[Shikimic]

I've just run an experiment for my O-chem class and I'm unsure about some theory on TLC. I was given a mixture containing two unknown compounds and I had to find an appropriate solvent system that would separate the two well. I was told to use a less polar solvent system if I wanted a greater separation.

I'm aware that a more polar solvent system would bring about greater migration on the TLC plate (solvent better able to compete for the polar silica adsorbent sites) but I haven't been able to find anything on the effects of a less polar solvent (in regards to separating compounds better). So I'm wondering...would a less polar system make the polarity differences between the two compounds more pronounced? I'd really like some literature on this.

Thanks in advance.

[Dan]

The key is the relative difference in retention factors (R_F) of the two components. To achieve better separation you want one component to travel faster relative to the other.

Let's look at an example, a mixture of two compounds A (less polar) and B (more polar), and their R_Fs in three solvent systems of decreasing polarity, X, Y and Z. Admittedly I have invented these data to illustrate a point, but I think they're quite typical in my experience.

In solvent X, A has R_F 0.80 and B 0.50
In solvent Y, A has R_F 0.25 and B 0.10
In solvent Z, A has R_F 0.10 and B 0.02

When considering which solvent system will be most effective it is quite common for students to initially choose solvent X because the R_F difference is the largest (0.30 compared to 0.15 or 0.08). However, if you consider the the mobility of A relative to B (ie the ratio of R_Fs), you get a different story. In solvent X, the relative mobility of A (compared to B) is 1.6, in solvent Y is 2.5 and in solvent Z is 5. So, although the absolute mobility in solvent system Z is the least, the mobility of A relative to B is greatest in solvent system Z.

[Shikimic]

A perfect reply and it completely makes sense. Thanks for giving me a detailed explanation!