Column Chromatography

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. You place an extract of green leaves on top of a column packed with a solid, and let a solvent flow down. After a while, a yellow band races ahead down the column, while the green bands move slowly behind it. Same solvent, same column. So why does the yellow band win the race? Hold on to your answer; by the end, you will answer it in one line. Column chromatography is a standard part of JEE questions on purification. So, let's begin.

0:33Column chromatography: the set-up

So, look at the figure. We take a long column and pack it with an adsorbent, a solid such as silica gel or alumina, labelled on the left. It stays fixed, and holds compounds on its surface, some strongly, some weakly. Okay? Now we place the mixture on top of the column, and let a solvent flow down through it. This flowing solvent is called the eluent. In the figure, it is at the very top, labelled eluent, the mobile phase. With me? Now let me ask you this. Do all the components move down at the same speed? No, they don't, and that is the whole trick! Each component is held by the adsorbent to a different extent, so each one moves at its own speed. The mixture spreads out into separate bands, like the three coloured stripes you see in the column. Clear? Finally, the bands reach the bottom and drip out, one after another, into the beaker below. We collect them in turn, as separate fractions, and this is called elution, as the figure says at the bottom. Got it?

1:35Who comes out first?

Okay, now the big question. Silica gel and alumina are polar solids. So, which component will reach the bottom first: a polar one, or a non-polar one? Yes, exactly, it is the least polar one! The polar adsorbent hardly holds it. It is weakly adsorbed, so it travels with the eluent and comes out first. In the figure, that is the blue band, lowest down, labelled least polar, moves fastest. Right? Now look at the most polar component. It is held most strongly by the adsorbent, so it moves the least, and it comes out last. That is the orange band near the top, labelled most polar, held most strongly. And the green band in the middle is intermediate, in between the two. Clear? So here is a neat way to read any column at any moment. The band lowest in the column is the least polar one, the one held the least. With me? And it is the band that spends the least time in the column, because it comes out first. Keep this picture in your head; we will use it in our examples. Okay?

2:44Order of elution on silica or alumina

Alright, now the course gives us the typical order on silica or alumina. Read each less-than sign on screen as less polar than. So alkanes come first, then alkenes, then aromatic hydrocarbons, then halides, then ethers, then esters. Right? The list goes on: ketones and aldehydes together, then amines, then alcohols, then phenols, and finally carboxylic acids, the most polar of all. Okay? So let me ask you this. If a mixture has an ester and an alcohol, which one comes out of the column first? Yes, it is the ester that comes out first! It sits to the left of the alcohol, so it is less polar. Look at the last line: the further right you go, the more polar the compound, the more strongly it is held, and the later it comes out. Got it?

3:35Topper's trick: the link with TLC

Now let me share a trick that toppers use. In thin-layer chromatography, TLC, which comes in the next lesson, each compound gets an R f value. It is written as a capital R with a small f below it. On the plate, a higher R f value means the spot moves further up. Okay? Here is the link. With the same adsorbent and the same solvent, the order of elution from a column is the same as the order of R f values on the plate. The equals sign here means the two orders match. With me? So tell me this. From the column, which comes out first: the compound with the highest R f value, or the one with the lowest? The highest R f value comes out first, correct! It is held the least, so it moves furthest on the plate and rushes through the column. So if you know the TLC spots, you already know the column order, and that's the whole trick!

4:31Stuck near the top? Make the eluent more polar

Now one practical problem. The less polar bands have all come out, but the most polar compound is still sitting near the top, held very strongly. We cannot change the adsorbent now. So, what can we change? Yes, exactly, we change the eluent! The course says the eluent is often made gradually more polar as the run goes on. Right? And here is why this works. The more polar eluent competes with the compound for the adsorbent, so the strongly held compounds come off and move down too.

5:06Example 1: leaf pigments, which band is less polar

Clear? Now, Example one, and this is our puzzle from the start. Chlorophyll extract from leaves separates on a column into a yellow band, the carotenes, that moves fast, and green bands that move slowly. Which pigments are less polar? Pause the video and try it first. Done? Okay, let's check. The yellow band moves fast. The double arrow on screen means so, or implies. Moving fast means the adsorbent holds it weakly, so it is less adsorbed. Got it? And by today's rule, the weakly adsorbed component is the less polar one. So less adsorbed means less polar. Okay? So the carotenes, the yellow pigments, are less polar than the green pigments, which move slowly because they are held more strongly. So the yellow band wins the race because it is held less. Puzzle solved, right?

6:00Example 2: three bands on silica, order of elution

Now Example two, a little harder. Three bands form on a silica column: small a at the top, small b in the middle, and small c at the bottom. Which band is the most polar, and in what order will they come out? Pause the video and try it first. Done? First, let me test you. Small a is the first band, at the top, so the order is small a, then b, then c, right? No, no, careful, that is exactly the trap in this question! The column empties from the bottom, not the top. All three bands started together at the top, and band small a has moved the least. So it is held most strongly by the adsorbent. Okay? On silica, held most strongly means most polar. So small a is the most polar band. With me? Now the order. The lowest band, small c, is the least polar, so it comes out first, then band small b, and last of all, band small a, the most polar one. Clear? And one follow-up from the course. In this same column, which band spends the longest time inside? It is band small a, very good! It is eluted last, so it stays in the column the longest. Right?

7:19Quick revision: polarity, elution order, Rf link, eluent

So, quick revision. The least polar component is weakly adsorbed, and comes out first. The most polar is held most strongly, and comes out last. The order is the same as the R f order on TLC, so the highest R f value comes out first. And if a band is stuck, make the eluent gradually more polar. Now go and try the three practice problems on this page. All the best!