Liquid-Liquid Extraction

Liquid-Liquid Extraction - Liquid/Liquid Extraction...

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Liquid/Liquid Extraction: Liquid/liquid extractions come in two general forms: organic solvent extractions and acid/base extractions. These types of separations are performed using two immiscible liquids to partition the components of a mixture. The typical two-liquid system is comprised of an aqueous phase and an organic phase. Most organic solvents are immiscible with water resulting in the formation of two distinct liquid layers when mixed in a separatory funnel. The more dense liquid forms the bottom layer, while the less dense liquid forms the top layer. Some of the most commonly used organic solvents are listed below: Methylene chloride (CH 2 Cl 2 ) Chloroform (CHCl 3 ) Diethyl ether (CH 3 CH 2 OCH 2 CH 3 ) Ethyl acetate (CH 3 CO 2 CH 2 CH 3 ) Toluene (methylbenzene) The majority of organic solvents are less polar than water. As a general rule, polar compounds are more soluble in aqueous solutions and nonpolar compounds are more soluble in organic solutions. These fundamental differences in solvent polarity and compound solubility form the foundation of all liquid/liquid extractions. A compound that is moderately soluble in water can be removed from the aqueous phase when it is extracted with an organic solvent for which the compound has a greater affinity. As the liquids are mixed together, the compound will slowly leech into the organic layer so that it may be isolated after separation. In your lab this week, you will be utilizing an acid/base extraction to separate the components of a complex mixture. The details of this specific type of liquid/liquid extraction are presented in the following section. Acid/Base Extraction: We classify organic compounds as being neutral, acidic, or basic according to the functional groups present in their molecular structures. Many functional groups are capable of acting as either Br nsted-Lowry acids or bases (proton donors or acceptors). Several examples of acidic and basic functional groups are provided below: The alkyl groups (R) that comprise the bulk of these structures typically result in the compounds being largely nonpolar and hence only sparingly soluble in water. By contrast, they are normally quite soluble in a variety of nonpolar, organic solvents (ie. methylene chloride, diethyl ether, etc.). Remember the axiom like dissolves like. Their aqueous solubility can be manipulated,
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however, using simple Br nsted-Lowry acid/base reactions. Acid/base extractions exploit a compound s inherent acid/base characteristics to create an ionic salt (polar) which is soluble in aqueous solutions and can be isolated therein. Consider the following acid/base reactions: Depending on the reaction conditions used (acid or base) the two compounds shown above can be selectively moved from the organic layer of an extraction mixture to the aqueous layer. In acidic conditions the carboxylic acid would be unaffected and remain in the nonpolar organic layer while the amine would be protonated to form a charged ammonium salt (RNH
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Liquid-Liquid Extraction - Liquid/Liquid Extraction...

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