An Organic Lab Question to Pharmacist Student

[WorcesterPHOBoy]

You guys are doing organic lab right now and you might know to answer this: Why would it be be diffcult to visualize the separation of cis- and trans-2-butene by thin-layer chromatography?

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[Trancelucent1]

Sorry can't help you. I don't know any pharmacy school that has organic lab in their curriculum.

 

[FungManX]

I'm not in pharm yet.. but i'm doing organic chem right now.
And I can tell you that the answer is..
You can't tell because whether its cis and trans are pretty much the same molecule (Same compounds, just different stereochemistry... or arrangments of molecules in space if you like.)

T.L.C can find different molecules only,stereoisomers can't be found..
just like how you won't be able to see (R) and (S) configurations in TLC.. the only way to observe (R)/(S) configurations is to analyze the way that it bends light

 

[bananaface]

This is a question those in pre-pharmacy are best suited to answer as they are taking or have taken the course more recently. 🙂 Moving...

 

[QofQuimica]

You can't tell because whether its cis and trans are pretty much the same molecule (Same compounds, just different stereochemistry... or arrangments of molecules in space if you like.)

T.L.C can find different molecules only,stereoisomers can't be found..
just like how you won't be able to see (R) and (S) configurations in TLC.. the only way to observe (R)/(S) configurations is to analyze the way that it bends light

Your explanation is actually not exactly right, but you are on the right track. It is true that *enantiomers* cannot be resolved by TLC, but *diastereomers* can be. Cis/trans isomerism is a type of diastereomerism, so in theory those two compounds actually *should* be separable by TLC. As to why they are not in practice, my guess would be that if you are using normal phase TLC (where you have a polar stationary phase and a nonpolar solvent), these two highly nonpolar compounds would probably both have such a high Rf (travel to the top of the plate along with the solvent front) that you would not see the separation. However, if you were to use reverse-phase plates (where the stationary phase is nonpolar and the solvent is polar), you should be able to see them separate under the right conditions. And for sure you can definitely separate them by GC (gas chromatography).

 

[QofQuimica]

Your explanation is actually not exactly right, but you are on the right track. It is true that *enantiomers* cannot be resolved by TLC, but *diastereomers* can be. Cis/trans isomerism is a type of diastereomerism, so in theory those two compounds actually *should* be separable by TLC. As to why they are not in practice, my guess would be that if you are using normal phase TLC (where you have a polar stationary phase and a nonpolar solvent), these two highly nonpolar compounds would probably both have such a high Rf (travel to the top of the plate along with the solvent front) that you would not see the separation. However, if you were to use reverse-phase plates (where the stationary phase is nonpolar and the solvent is polar), you should be able to see them separate under the right conditions. And for sure you can definitely separate them by GC (gas chromatography).

Oh, and one other difficulty with using TLC to separate the 2-butene isomers is that they both have very low boiling points. So you'd have to be running your TLC in a cold room below 0 C. 😛

 

[WorcesterPHOBoy]

Thank you guys! you guys are so knowledgeable than I am!! thank you for those great answers!