Extraction - Online Tutor, Practice Problems & Exam Prep

Extraction involves separating a solid from a liquid using selective precipitation in a new solvent, often through acid-base reactions. By adjusting pH with strong or weak acids or bases, the solubility of components changes. For instance, adding a base to acetic acid converts it to an acetate ion, which is polar and moves to the aqueous layer. This process can be repeated with different bases to isolate various compounds based on their pKa values, allowing for effective separation of mixtures in organic chemistry.

Extraction

Hey everyone, so in this video let's take a look at extracting. Now we're going to say under extraction we have the separation of a solid and a liquid. We're going to say the removal of a component from a mixture is done by selective precipitation in a new solvent. Here I already partially colored this in so here we're doing extraction, and here we have it with a separatory funnel. Now, we're going to say that this blue portion here is the more dense liquid portion, so that's why it's on the bottom, and let's say that this represents water. So water here would be our aqueous phase or aqueous layer. And then we're going to say up here is our organic layer.

Now let's say that we have two compounds in here in the organic phase, let's say that one of them was ammonia, which is NH₃, and let's say that the other one was acetic acid. Now typically with extractions, it's customary to do with acid-base reactions. We're going to say, acid-base extractions are the most commonly used form. And we're going to say the pH of the system is selectively varied by adding strong or weak acids or bases. And we're going to say depending on the pKa of the component its solubility in aqueous organic solvents will change. So here we're talking about, changing one of the two compounds A and B by adding either an acid or a base to the organic phase.

Now, here we're going to say formation of an ion equals increased solubility in the aqueous solvent layer because aqueous water is polar. Ions are polar, so they mix together. A non-ionic form equals increased stability in the organic layer. The organic layer tends to be more non-polar, not having a charge tends to make it non-polar, so they'll mix together. So what we do here, so for example let's say we had these two within the organic layer A and B, and let's say I wanted to remove acetic acid from this separatory funnel. What could I do? What I would do here since acetic acid is acidic, I can add some base to the separatory funnel. It would seep into the organic layer, and since I'm using a base, the base would remove the proton or H⁺ ion from acetic acid and change it into the acetate ion. So that the acetate ion now has a charge, it's polar, so it would travel down out of the organic layer into the aqueous layer.

And now that it's in the aqueous layer, I would drain this aqueous layer into a beaker. And there goes the acetic acid as the acetate ion. It's still mixed in there, it's soluble because it's polar. I want to keep it as a solid. What do I do? Well, you can introduce some acid into here. Introducing acid would give back an H⁺ ion to the acetate ion and change it into acetic acid. Since acetic acid is neutral it will precipitate. And since it's precipitating it's a solid I can just drain the aqueous portion and have left some of this precipitate at the bottom. And in that way, I've separated my two compounds A and B from each other by using extraction with the use of a separatory funnel.

Now, how can I make sure that all of the acetic acid is no longer in the organic layer? Well, you're adding base. If there's any acid still remaining, the pH of the organic layer would still be a little acidic. So you just use pH strips, you dip it in there to make sure that the organic layer is completely basic. That means the base has completely neutralized any potential acetic acid still remaining. Another thing about this, since this is an acid-base reaction, you're mixing acids and bases together, they would build up pressure. Because to ensure thorough mixing, you would invert the separatory funnel. This will cause a buildup of pressure. As you're inverting it, mixing it thoroughly, you would make sure you have the end portion of the separatory funnel pointed up, twist this opening here to allow some gas to exit. So it's pretty involved work when you're trying to use a separatory funnel by using extraction of these two types of compounds. In the next video, we'll take a closer look at what would happen if we had even more compounds mixed within a solution. How could we use this acid-base extraction in order to separate each one of these compounds from one another?

Acid-Base Extraction

In this video, we're going to take a look at an acid-base extraction. Now, we're going to say whenever you do an acid-base extraction, typically, it is better to add a weak base before a strong base. That's because the most acidic component will react with that weaker base. If we were to use a strong base from the very beginning, all the acidic compounds would all react with it. And in that way, we wouldn't be able to thoroughly separate them from one another. So, we're going to say this allows you to only isolate the strongest acid. And that won't become negatively charged and, therefore, become aqueous or become soluble in the aqueous layer.

Now, here I'm giving us different compounds and their pKa values. Remember, the lower your pKa is, the stronger the acid. This is going to require you guys to know some basics about acids and bases. So, make sure you've looked at my videos dealing with acid-base identification. Now, if we take a look at these, we have here, the first compound is called propane. Its pKa is 60, so it's pretty high. Next, we have methylamine, which is 10.64. Next, we have what's called phenol. Let me write them all down. You don't need to know how to name them. I'm just giving you the names for them. And then we have acetic acid finally. So, the lower your pKa, the more acidic you are. We can see that acetic acid has the lowest pKa present, so it's going to be the most soluble.

Now, here, at the moment, they're all found within the organic layer of your separatory funnel. Right? Your separatory funnel, not the best drawer, but hopefully, you guys understand what I'm writing. So what we have here is our organic layer, which has all four components in it. And then below that is our aqueous layer, which is more dense, heavier. That's why it's on the bottom. Right. So, in the organic layer, they're all neutral, so they're all going to be found in the nonpolar organic layer. What you're trying to do is react the solution with an acid or a base at each step. This will cause some of the compounds to gain charges. Once they gain a charge, they're going to slip down into the aqueous layer because the aqueous layer is polar, and when you gain a charge, you become polar. And once they're in the aqueous layer, we can drain out that aqueous layer and separate that component from the rest of them. Then all we do is we re-add more water, which will naturally seep down to the bottom and do the process over again.

So here, what we do first is we react these four guys in the organic layer with 0.25 molar sodium bicarbonate, which is just baking soda, which is weakly basic. So this would react with the strongest acid present, and the strongest acid present is the acetic acid. It would react with the base, so it would donate an H⁺ to the base and become my acetate ion. So here goes your acetate ion. It's now in the aqueous layer because it's gained a charge. And then the remaining guys here, they're not as acidic, so they won't react with the sodium bicarbonate. So they're still in the organic layer.

Now, here in the aqueous layer that’s in, we want to make it a solid within that aqueous layer. At the moment, it can't be a solid because it's charged and making it polar, so it'll dissolve in a polar solvent. What we have to do here is to take away its negative charge, and we do that by giving back its H⁺. So we use some acidified water. This is acidified water. That acidified water is donating H⁺ to this, making it neutral again. And now it would exist as a solid in the aqueous layer.

Now, we've got to continue. So, we've gotten rid of the most acidic component out of the four. Now we have these three remaining. So now, we're going to use a stronger base. So we use sodium hydroxide. That stronger base will react with the next most acidic compound, which is the phenol. It takes away its H⁺, so it becomes C₆H₅O^-. We don't take away one of these here because those are connected to carbon, and a carbon-hydrogen bond is not very acidic. So those stay there. We're always taking hydrogens from either end. This H technically is not to the carbon. The formula is deceiving. That hydrogen is actually connected to the oxygen in phenol. So here, we get our phenolate ion. It has a charge, so now it is in the aqueous layer. And because it's in the aqueous layer, it's going to be polar. But we need to make it neutral again. So, again, we use acidified water, which is going to donate an H⁺ to the phenolate anion and give us back phenol. It's neutral again, so it becomes a solid within the aqueous layer.

So, what do we have left? We have left our propane and our methylamine. So, if you recall my videos on identifying acids and bases, remember that compounds made up of only carbons and hydrogens are nonpolar. Therefore, they're not acidic or basic. So this is always going to be found within the organic layer. So there's nothing we can do about that. And then methylamine, in the name, we hear the term amine. Remember, if you're a neutral amine, meaning you have no positive or negative charge, you are a weak base. Because you are a base, you will react not with a base but with an acid. So we switched it up a little bit here. Instead of using a base now, we are using an acid. That HCl will donate NH⁺ to the methylamine. And here, it becomes CH₃NH₃⁺. So it becomes Methylammonium ion. And because it has a charge, it's going to be in the aqueous layer. K? But here, now, we technically aren't going to use acidified water. What we're going to do instead, actually, we're going to use OH^-. We're going to use some base. The base will rip off a hydrogen here. It's going to accept an H⁺ so that this can become neutral again. And now, it's going to be a solid in the aqueous layer.

This is how we perform an acid-base extraction. It allows us to separate different solids from solvents by introducing a new solvent. Again, it's hard to do this if you don't understand the principles of identifying acids and bases in which we talked about. So make sure you take a look at the chapter when we talk about acids and bases. How do we identify them as being an acid or a base, weak or strong? What does a high Ka mean? What does a low pKa mean? The higher your Ka, the stronger the acid. The lower your pKa, the stronger the acid. All these things build up in terms of being able to do this laboratory technique in which we use different pHs and different affinities towards acids and bases to help separate solids from a liquid.

Here’s what students ask on this topic:

What is the principle behind acid-base extraction in organic chemistry?

Acid-base extraction relies on the principle of changing the solubility of compounds by adjusting the pH of the solution. By adding strong or weak acids or bases, the solubility of components in aqueous or organic solvents changes. For example, adding a base to acetic acid converts it to the acetate ion, which is polar and moves to the aqueous layer. This method allows for the selective separation of compounds based on their pKa values, making it a powerful technique for isolating specific components from mixtures.

How does a separatory funnel work in the extraction process?

A separatory funnel is used to separate immiscible liquids based on their densities. In an extraction process, the mixture is added to the funnel, and the denser aqueous layer settles at the bottom while the less dense organic layer remains on top. By adding acids or bases, specific compounds can be made to move between these layers. For instance, converting acetic acid to acetate ion makes it polar, causing it to move to the aqueous layer. The layers can then be drained separately to isolate the desired compounds.

Why is it important to use a weak base before a strong base in acid-base extraction?

Using a weak base before a strong base in acid-base extraction is crucial because it allows for the selective reaction with the most acidic component first. If a strong base is used initially, all acidic compounds would react simultaneously, making it difficult to separate them. By starting with a weak base, only the strongest acid reacts, allowing for its isolation. Subsequent steps with stronger bases can then be used to separate the remaining acidic components based on their pKa values.

How can you ensure that all acetic acid is removed from the organic layer during extraction?

To ensure all acetic acid is removed from the organic layer, you can use pH strips to check the pH of the organic layer. After adding a base to convert acetic acid to acetate ion, the pH should be basic if all acetic acid has been neutralized. If the pH is still slightly acidic, it indicates that some acetic acid remains. Continue adding the base and mixing until the pH strip shows a completely basic pH, confirming that all acetic acid has been removed.

What role does pKa play in the extraction process?

pKa is a measure of the acidity of a compound, and it plays a crucial role in the extraction process. Compounds with lower pKa values are stronger acids and will react with bases more readily. By knowing the pKa values of the components in a mixture, you can selectively add acids or bases to change their solubility. For example, a compound with a low pKa will react with a weak base first, allowing it to be separated from other components with higher pKa values. This selective reactivity is key to effective extraction.