> (HNO3/H2SO4) --> Nitrobenzene, Dinitrobenzene Trinitrobenzene

1. Assuming the reaction did not go to completion and starting material is present in

addition to the three product, which fraction would contain which compound? Explain

your answer. (4 pts)

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. (4 points)

O

OH

O O

HO O

ibuprofen aspirin

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. (3 points)

Answer 1

1) The stationary phase is silica and the mobile phase is a 75:25 mixture of hexane:ethyl acetate. Hexane is less polar than ethyl acetate and since we have hexane present in a greater proportion, the mobile phase is non-polar. We have the starting material (benzene) and a mixture of the three nitrobenzenes (mono, di and tri). As the number of nitro groups on the benzene ring increases, the polarity increases. This is because the nitro group is highly electron-withdrawing and draws the charge density from the benzene ring, leading to a somewhat polarized benzene nucleus. Therefore, trinitrobenzene is the most polar and the polarity order is trinitrobenzene> dinitrobenzene> nitrobenzene> benzene. Infact, nitrobenzene is not highly polar and is almost as much polar as benzene. Therefore, when 75:25 mixture of hexane:ethyl acetate is used, benzene and nitrobenzene will go into the hexane layer and dinitrobenzene, trinitrobenzene will go into the ethyl acetate layer.

2) Now the mobile phase is more polar (we increased the percentage of ethyl acetate to 50%); the mobile phase can now partition the nitrobenzene into the polar layer and therefore, only benzene remains in the hexane layer while all the three nitrobenzenes go into the ethyl acetate layer.