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ACID-BASE EXTRACTION: SEPARATION OF AN ORGANIC ACID, AN ORGANIC BASE, AND A NEUTRAL COMPOUND (5/27/09) Extraction is one of the oldest chemical operations, and it is, undoubtedly, the most frequently performed chemical operation. One or more extractions are performed during the isolation of the products from practically every organic chemical reaction. Whenever you make a pot of coffee, a cup of tea, or pot of vegetable soup you are performing an extraction (more precisely, a solid-liquid extraction). The isolation of trimyristin from nutmeg which you performed earlier is another example of a solid-liquid extraction. Solid-liquid extraction is commonly used by organic chemists to isolate natural products. However, organic chemists employ liquid-liquid extractions even more often than solid-liquid extractions. There are actually two types of liquid-liquid extractions: chemically passive and chemically active. In chemically passive extractions, distribution of a compound between two immiscible solvents occurs because of different solubilities in the two solvents, whereas in chemically active extractions, a compound is chemically altered to change its distribution between the two different solvents. This chemical alteration of a compound to change its distribution between the two solvents is most commonly done through an acid-base reaction. Sometimes, this type of extraction is referred to as an acid-base extraction. For an example of how a chemically active extraction works, let us consider the separation of a mixture of a neutral organic compound and a carboxylic acid. Let us assume that both compounds are soluble in diethyl ether but insoluble in water. A passive extraction using these to solvents obviously will not work. However, if the carboxylic acid were converted into its conjugate base, an anion which would be soluble in water but insoluble in diethyl ether, then a clean separation of the carboxylic acid (as its salt) from the neutral compound could be effected. Consider a mixture of a hydrocarbon and a carboxylic acid, both of which are soluble in ether but insoluble in water. If an ether solution of the mixture is treated with an aqueous solution of a base, Ether-soluble, water-insoluble mixture: COHOa hydrocarbon (neutral)a carboxylic acid (acidic)such as sodium hydroxide or sodium carbonate, the carboxylic acid will react to form its water soluble carboxylate anion, but the neutral hydrocarbon will not react. The hydrocarbon will remain ether-soluble and water-insoluble. By an acid-base reaction, we have converted the carboxylic acid (ether-soluble, water-insoluble) into its carboxylate anion (ether-insoluble, water-soluble).

Formation of a water-soluble salt: COHOether-soluble water-insoluble+OHCOO+H2Oether-insoluble water-solubleBy extraction of an ether solution containing a neutral compound and a carboxylic acid with an aqueous solution of sodium hydroxide, we can separate the two compounds. The neutral compound remains in the ether layer, while the carboxylic acid reacts with hydroxide ion to formthe carboxylate anion, which dissolves in the water layer. The two layers are separated using a separatory funnel, and the aqueous layer is then acidified to regenerate the water-insoluble carboxylic acid. Recovery of the carboxylic acid: COHO+COOHcarboxylate anioncarboxylic acidUsing a similar acid-base reaction, amines (organic bases) can be separated from neutral or acidic compounds by conversion into their conjugate acids, water-soluble cations. Thus treatment of an ether solution of a mixture containing an amine with an aqueous acid (e.g., hydrochloric acid) produces a water-soluble cation that can be separated from water-insoluble compounds. Formation of a water-soluble salt: RNH2+HRNH3ether-soluble water-insolubleether-insoluble water-solubleAfter separation of the ether and water layers, the water layer is then treated with aqueous base to regenerate the water-insoluble amine.