Calculating retention factors for TLC

Remember that when you run a TLC plating lab you have twp phases, the stationary phase shown as this blue silica gel on the plate and a mobile phase. The mobile space is a solvent that's less polar than the solid stationary phase. Silica gel is very, very polar. Let's say that you had a plate that looked something like this. You had initially spotted two compounds. We'll call them A and compound B. And then what you saw on the plate was that your mobile phase had traveled up to about here, A had traveled to about here, and B had traveled this far. But what does that really mean? How can we even report these values? The way we'd report them if we were writing up a lab report or writing a manuscript, you'd need something known as the retardation factor, also known as the retention factor or RF for short. RF is equal to the distance traveled by solute over the distance traveled by the solvent. So the first step you need to do is measure these distances for the different compounds and also for the solvent, also known as the mobile phase. So let's put a ruler next to our TLC plate, much like you would if you were sitting in lab. We'll say that this is 1 unit, 2 units, 3 units, and 4 units. So we can measure the distance that A has traveled, and that's from the starting line to the center of the spot. That's two units. And for compound B, again from the starting line to the center of the spot, that's 3 units. And for the solvent, the starting line to this finish line, that is 4 units. So let's plug that into our equation. If we wanted to solve RF of A, you need the distance traveled by compound A over the distance traveled by the solvent, so let's say A over S. Here, that would be equal to 2 over 4, and the convention is to report these values as decimal points, so we'll say that this is 0.5. Now, we'll do the same for compound B. RF of B is equal to distance traveled by B over distance traveled by S. In this case, that's equal to 3 over 4, or 0.75. So what can we tell about these two compounds? If we remember from talking about the mobile phase and stationary phase, compounds that travel really far must be more attracted to the mobile phase, and therefore are less polar. So we can say that compound B is less polar and travels faster. The opposite is true for compound A. Since this doesn't move as much, it's more attracted to the polar silica gel, and hence it's more polar than compound B and travels slower. Think about it like it's getting stuck in the stationary phase and doesn't really want to move away from it. So there we've done our first example. Let's do another one. In this example, we can see that our initial reaction mixture separated into four different compounds. Let's label these as A through D, with A being the orange spot, B as the yellow one, C as the green one, and D as the purple one. Again, we'll use the same process that we used earlier. So the first step is to take a ruler and put it next your TLC plate. This is 1 unit, 2 units, 3, 4, 5, and 6. So let's calculate the RF of A. This is equal to the distance traveled by A over the distance traveled by the solvent, so we need to measure these. First, we can see that A has traveled 1 unit, equal to 1, and the solvent has traveled about 6 units. So we'll say that's 1 over 6 then. Let's convert that to decimals and you have 0.17. We can do the same for each these compounds. Next, we'll take B. This is again equal to B over S, which equals this distance is about 3 units. So we have 3 over 6, which is equal to 0.50. Next, we'll measure this for C. The RF of C is equal to the distance traveled by C over the distance traveled by S, which equals-- distance traveled by C is 4-- so that's going to be 4 over 6, which is equal to 0.66. And lastly for D, again we'll have to measure the distance traveled by D over distance traveled by S. In this case, this distance is 5, so this would be 5 over 6, which is equal to 0.83. Now what can we say about these overall trends? Again, we said that compounds that travel really, really far are pretty nonpolar, and compounds that don't travel very far at all are more attracted to the stationary phase and hence are more polar. So if we look at these RFs, we can show that there really is a trend here. Compounds with a smaller RF are more polar, since they're more attracted to the stationary phase. And compounds with a bigger RF are less polar, since they're more attracted to the mobile phase. Let's review quickly what we've learned today. We learned how to calculate the RF value, also known as the retention factor or retardation factor, and how you would report that when presenting in a lab report or in the literature. We showed that compounds with big RFs are less polar, and compounds with pretty small RFs are more polar.