Purification of Organic Compounds: Methods & Applications

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The primary focal points of this lesson will be on some of the common methods and applications of purifying organic compounds, including sublimation, recrystallization, distillation, selective extraction, and chromatography.

Mining for Nature's Gold

You've just been give an opportunity to work with one of the nation's top drug discovery chemists and today, you're going out into the rainforest to collect plant samples to evaluate them to see if they have any medicinal properties. Extracting medicinal compounds from natural sources has been going on for centuries, and sometimes it's referred to as bio-prospecting. It's kind of like mining for gold! After you've collected your plant samples and tested them in the lab, you're delighted to find out that one of them may be active against cancer!

Now comes the long and arduous task of trying to isolate and determine the identity of the one compound in your sample that's responsible for this exciting activity. Before you get started on your project, let's take a moment and introduce you to some of the methods and techniques you might use during the purification process.

Sublimation

The process of sublimation occurs when a substance goes through a phase transition and goes from the solid state to the gas state without ever being a liquid. Sublimation is a great way to purify an organic compound, but one drawback is that your compound has to be a solid or it typically won't work. If you wanted to sublime a compound for purification, you could take your compound and put it in a Petri dish and heat it gently on a hot plate. After the compound goes from a solid to a gas, it will cool and condense back to a solid on the top of the Petri dish, yielding a pure product.

Recrystallization

If you have a crude (not pure) organic solid and you know a sublimation won't work, performing what's called a recrystallization might be a good way to try and purify your compound. In a recystallization, you take your crude compound and dissolve it in a hot solvent. Ideally, as the mixture starts to cool, your pure compound will crystallize out of solution and the impurities will remain dissolved. Notice in the last picture in the diagram the solid particles that form once the temperature cools back down. You can then simply filter the solid to isolate it from the solvent and other impurities.


Process for recrystallizing an impure solid
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A very important task in any recrystallization is choosing the correct solvent. In choosing a solvent, you want to make sure that your compound is soluble in the solvent at high temperatures but insoluble at room temperature. For the impurities, they need to be soluble (stay dissolved) at all temperature ranges so they can be separated from your desired product. Like sublimation, being able to use recrystallization for your purification hinges on your compound being a solid.

Distillation

The two previous purification techniques we've learned about rely on your compound of interest being a solid. But what if that anti-cancer compound you're after in your plant sample is a liquid? Conducting a distillation is a great way to purify organic liquids. A distillation involves purifying liquids by selective evaporation and condensation of each liquid in the mixture, and this is done by taking advantage of the different boiling points of the individual compounds. For example, say you had a mixture in which one compound's boiling point was 50 degrees and the other's was 150 degrees. You would heat up the sample and as the temperature reached 50 degrees, the first compound would evaporate (boil) and travel up and down a column filled with cool water. The vapors would condense back to liquid form and you could collect the first compound in pure form.

Once the temperature got hotter and reached 150 degrees, the second compound would start to come off and you would switch collection containers as you isolated the second compound in pure form.


Diagram of a common distillation apparatus
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