Solubility-Based Methods

Separating and purifying compounds is the final piece of MCAT organic chemistry, and it breaks down into three families of technique: solubility-based methods, which use differences in how compounds dissolve; distillation, which uses differences in boiling point; and chromatography, which uses differences in how strongly compounds interact with a stationary surface. This article covers the first family — solubility-based methods, including extraction, filtration, and recrystallization — each of which exploits how a compound's solubility differs from the solubility of everything else in the mixture.

Key Takeaways

• Ch12 covers three separation-method families: solubility-based methods (this article), distillation (boiling point), and chromatography (interaction with a stationary surface).

• Extraction transfers a dissolved compound from one solvent into another in which it's more soluble, using two immiscible liquids — a polar aqueous phase and a nonpolar organic phase.

• Extraction is performed with a separatory funnel; layers separate by density and are drained one at a time via the stopcock.

• Multiple small-volume extractions recover more compound than one large extraction of the same total solvent volume — a common MCAT misconception to watch for.

• A rotary evaporator (rotovap) removes solvent to isolate the compound after extraction.

• Filtration separates solid (residue) from liquid (filtrate): gravity filtration is used when the filtrate is the desired product; vacuum filtration is used when the solid is the desired product.

• Recrystallization purifies solids by dissolving in minimal hot solvent, then cooling so the desired compound crystallizes out while more-soluble impurities stay dissolved.

Extraction

Extraction is the process of transferring a dissolved compound from one solvent into another solvent in which that compound is more soluble.

Extraction typically involves two immiscible liquids — meaning they don't mix, like oil and water. One of these liquids is polar and one is nonpolar:

• The polar solvent is the aqueous phase.

• The nonpolar solvent is the organic phase.

Extraction works because compounds tend to dissolve better in solvents that match their own polarity. If the compound of interest is more soluble in the organic layer, it transfers into that layer; if it's more soluble in water, it stays in the aqueous layer.

The Separatory Funnel

Extraction is carried out using a separatory funnel — glassware shaped like an inverted cone with a stopcock at the bottom. After both solvents are added and the mixture is shaken, the two layers separate on their own due to differences in density: the denser layer settles at the bottom, and the less dense layer floats on top. Opening the stopcock then drains one layer at a time, selectively.

Extraction Efficiency: Why Multiple Small Extractions Win

MCAT Callout — Common Misconception: It might seem like one large extraction should recover the most compound — but the opposite is true. Multiple small-volume extractions are more effective than a single extraction using the same total volume of solvent. Each extraction only removes a portion of the compound remaining in the original layer, so repeating the process with smaller portions recovers more of the compound overall than doing it once with all the solvent at once.

Isolating the Compound

Once the compound of interest has been transferred into the desired solvent, a rotary evaporator ("rotovap") is used to remove that solvent and isolate the compound. Additional washing steps sometimes follow to further purify the sample.

Filtration

Filtration separates a solid from a liquid. The solid that gets trapped on the filter paper is called the residue, and the liquid that passes through is called the filtrate. There are two types to know:

• Gravity filtration is used when the compound of interest is dissolved in the liquid — that is, when the desired product ends up in the filtrate. The liquid simply passes through the filter under gravity.

• Vacuum filtration is used when the solid is the compound of interest. A vacuum pulls the liquid through more quickly, leaving the solid behind on the filter paper.

Recrystallization

Recrystallization is a method for purifying solids. The impure solid is dissolved in a minimal amount of hot solvent. As the solution cools, the desired compound begins to crystallize out, while impurities — assuming they're more soluble — stay dissolved in the solvent. This method works best when the compound of interest has meaningfully different solubility characteristics than the impurities present.

Extraction, filtration, and recrystallization are all rooted in solubility and the physical properties of a compound — each takes advantage of differences in how substances interact with solvents to pull a desired compound out of a mixture.

Common MCAT Mistakes

• Mixing up which phase is which. The polar solvent is always the aqueous phase; the nonpolar solvent is always the organic phase — not the other way around.

• Assuming one large extraction beats several small ones. For the same total solvent volume, splitting it into multiple smaller extractions recovers more compound than doing it in a single pass.

• Confusing gravity and vacuum filtration. Use gravity filtration when the desired product is dissolved in the filtrate; use vacuum filtration when the desired product is the solid residue.

• Assuming recrystallization always works. It only purifies effectively when the compound and its impurities have a meaningfully different solubility — if their solubilities are too similar, the impurities crystallize out along with the compound.

MCAT-Style Concept Check

Question: A chemist has 30 mL of organic solvent to extract a compound from an aqueous mixture. Which approach recovers the most compound?

• A) One extraction using all 30 mL at once

• B) Three separate extractions of 10 mL each

• C) Two separate extractions of 15 mL each

• D) It makes no difference how the 30 mL is divided

Answer: B

Explanation: Each extraction only removes a portion of the compound remaining in the original layer, so dividing the same total solvent volume into more, smaller extractions recovers more compound overall. Of the options given, three 10 mL extractions divide the total volume into the most passes, making it the most effective choice.

FAQ

What's the difference between the aqueous phase and the organic phase in extraction?

The aqueous phase is the polar solvent (typically water), and the organic phase is the nonpolar solvent. Extraction works because a compound distributes itself between the two phases based on which one better matches its own polarity.

Why are multiple small extractions more effective than one large extraction?

Each extraction only removes a portion of the compound remaining in the original layer. Repeating the process with smaller volumes of solvent recovers more of the total compound than performing a single extraction with all the solvent at once.

What's the difference between gravity filtration and vacuum filtration?

Gravity filtration is used when the desired product is dissolved in the filtrate — the liquid that passes through the filter. Vacuum filtration is used when the desired product is the solid residue left behind on the filter paper.

How does recrystallization purify a solid?

The impure solid is dissolved in a minimal amount of hot solvent, then the solution is allowed to cool. As it cools, the desired compound crystallizes out of solution while the more-soluble impurities remain dissolved, separating the two.