Acid and Base Extraction

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What is an Acid and Base Extraction? 

An acid base extraction is used to separate organic compounds from each other based on their properties [1]. This is based on the fact that most organic compounds have a higher solubility in organic compounds than water or or other aqueous solutions [1]. They involve 2 solvents that are mostly insoluble in each other, with one of the solvents being water or an aqueous solution [1]. Since the solvents are insoluble in each other, this would result in them separating into 2 different layers [1]. The less dense solvent would form the layer at the top, with the denser solvent at the bottom [1]. A common daily example of acid and base extraction would be making tea or coffee. When making tea or coffee, the chemicals that are soluble including flavour chemicals, odour chemicals and caffeine are extracted from tea leaves or coffee beans into water [2]. Insoluble plant material is left behind in a filter or bag [2]. 

Extracting Bases

Basic compounds can be extracted from organic compounds by shaking them with acidic solutions to convert them into water soluble salts; they can then be extracted from an organic layer to an aqueous one [3]. The following reaction is an example of an amine being extracted into an ammonium salt. 

Figure 3: A Basic Amine Reacts with Hydrochloric Acid to Produce a Soluble Ammonium Salt [3]

Figure 3: A Basic Amine Reacts with Hydrochloric Acid to Produce a Soluble Ammonium Salt [3]
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The ammonium salt goes through the following processes in the diagram below. The ammonium salt is added into an Erlenmeyer flask along with 5% aqueous HCl. This leads to 2 layers, one composed of neutral compounds and one containing HCl. Neutral compounds are filtered out, while NaOH is added to the HCl and amine solution to basify the solution. The basic compounds are then extracted into organic solvents, and filtered out for solid compounds.

Figure 4: The Ammonium Salt is Extracted into an Organic Solvent [3]

Figure 4: The Ammonium Salt is Extracted into an Organic Solvent [3]
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Extracting Acids

Acidic compounds can be extracted from organic compounds in a similar way to basic compounds. NaOH is added to a neutral solution made of amines and carboxylic acids, which splits into a layer of NaOH and a layer of neutral compounds. The solution is then separated into neutral compounds, and a solution containing NaOH and carboxylic acids. The NaOH containing the carboxylic acids is then filtered removing the solid compounds, while the acidic compounds are extracted into the organic solvent.

Figure 5: Extraction of Acidic Compounds into an Organic Solvent [3]

Figure 5: Extraction of Acidic Compounds into an Organic Solvent [3]
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Extracting Carboxylic Acids and Phenols

Carboxylic acids and phenols can be extracted from an organic layer to an aqueous one by shaking them with a basic solution, which converts them into water-soluble salts. Phenols are less acidic than carboxylic acids, hence it is necessary to use a stronger base to completely react with it. A solution of bicarbonate (HCO3^-) can be used to completely react to both phenols and carboxylic acids [3]

Figure 6: A carboxylic acid reacts with NaOH to create a water-soluble salt  [3]

Figure 6: A carboxylic acid reacts with NaOH to create a water-soluble salt  [3]
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