[Violagirl]

We had to do an experiment involving column chromatography with ferrocene and acetylferrocene. In doing the TLC test, we had to analyze the Rf values from three different solvent systems: 100% hexane, 25% EtOAc/Hex, ande 75% EtOAc/Hex. The Rf values that I obtained for the ferrocene were .3125 for 100% hexane, .0465 for 25% EtOAc/Hex, and .79 for 75% EtOAc/Hex. For the actylferrocene, I got .65 for 100% hexane, .55 for 25% EtOAc/Hex, and .61 for 75% EtOAc/Hex. For two of my fractions, both of which were hexane, I got 0 for all solvent systems for the Rf values. Do these values seem to make sense for anyone that has done this lab before? Also, does the melting point have an impact on high an Rf value will be along with the chemical structure? Finally, I have to determine the solvent system that gave me an "ideal" range of Rf values between .30-.60 and in this case, it looks to be the solvent system of 100% hexane right?

[Nosterius]

Are you sure about your Rf values?

I have doubts about your valur for acetylferrocene in 100% hexanes and ferrocene in 25%EtOAc/hexanes. I would have guessed that the more polar your eluant (more EtOAc), the higher the Rf you would get.

You need to have the highest difference in Rf between your two compounds, in the same solvent system to achieve a good separation. Also, consider that, the higher your Rf, the faster it will elute from your column. If it is too fast, you will wash out your compound(s) without good separation.

[Violagirl]

Actually you are correct, my partner and I had calculated the wrong values but she has the correct ones so I'll get them from her. Thanks for telling me!

Otherwise I have to draw a picture that best represents an ideal range of Rf values that should be between .30-.60 for the solvent system out of the three (100% hexane, 25% ethyl acetate/hexane, and 75% ethyl acetate/hexane) that I had mentioned and draw what the 4 fractions should look like; i.e. my starting mixture of hexane, my second fraction of ferrocene, my third fraction of hexane, and my fourth fraction of acetylferrocene. Since actylferrocene is more polar in ethyl acetate, it sounds like it will be farthur up the plate then ferrocene. However, is there a way to tell which solvent system is best without Rf values or are going to be needed to figure that out?

[Nosterius]

In TLC, your compounds interacts with both the silica gel and the eluant. The silica gel is polar; there are many Si-OH bonds lyings around, waiting to interact with your compound. Thus, to more polar your compound is, the more it will interact with the silica gel, and it will "stick" to it. As such, your more polar compound (acetylferrocene) should be lower on your TLCs than ferrocene itself.

To make it move up the TCL plate, you need to break those polar interactions between your compound and the silica gel. The way to achieve this is to use more polar eluants that will compete with the silica gel to interact with your compound.

You will need this:

1. A TLC which has your two compounds well separated.
2. A TLC of your column fractions containing only ferrocene (it should be the uppermost compound on your TLC plates)
3. A fraction that has nothing in it (meaning that ferrocene and acetylferrocene were well separated)
4. A TLC showing only acetylferrocene.

If I remember correctly (I did this experiment during my undergrads), ferrocene elute much more rapidly than acetylferrocene. It should be possible to elute it with a non-polar solvant such as 100% hexanes while acetylferrocene sticks to the silica gel (Rf = 0). Once all the ferrocene is collected, you can change the eluant for a more polar one, to flush acetylferrocene from the column. This is a slightly more advanced technique and you should ask your TA's advice before testing it.

Good look