Hello everybody, my name is Iman. Welcome back to my YouTube channel. Today we've made it to the last chapter for MCAT Organic Chemistry. This chapter is all about separations and purifications. Round of applause.
Okay, let's get started. We're going to cover three main objectives here. That first objective is going to be discussing solubility-based methods.
We're going to talk about extraction and a couple of... other methods here. Then we'll move into objective two, which is all about distillation.
We'll talk about simple, vacuum, and fractional distillation. And then the last and final objective in this chapter is chromatography. Here we're going to talk about thin layer and paper chromatography, column chromatography, gas chromatography, and high performance liquid chromatography, or also known as HPLC. Now, all of these extraction methods, there is a lot...
to say. Just like for spectroscopy, in that video, we covered three spectroscopy techniques. We could spend hours talking about each one.
Same goes here for the separation and purification techniques. There is a lot to talk about. However, we are going to be focused on only the material that we need to know specifically for the MCAT.
So it might seem a little bit brief when we talk about certain things, but that's because we're not going to need to know too many grand details about some of the techniques we covered today for the MCAT. All right, with that being said, I want to set up the stage for this chapter. So far in our course leading up to this last and final chapter, we've discussed a lot of different ways on how to theoretically get various products from a range of reagents. However, if you've taken an organic chemistry lab to supplement your lecture, you've probably learned that chemistry isn't as straightforward in the real world as it is on paper. And much of the time you spend in lab is dedicated to isolating and purifying some desired product after the reaction has occurred.
And so in this chapter, we're focused on talking about several techniques. for this. All right, now we're going to start off with objective one, which is solubility-based methods.
And one of the simplest ways to separate out a desired product is through extraction. All right, extraction, the transfer of a dissolved compound, your desired product, from a starting solvent into a solvent in which the product is more soluble. Now, extraction is based on the fundamental concept that like dissolves like. And this principle tells us that a polar substance will dissolve best in polar solvents and a nonpolar substance will dissolve best in a nonpolar solvent. And we're going to take advantage of those characteristics in order to extract our desired product, leaving impurities and other things behind in that first solvent.
Now, when we talk about extraction, when we perform extraction, it's important to make sure that the two solvents are immiscible, meaning that they form two layers that do not mix, like water and oil. The two layers are going to be temporarily mixed by shaking so that the solute can pass from one solvent to another. All right, so that's really important. Now, after the two layers are mixed together, how do we then get the desired product out?
The water, the aqueous part, and the ether, for example, let's take that as an example, the organic phase will separate out on their own given time to do so. So after you shake two layers, say an aqueous and an organic layer, and then you leave it out, they will separate with time. In order to isolate these two phases, we can use a piece of equipment that's called a separatory funnel.
Now, gravitational forces are going to cause the denser layer to sink to the bottom, all right, and the heavier layer to be on top, all right? So gravitational forces cause the denser layer to sink to the bottom of the funnel, where it can then be removed by turning this stop cock, all right, at the bottom. And usually it's more common for the organic layer to be on top, although the opposite can also occur, all right?
Now, remember that the position of the layers is really determined by their relative densities. All right. So then you shake it, you let it settle.
You're going to have two layers that isolate. All right. And then you can use this stop call.
tool open and then you can drain out one layer. You can also continue to add water and mix up so you can do multiple extractions to maximize the separation between these two layers as best as possible. Now once the desired product has been isolated in the solvent, what you can do is you can obtain the product alone by then evaporating the solvent. And you can do that through a process or an instrument called the rotary evaporator, rotovap. Now, another way to take advantage of solubility properties is to perform the reverse of the extraction we just described in order to remove unwanted impurities.
And in this case, a small amount of solute is used to extract and remove impurities rather than the compound of interest. And this process is called a wash. So that's extraction. Now, in addition to extraction, there are other methods we can talk about, predominantly filtration and recrystallization. All right, these make use of solubility characteristics to separate compounds from a mixture.
So filtration isolates a solid from a liquid. So in the chemistry lab, you can pour a liquid-solid mixture onto a paper filter, and that paper filter is only going to allow the solvent to pass through, very much like a coffee filter. And then at the end of the filtration, you're going to be left with the solid called the residue. And then the flask is going to be full of liquid that passes through the filter known as the filtrate. Now, filtration can be modified also depending on whether the substrate of interest is the solid or is dissolved in the filtrate.
So you can have gravity filtration in which the solvent's own weight pulls it through the filter. and is more commonly used when the product of interest is in the filtrate. Hot solvent is generally used to keep the product dissolved in liquid as well. You can also have vacuum filtration.
This is in which the solvent is forced through the filter by a vacuum that's connected to the flask. And that's often used when the solid is the desired product. And then, of course, we can also talk about recrystallization.
This is a method for further purifying crystals and solutions. And in this process, we dissolve our product in a minimum amount of hot solvent and let it recrystallize as it cools. The solvent that's chosen for this process is going to be one in which the product is soluble and only at high temperatures. And so when the solution cools.
Only the desired product is going to crystallize out of solution, excluding any of those impurities you didn't want to begin with. So that is extraction. That is our first objective.
Now we can talk about our second objective. Our second objective is distillation. Now, as we notice in that first objective, extraction requires two solvents that are immiscible. in order to separate the product.
But then what happens when the product itself is a liquid that's soluble in the solvent? Well, this is where distillation comes in handy. Distillation takes advantage of differences in boiling points to separate two liquids by evaporation and condensation. So the liquid with the lower boiling point is going to vaporize first, The vapors will rise up the distillation column to condense in a water-cooled condenser.
Then this condensate drips down into a vessel and the end product is called the distillate. The heating temperature is kept low so that the liquid with the higher boiling point is not going to be able to boil. and therefore will remain liquid in the initial container.
So you can separate two things out based off of boiling points. This is the process that's used to make liquor at a distillery. All right. So there's a couple of things to talk about here under the category of distillation. We could talk about simple distillation.
As the name indicates, this is the least complex version of distillation. And it proceeds precisely... As we just described, the technique should only be used to separate liquids that boil below 150 degrees Celsius and have at least a 25 degree Celsius difference in boiling points. These restrictions prevent the temperature from becoming so high that the compounds degrade, and it provides a large enough distance and difference in boiling points. that the second compound won't accidentally boil off into the distillation.
Now, the apparatus that's used for this kind of technique, I'm going to try to draw out. It's very similar to what we see here, right? We have our initial round bottom flask that has a solution, all right, that we want to separate things out of. We can add a clamp here and the following. tube.
All right, now you have a heat source down here that's going to have something boil and then go through the condenser. Here's the condenser and then out into a different round bottom flask that I cannot draw. All right, so it's essentially you have your your distillery flask, you have this condenser right here in the middle.
All right, where you have water in and out to keep it cool and then it will... drip down here. So just like we described it.
So this condom, you're going to have liquid with the lower boiling point vaporize first, the vapor will rise up the distillation column and then condense through the condenser and then out into a new round bottom flask and we get to separate two things this way. All right. So that is simple distillation, right?
We have a distilling flask that contains the compound liquid solution. a distillation column consisting of a thermometer and a condenser, and then of course our receiving flask to collect the distillate. Now in addition to simple distillation we can also talk about vacuum distillation. We use vacuum distillation whenever we want to distill a liquid with a boiling point over 150 degrees Celsius. And by using a vacuum what we're going to do is we're going to lower the ambient pressure thereby decreasing the temperature that the liquid must reach.
in order to have sufficient vapor pressure to boil. This is going to allow us to distill compounds with higher boiling points at lower temperatures so that we really don't have to worry about degrading the product, right? That's an important concern to have here. And so this is one way to bypass that. So that's vacuum distillation.
The last kind of distillation we need to talk about is fractional distillation. And we can see the setup right here. To separate two liquids that have a smaller boiling point with smaller boiling points, so less than 25 degrees Celsius apart between the two things you're trying to separate, we can use fractional distillation. So in this technique, A fractionation column connects the distillation flask to the condenser. And this fractionation column is a column in which the surface area is going to be increased by the inclusion of inert objects like glass beads or steel wool.
And so as the vapor rises up the column, it condenses on these surfaces and refluxes back and forth until rising heat causes it to evaporate again. only to condense again higher in the column. And each time the condensate evaporates, the vapor consists of a higher proportion of the compound with the lower boiling point. And so by the time the top of the column is reached, only the desired product drips down to the receiving flask.
All right, so that is fractional distillation. So we covered the first two objectives. All right, we covered solubility-based methods.
We talked about extraction, how it combines two immiscible liquids, one of which each easily dissolves the compound of interest. And we talked about filtration as well. Filtration isolates a solid from a liquid.
And we talked about gravity filtration and vacuum filtration, in addition to recrystallization. And then we moved into our second objective all about distillation which separates liquids according to differences in their boiling points. We covered the differences between simple, vacuum, and fractional distillation.
Now with that what I'm going to do is end the video here and in the next video we're going to talk about our last and final objective for this chapter which is chromatography. Let me know if you have any questions, comments, concerns down below. Other than that good luck happy studying and have a beautiful beautiful day future doctors