Separation Of a Sample Mixture By Liquid-Liquid Extraction
Reading assignment: Techniques in Organic Chemistry 2nd ed pages 75-99. 3rd ed pages 113-140.
Topics and Techniques i) identification of solvent layers of two immiscible solvents ii) partioning of a compound between two immiscible solvents and determination of KD iii) liquid-liquid extraction with aqueous acids and bases with organic solvents. iv) use of drying agents Introduction Liquid-liquid extraction is a method used for the separation of a mixture using two immiscible solvents. The ability to separate compounds in a mixture using the technique of liquid-liquid extraction depends upon how differently the compounds of the sample mixture partition themselves between the two immiscible solvents. The technique of liquid-liquid extraction often involves a number of processes. First, the component mixture is dissolved in a suitable solvent and a second solvent that is immiscible with the first solvent is added (e.g., two solvents that are immiscible are diethyl ether and water). Next, the contents are thoroughly mixed (shaking) and the two immiscible solvents allowed separating into layers. The less dense solvent will be the upper layer, while the more dense solvent will be the lower layer. The components of the initial mixture will be distributed amongst the two immiscible solvents as determined by their partition coefficient. The relative solubility that a compound has in two given solvents can provide an estimation of the extent to which a compound will be partitioned between them. A compound that is more soluble in the less dense solvent will preferentially reside in the upper layer. Conversely, a compound more soluble in the more dense solvent will preferentially reside in the lower layer. Lastly, the two immiscible layers are separated, transferred and the component in that solvent is isolated by solvent evaporation and/or crystallization. The technique of liquid-liquid extraction is frequently used for the separation of an organic product from a reaction mixture after an aqueous work up, or for the isolation of naturally occurring substances. Organic solvents frequently employed for liquid-liquid extraction procedures include petroleum ether (which is a mixture of low molecular weight alkanes), ethyl acetate, methylene chloride (dichloromethane) and toluene. Diethyl ether (ether) is an effective extraction solvent and with its low boiling point (37 °C) is widely used. However, the high volatility and extreme flammability of diethyl ether is a deterrent in employing ether as a solvent for extraction. In the experiment below, methyl tertiary-butyl ether (MTBE) is used as a replacement for diethyl ether since MTBE is less of a fire hazard. Extraction under basic and acidic conditions As mentioned above, the ability to separate compounds of a mixture using liquid-liquid extraction procedures depends upon the relative solubility that each compound has in the two immiscible solvents. A change in the pH of the solvent (the addition of acid or base) can change the solubility of an organic compound in a solvent considerably. For example, the solubility of a carboxylic acid (RCO2H) in water is increased considerably by the addition of a base e.g., NaOH(aq) or NaHCO3(aq). The increase in water solubility is due to an acid base reaction that converts the less water soluble carboxylic acid (RCO2H) into the more water soluble sodium carboxylate (RCO2Na) equation (a). The water solubility of other organic compounds having acidic hydrogens e.g., Phenols can also be increased by addition of base equation (b).
Noteworthy is that an aqueous solution of NaHCO3 converts carboxylic acids to their sodium carboxylates equation (c), but NaHCO3(aq) is not a strong enough base to form sodium salts of phenolic compounds equation (d). Thus two weak organic acids can be partitioned or separated providing their pKa values differ by five of so pKa units and the...
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