Chances are, you’ve already seen acetone at work as an extraction solvent, since it is commonly used to “extract” nail polish right off of your nails. Thanks to its intermediate polarity and its prevalence in nature, acetone is a common and versatile solvent.
And yet, when it comes to extraction, there are a lot of solvents to choose from. Some of them, like water and ethanol, are even more familiar than acetone. Choosing the right solvent is possibly the most important extraction parameter. When – other than nail polish remover – is acetone the best choice?
First, some extraction basics
First, let’s recount some basic chemistry. Extraction is the process of separating out a compound or group of compounds from a matrix. For example, an analytical chemist might simplify a matrix prior to an analytical method like GC-MS, to keep the chromatogram (and the instrument) from getting too messy. Many medicines and supplements are created by extracting bioactive compounds from botanicals. Flavor chemists use aroma extracts from botanicals to create fragrances.
Extraction follows the old principal of “like dissolves like.” That means that the extraction solvent is going to extract the solutes that are most chemically similar to it. Keeping this in mind, let’s take a look at the molecular make-up of acetone.
What can you extract with acetone?
Acetone is the simplest of ketones, consisting of one methyl group on each side of the carboxyl functional group. This makes it less polar compared to water, which has a polarity index of 10.2, but more polar than another common solvent, hexane, which has a polarity index of practically 0. In fact, acetone falls smack in the middle, at 5.1.
Why is this important? Recall that “like dissolves like.” That means that acetone will be ideal for extracting solutes or analytes that have intermediate polarity. In other words, if water is too polar, and hexane is not polar enough, acetone might be just right.
Another useful feature of acetone’s intermediate polarity is that it is miscible with a wide range of solvents. Not only is fully miscible with water, but it can also be mixed with hydrophobic solvents like hexane and petroleum ether. For example, a mixture of acetone with petroleum ether creates a new kind of extraction medium, this time with a polarity index around 2.5.
Some case studies
Now that we’ve mastered the fundamentals, let’s take a look at the literature to see how it applies.
Case study 1:
In a study investigating the extraction of lipophilic components from krill meal, acetone was found to be the best solvent for the extraction of carotenoids and sterols. Carotenoids are the pigments that give carrots and tomatoes their characteristic color. They have a long hydrocarbon chain that makes them predominantly hydrophobic, but many also have a carboxyl or hydroxyl functional group. In this way they are chemically very similar to acetone. Sterols also have a hydroxyl group. Since like dissolves like, it makes sense that acetone would extract both of these compound classes efficiently.
Conversely, in that same study, compounds that were more lipophilic were poorly extracted by acetone compared to the other extraction mediums tested. That makes sense too – acetone is a little bit too polar to cooperate well with truly hydrophobic compounds.
Case study 2:
Remember how we said that acetone is miscible with nonpolar solvents? When acetone is mixed with petroleum ether, it creates a new extraction medium, which was used to extract powerful bioactive compounds from ginkgo plant.
Ginkgo biloba is a plant used in traditional medicine because it has a plethora of potent anti-bacterial, anti-oxidant, anti-cancer, and immune-stimulating compounds. Because of this, it’s a great example of a plant that is often extracted for analytical and medicinal purposes.
In a recent study, acetone was used in combination with petroleum ether to extract bioactive compounds, ginkgols and bilobols, that have anti-tumor activity.
Case study 3:
In some cases, acetone alone was found to be the best solvent for extracting lipids. But wait a minute – didn’t we say that acetone is too polar to effectively extract hydrophobic compounds?
It turns out that in some matrices – in particular food matrices – that have high water content, the water gets in the way of the lipid extraction. In such cases, it’s actually helpful to have a slightly less lipophilic solvent. Something like acetone is able to mix with the watery matrix, but is still sufficiently lipophilic to extract the lipids.
Thanks to its intermediate polarity, acetone is a perfect choice for extracting solutes that fall somewhere between water and lipids. Furthermore, it is useful for extracting lipids from matrices of high water content. Finally, it can be mixed with a wide range of solvents to create a new extraction medium with the perfect properties.
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