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After determining that the compound is present in the extract, you see that it is the most polar compound in the mixture. How, with respect to solvent system select...

Question

After determining that the compound is present in the extract, you see that it is the most polar compound in the mixture. How, with respect to solvent system selection, might you go about isolating the compound on an alumina column?

After determining that the compound is present in the extract, you see that it is the most polar compound in the mixture. How, with respect to solvent system selection, might you go about isolating the compound on an alumina column?


Answers

In flushing and cleaning columns used in liquid chromatography to remove adsorbed contaminants, a series of solvents is used. Hexane $\left(\mathrm{C}_{6} \mathrm{H}_{14}\right),$ chloroform $\left(\mathrm{CHCl}_{3}\right),$ methanol $\left(\mathrm{CH}_{3} \mathrm{OH}\right),$ and water are passed through the column in that order. Rationalize the order in terms of intermolecular forces and the mutual solubility (miscibility) of the solvents.

Chromatography is a technique that is used to separate two substances that comprise a mixture. It can also separate multiple substances, depending on the conditions of the separation. If you're separating two species or two substances, how would you know whether the separation is successful? Well, it depends on the type of chromatography you're using. If you're using thin layer chromatography, you might see a visual separation on a piece of paper or on some stationary phase. If you're separating using a column, then the data is usually represented as two peaks. Either way, the quality of the separation is expressed as a ratio of the average width of the spots or the width of the peaks and the distance, or the time between the spots or the peaks. This term slightly modified, is called resolution. The quality of the separation then is expressed in resolution units, which, as I mentioned, is a function of the ratio of the width of the, of the spots of the peaks, divided by the distance between the spots or the peaks.

This is the answer to Chapter 20 problem number three from the McMurray Organic Chemistry textbook. This problem is a very common organic laboratory problem. So if you're taking on organic chemistry course and it has a laboratory component, this is very often the first or second lab that's conducted in the semester. So this problem says, assume you have a mixture of Napoli in a man's OIC acid that you want to separate how much you take advantage of the acidity of one component in the mixture to affect a separation. Okay, so we have a mixture of these, uh, these two salads there, um, you know, both just white powders. So when they're mixed together, they're impossible to, you know, physically separate. Um, So what we're actually gonna do is what's known as an acid base extraction. So benzo gas. It obviously is an acid napfa lean. You know, when you look at it, it's totally aromatic. Doesn't really have any functionality beyond its era Metis ity on. And so we're going to have to do something that takes advantage of the acidity of Ben's OIC acid. And so basically the first thing that we would do would be to dissolve both of these solids into an organic solvent, so dissolve into Ah, I don't know. Let's say either it's or dissolve into e t 20 s o di ethyl ether. Um And then once these two solids are dissolved into the ether, what we're gonna do is actually add some acquis base. So it's a Equus. Ah, sodium hydroxide. So a nice strong base, Um, and so that's going to deep protein. Ate the benzo gas it. So after the addition of a quiz sodium hydroxide, what we will have will be, uh, this So the Napa lean is gonna be unaffected, so it's going to look exactly the same as it did. Um, but the Benz OIC acid is actually going to be in the form of the benzoate ion. So it well look like this. So it's been deep protein ated, um, and it will be associated with the positive sodium ion. So have a negative charge here on this oxygen in a positive charge here on sodium ion. Ah, and then we also of course, I made some water when the hydroxide ion t pro donated the benzo kassid. Okay, um and so now we're gonna have organic solvent and water. Um, in our vessel. Whatever. We're doing this reaction in hopefully a separate Torrey funnel. Um and so over here, this is going to be what's going on in the, uh, acquiesced layer and the Napa lean eyes Still going to be in the organic layer. So the dia feel either that we used ah, in the beginning. Okay, um and so now we have these in the two separate layers, and I'm so basically, what we would want to do would be to separate layers. Okay. Ah. And so when we separate the layers, um, if we take the quiz layer that had our, um, benzoate ion in it. So here's our benzoate ion again complex with the sodium counter ion. And so now we will actually treat this with some acid. So let's a hcl uh, hopefully dilute hcl we find. And if we use enough of it, that's going to reassert If I, um our benj elite ion back to Ben's OIC acid. Okay, so we have Arbenz OIC acid back. Ah, and then this will actually, since it's not soluble in water, this will crash out. Ah, as a solid and so weaken do some kind of vacuum drying or air drying overnight. And we should be left with solid, relatively pure Ben's OIC acid. Okay? And so that's how we would take advantage of the acidity of Ben's OIC acid in this mixture in order to affect a separation from Napa lean. And that's the answer to Chapter 20 Problem number three.

Let's look at the inter molecular forces that can be present here. Uh, vaccine exhibits. Um, London forces, sometimes called Vander Wal's forces. The chloroform exhibits dappled I poll and London forces. Ah, methanol exhibits, uh, hydrogen bonding in water exhibits, hydrogen bonding.

Okay, so this question actually has your understanding on the structure relations between the molecular interactions and the scalability. So first let's look at uh molecular interactions. The six molecules. The four molecules is given which is a hack scene chloroform and the mass. No and the water. So the first thing we need to determine is what kind of interactions those molecules are, what is dominant, What kind of force is dominant Among the molecules. So for six age 14 that have seen it's a non power molecule. So the dominant interaction should be simply the vent Awards London force which is the weakest force. And for the chloroform it's non polar molecules Pollard but it does not have any other things. Right? So it's also a Band Awards force. However, it's time it's a polar polar interactions And for the mass. No, you say it has O. H. Group and the Rh group can form the hydrogen bond. So here the hydrogen bond is a dominating force. And same for the water. So from here we can already kind um just other those other make another for the interactions because now we already know the same six age 14 has weakest interaction because he used non popular London force man words interaction and smaller than the kurban form. Use a polo pony direction. And the polar polar banned words in question is actually weaker than the hydrogen bombs. But how can we compare the water and uh medicinal, we look at the water. We find each water molecule actually has 20. H. Group, Which means one water molecule can For more than one hydrogen bombs. But for the mass. No there's only one or each group. So it can only form one hydrogen bonds of one barbecue. That means the concentration of the hydrogen bond inside the water is much larger than the mass. No, so definitely the interactions in the water higher or stronger. So then it follows on the mass. No and follows the water. So now we can work on the scalability for scalability. We have to follow. The one principle is similar structure and the property leads to a good mutual scalability. What means similar structure or similar property? For example if they have similar functional groups then they are probably pretty in terms of mutual stability. The two molecules should be pretty good if they are very far away from each other in terms structures or some like uh functional groups there. The scalability would be much um will be actually much against a smaller. So now let's say first let's look at the vaccine dissolved in the stage three. Uh Sorry I'm just right down dissolve in the Sage Sale three. Stage 3 or age and water. So which one which is strongest. Which one has a strongest interest or ability with the vaccine. We look at which one has the most similar structure and you can see the water has the O. H. Group messaging has approach group they don't have. And also yeah the O. H. Group is a hydro filic, we know the hydra. It's a hydro filic group function group but they don't have. So we can find that definitely the Sage cl three should be strongest in terms of mutual vulnerability. Will say six age 14. And then let's compare the water and the mass. No you can see for the vaccine and the mass. No they all have the carbon groups. Has a math in groups. So definitely they have similar structure right? They have similar functional group rather than water. So the water is the weakest Now for the stage Cl three ways say 6 14. What? So now the same thing. Both sage Cl three and say six age 14 hydrophobic. So they are most soluble with each other and followed by the math. No because they both have the same age groups. They have similar structure and then follows by three. And now we can go to my personal. Okay so which one has the strongest solidarity? You see the mass. No, can form hydro bond with water. So there's a strong directions. So it's one and it follows Which one with this too. Which one has a similar structure with massive definitely is a chroma fall. So this is too. And that is three now in terms of water right? And same which was the strongest in terms mutual scalability its mass. No because they all have oh age groups. And then We can consider both water and save cl three is pollard. So this is kind of poverty that the shell so maybe a little bit solvable and then followed by the vaccine most insolvable. And this is the answer.


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