Liquid-Liquid Extraction

Liquid-liquid extraction is a separation technique that uses a two-phase system, typically an organic and an aqueous phase, to separate compounds based on properties like solubility, polarity, and reactivity. This allows for the isolation of a desired compound by moving it from one solvent to an immiscible one.

The separatory funnel is a key tool for this technique, designed to separate two immiscible liquids of different densities. Usually, the aqueous layer is denser and settles at the bottom, but it's important to verify layer densities. The funnel has a stopper at the top and a stopcock at the bottom for controlled separation.

To set up, secure the separatory funnel on a ring stand and check the stopcock for leaks. The tutorial then presents a scenario: separating benzoic acid and 2-methoxynaphthalene dissolved in diethyl ether. The strategy involves adding an aqueous base to deprotonate benzoic acid, making it water-soluble and moving it to the aqueous phase, while 2-methoxynaphthalene remains in the organic phase.

Transfer the mixture to the separatory funnel. Add 30 milliliters of 1 molar sodium hydroxide to react with benzoic acid. Mix the contents by inverting and swirling the funnel, intermittently opening the stopcock to vent gases. After mixing, allow the layers to separate. Carefully collect the bottom aqueous layer in a clean flask, stopping before the organic layer drains. The top organic layer is then poured out from the top of the funnel.

To maximize yield, perform multiple extractions by returning the organic layer to the funnel and adding more aqueous base. Once extractions are complete, the organic layer, containing 2-methoxynaphthalene, will likely contain residual water. A drying agent like sodium sulfate is added until a clear solution with clumped solid is observed. The dried organic layer is then subjected to gravity filtration to remove the drying agent, and the solvent is evaporated to yield pure 2-methoxynaphthalene.

The aqueous layer contains sodium benzoate. To obtain benzoic acid, add 6 molar hydrochloric acid until the solution becomes acidic (pH around 2), which protonates the benzoate, causing benzoic acid to precipitate. The precipitation can be encouraged by an ice bath. Finally, the precipitated benzoic acid is collected by vacuum filtration and allowed to dry.

This liquid-liquid extraction successfully separated two organic molecules based on their differing reactivity to an acid-base reaction. This technique is critical in organic chemistry and widely used in industry for various extractions, from caffeine to metals.