acid-base separation of organic compounds

Meaning: Acid-base separation is a liquid-liquid extraction that splits a mixture by turning acidic or basic compounds into water-soluble salts. The mixture is dissolved in an organic solvent and shaken with aqueous acid or base in a separatory funnel. Carboxylic acids form carboxylates with base, and amines form ammonium salts with HCl, and each salt moves to the water layer. Neutral compounds such as esters and hydrocarbons stay in the organic layer. Adding the opposite reagent to the water layer regenerates the neutral compound.

Equation: pH = pKa + log([A⁻]/[HA]), where pH and pKa are dimensionless and [A⁻] and [HA] are the deprotonated and protonated acid concentrations in mol/L. Above the pKa, the acid is mostly deprotonated.

On the MCAT: A passage gives a mixture, such as benzoic acid, phenol, aniline and naphthalene, and asks which reagent extracts which compound. NaHCO₃ removes carboxylic acids (pKa near 4 to 5) but not phenols (pKa near 10); NaOH removes both; HCl removes amines.

Common confusion: Students assume the organic layer is on top. Density decides: ether floats on water, dichloromethane sinks. The constant rule is that the ionic salt goes to water and the neutral form stays in the organic solvent.