extraction

Meaning: Extraction moves a solute from one liquid phase into a second, immiscible one, based on "like dissolves like." Neutral, nonpolar compounds favor the organic layer. Ions and very polar species favor water. In a separatory funnel the layers separate by density: diethyl ether is less dense than water and sits on top, while dichloromethane is denser and sits on the bottom. Several small portions of solvent remove more solute than one large portion of the same total volume.

Equation: K = Corganic / Caqueous, where K is the partition coefficient (unitless) and C is the equilibrium concentration of the solute in each layer (mol/L).

On the MCAT: A passage describes separating a mixture, such as a carboxylic acid, a phenol, an amine and a neutral compound, dissolved in ether. You predict where each compound ends up after each wash. Aqueous NaHCO₃ deprotonates the carboxylic acid but not the weaker acid phenol. NaOH deprotonates both. HCl protonates the amine to an ammonium ion. Each charged species moves to the water layer. Adding acid or base to that aqueous layer regenerates the neutral compound, which can be re-extracted or may precipitate.

Common confusion: Students assume the organic layer is on top. Density decides the position, so check the solvent. Students also forget that the ionic form is the one in water, and that it must be neutralized to recover the original compound.