A chemist performed the reaction shown below. He then wanted to separate the mix

Question

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). H2SO4 NO2 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 hexanesethyl acetate rather than 7 Would the order of elution change (the order in which the compounds come out ofthe column)? lain 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)

Explanation / Answer

1. Column chromatographic separation

The starting material is most non-polar and would elute out with pure hexane. This is followed by nitrobenzene being the slightly more polar than the strating material. The dinitroproduct would elute out next and the last would be the most polar trinitroproduct.

2. If the eluent is changed to 50:50 mixture of hexane and ethylacetate, the elution order would be the same but the chances of getting mixture of compounds increases as the separation of compounds on the column would be minimal.

3. On the tlc separation, the spots separate in a much better way and the elution rate is much lower when compared to column chromatography wherein, the closer spots often elute together with almost no separation.

4. changing the polarity of the eluting solvent would result in lower retention time for the compounds on the column and thus lower separation effeciency.

5. Ibuprofen being the less polar component of the two would elute out first from the column. When the eluting solvent plarity is shifted to a more polar side by using DCM and acetic acid, aspirin would elute out first as the solvent polarity matches with the compound (lie-dissolves-like).

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