Extraction - MCAT Organic Chemistry

Extraction

Updated Apr 17, 2026

Liquid-liquid extraction separates mixtures based on polarity differences. A mixture is shaken with two immiscible solvents (typically water + an organic solvent like ether or dichloromethane). Polar compounds go to the aqueous layer; nonpolar compounds go to the organic layer. The layers are separated in a separatory funnel.

Separatory funnel showing two immiscible layers (aqueous and organic) with differentially partitioned compounds — Separatory funnel for liquid-liquid extraction. The aqueous layer (often at the bottom if denser) and organic layer (often at the top if less dense) are separated by gravity after shaking. Different compounds partition preferentially into one layer based on polarity. *Credit: Wikimedia Commons, CC BY-SA*

Common Solvent Pairs

Organic	Density	Notes
Diethyl ether	0.71	Lighter than water (top layer); volatile and flammable
Ethyl acetate	0.90	Lighter than water (top)
Hexane	0.66	Very nonpolar; good for nonpolar extractions
Dichloromethane (DCM)	1.33	Denser than water (bottom); good solvent for many organics
Chloroform	1.49	Denser than water (bottom); less common
Petroleum ether	~0.65	Very nonpolar, volatile

Always know which layer is the organic (top or bottom) based on density.

Practical Protocol

Load the mixture into a separatory funnel.
Add the two solvents.
Stopper, invert, vent (release pressure; important for volatile solvents).
Shake vigorously to maximize mixing.
Let the layers separate (1-2 minutes for simple systems; longer for emulsions).
Drain the bottom layer out the stopcock.
Pour the top layer out the top.

Multiple Small Extractions

For maximum recovery, do THREE smaller extractions rather than one large one. This is because the partition coefficient is a ratio, not a fraction:

If a compound partitions 1:4 (organic:aqueous) and you extract 100 mL aqueous with 100 mL organic:

Single extraction: 20% of the compound goes to the organic layer.
Three 33 mL extractions: ~49% ends up in combined organic layers (better recovery).

Rule: multiple small extractions > one big extraction for the same total volume of organic solvent.

Partition Coefficient and Equilibrium

K = [A]_organic / [A]_aqueous.

For a single extraction, the fraction remaining in the original (aqueous) layer is:

f = 1 / (1 + K × $V_{\text{org}}$ / $V_{\text{aq}}$ ).

For n extractions with equal volumes of organic:

$f_{n}$ = [1 / (1 + K × $V_{\text{org}}$ / $V_{\text{aq}}$ )]ⁿ.

Smaller $V_{\text{org}}$ per extraction but more extractions give a lower final $f_{n}$ (more compound removed).

When Extraction Fails

Some mixtures cannot be separated by simple extraction:

If all components are very polar (all in aqueous) or all nonpolar (all in organic).
If the compound decomposes during the extraction.
If an emulsion forms and does not break.

For these cases, use chromatography or other techniques.

Drying Organic Layers

After extraction, the organic layer often has traces of water. Dry with an anhydrous desiccant like Na₂SO₄ or MgSO₄. The desiccant absorbs the water; the dry organic solution is then filtered or decanted for further use. Solvent is then removed on a rotary evaporator (“rotovap”):

Rotary evaporator with rotating flask submerged in heated water bath and condenser to collect distilled solvent — Rotary evaporator: the sample flask rotates in a heated water bath under reduced pressure; solvent evaporates rapidly and condenses into a collection flask. The standard way to remove solvent from an organic product after extraction or column chromatography. *Credit: Wikimedia Commons, CC BY-SA*

In a liquid-liquid extraction using 100 mL water and 100 mL diethyl ether, why would three 33 mL ether extractions give better recovery of a compound than one 100 mL extraction?

Click to reveal answer

The partition coefficient is a ratio, not a fraction. Each extraction removes a fixed proportion of what is left in the aqueous layer. After n extractions with the same total volume of ether split into n portions, the fraction remaining in water is [1/(1+K× $V_{\text{org}}$ / $V_{\text{aq}}$ )]^n. With three 33 mL extractions vs. one 100 mL extraction, you multiply the "fraction remaining" three times instead of once - but each individual fraction is closer to 1, so the overall product is smaller. Mathematically: more small extractions always beats one big extraction for the same total organic volume.