MCAT Organic Chemistry Review

Separations and Purifications

12.1 Solubility-Based Methods

One of the simplest ways to separate out a desired product is through extraction, the transfer of a dissolved compound (the desired product) from a starting solvent into a solvent in which the product is more soluble. Extraction is based on the fundamental concept that like dissolves like. This principle tells us that a polar substance will dissolve best in polar solvents, and a nonpolar substance will dissolve best in nonpolar solvents. These characteristics can be taken advantage of in order to extract only the desired product, leaving most of the impurities behind in the first solvent.

KEY CONCEPT

Like dissolves like is a fundamental concept on the MCAT. Remember that polar substances will associate with other polar substances, and nonpolar with nonpolar.

EXTRACTION

When we perform extractions, it is 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 temporarily mixed by shaking so that solute can pass from one solvent to the other. For example, in a solution of isobutyric acid and diethyl ether, shown in Figure 12.1, we can extract the isobutyric acid with water. Isobutyric acid, with its polar carboxyl group, is more soluble in a polar solvent like water than in a nonpolar solvent like ether. When the two solvents are mixed together, isobutyric acid will transfer to the water layer, which is called the aqueous phase (layer). The nonpolar ether layer is called the organic phase (layer).

Figure 12.1. Isobutyric Acid and Diethyl Ether Isobutyric acid is more polar than diethyl ether and can exhibit hydrogen bonding, so it will congregate in the aqueous layer; diethyl ether will remain in the organic layer.

MCAT EXPERTISE

Think of the organic and aqueous layers as being like the oil and water in salad dressings: you can shake the mixture to increase their interaction, but ultimately they will separate again.

After the two layers are mixed together, how do we then get the desired product out? The water (aqueous) and ether (organic) phases will separate on their own, given time to do so. In order to isolate these two phases, we use a piece of equipment called a separatory funnel, as shown in Figure 12.2. Gravitational forces cause the denser layer to sink to the bottom of the funnel, where it can then be removed by turning the stopcock at the bottom. It is more common for the organic layer to be on top, although the opposite can also occur. Remember that the position of the layers is determined by their relative densities.

Figure 12.2. Separatory Funnel Used to separate solvents based on their relative densities; the denser solvent is always drained first.

KEY CONCEPT

Extraction depends on the rules of solubility and like dissolves like. Remember the three intermolecular forces that affect solubility:

1.    Hydrogen bonding: Compounds that can do this, such as alcohols or acids, will move most easily into the aqueous layer

2.    Dipole–dipole interactions: These compounds are less likely to move into the aqueous layer

3.    Van der Waals (London) forces: With only these interactions, compounds are least likely to move into the aqueous layer

In this example, we’ll assume that the aqueous layer is more dense and settles to the bottom of the separatory funnel. Once we drain the aqueous layer from the separatory funnel, we repeat the extraction several times. Additional water is added to the separatory funnel; it is shaken and allowed to settle, and the aqueous layer is once again drained off. This is done in order to extract as much of the isobutyric acid from the ether layer as possible because it does not completely transfer with the first extraction. Multiple extractions with fresh water are more effective for obtaining the most product, rather than a single extraction with a larger volume of water. You can imagine this as analogous to washing dirty clothes several times, rather than washing them with more water—the cleaner each volume of water is, the less dirt is likely to be left on the clothes afterward. Once the desired product has been isolated in the solvent, we can obtain the product alone by evaporating the solvent, usually by using a rotary evaporator (rotovap).

BRIDGE

You can use the properties of acids and bases to your advantage in extraction:

HA + base → A + H–base+

When the acid dissociates, the anion formed will be more soluble in the aqueous layer than the original protonated acid because it is charged. Thus, adding a base will help to extract an acid into the aqueous phase.

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. In this case, a small amount of solute is used to extract and remove impurities, rather than the compound of interest. This process is called a wash.

OTHER METHODS

In addition to extraction, filtration and recrystallization make use of solubility characteristics to separate compounds from a mixture.

Filtration

Filtration isolates a solid from a liquid. In the chemistry lab, one pours a liquid–solid mixture onto a paper filter that allows only the solvent to pass through, much like a coffee filter. At the end of filtration, one is left with the solid, called the residue, and the flask full of liquid that passed through the filter, known as the filtrate.

Filtration can be modified depending on whether the substance of interest is the solid or is dissolved in the filtrate. Gravity filtration, in which the solvent’s own weight pulls it through the filter, 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. Vacuum filtration, in which the solvent is forced through the filter by a vacuum connected to the flask, is more often used when the solid is the desired product.

Recrystallization

Recrystallization is a method for further purifying crystals in solution. In this process, we dissolve our product in a minimum amount of hot solvent and let it recrystallize as it cools. The solvent chosen for this process should be one in which the product is soluble only at high temperatures. Thus, when the solution cools, only the desired product will recrystallize out of solution, excluding the impurities.

MCAT Concept Check 12.1:

Before you move on, assess your understanding of the material with these questions.

1.    What must be true about the two solvents used for an extraction to work?

2.    When doing an extraction, would it be better to do three extractions with 10 mL of solvent, or one extraction with 30 mL?

3.    Would acid dissolve better in aqueous acid or aqueous base? Why?