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6. Liquid/Liquid Extraction

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6. Liquid/Liquid Extraction 6. Liquid/Liquid Extraction PreLab : Prepare a PreLab as you have for the last two experiments and do this exercise: Draw a flow diagram similar to that in Figure 6.10 for the substances 2,4, and 6 shown in Figure 6.15. Introduction Extraction is the drawing or pulling out of something from something else. A lawyer extracts the truth from a criminal; athletes try to extract the last ounce of energy from their muscles. Chemists extract compounds from solids or liquids using an aqueous or organic solvent. By far the most universal and ancient form of extraction is the brewing of tea or the making of coffee. Every pot of coffee or cup of tea involves solid/liquid extraction, the extraction of organic compounds from solid ground beans or leaves using hot water as the liquid. The lower molecular weight polar molecules such as caffeine dissolve in the hot water and are removed from the high molecular weight water-insoluble cellulose, protein, and lipid materials. Over 200 compounds, some in only trace quantities, are extracted from the solid into a cup of coffee or tea. Decaffeinated coffee is also an excellent example of solid/liquid extraction. Coffee manufacturers The brewing of tea extract the caffeine from the coffee to provide modern society with a decaffeinated version of an is one form of ancient drink. We will be demonstrating this chemical separation method in lab on a macroscale extraction by extracting caffeine from tea. O CH3 CH3 N N N O N CH3 Figure 6.1: The Structure of Caffeine. Over the centuries, humans have carried out solid/liquid extraction by brewing just about every common plant leaf, fruit, or root. In the process, they have isolated a number of extracts with pharmacological activity. Many of these compounds were used for medicinal purposes. For example, Sertuner first extracted morphine from poppy seeds in 1805. This drug and several derivatives, including codeine, are used as pain-killers today. Unfortunately, other derivatives such as heroine, have become drugs of abuse. Legal and Illegal O Drugs. HO CH3O CH3CO O O O NCH NCH NCH3 3 H 3 H H H H H HO HO CH3CO H H H O Morphine Codeine Heroin Figure 6.2: Morphine, Codeine, and Heroin. 73 Extraction will be While solid/liquid extraction is the most common technique used to brew beverages and isolate used in many of the natural products, liquid/liquid extraction is a very common method used in the organic laboratory. synthetic reactions. Organic reactions often yield a number of by-products, some inorganic, some organic. Also, since they do not go to 100% completion, some starting material is also often present at the end of an organic reaction. The real “work” in organic chemistry is not running the reaction, but rather in what is aptly called the “work-up” of the reaction mixture, that is, the separation and purification of the desired product from the mixture of by-products and residual starting material. Liquid/ liquid extraction is often used as the initial step in the work-up of a reaction, before final purification of the product by recrystallization, distillation or sublimation. A concrete example will help make sense of this. One of the synthetic reactions you will be carrying out this semester is a Grignard reaction involving the addition of phenyl Grignard reagent to benzophenone to form triphenylmethanol. MgCl ether HCl + O MgCl OH + MgCl2 H2O phenylmagnesium chloride benzophenone triphenylmethanol The final reaction contains the product, the reaction solvents ether and aqueous hydrochloric acid, and probably traces of benzophenone starting material. Since water and ether are immiscible, we will have two separate layers, one aqueous acid, the other organic ether. Since ether is less dense than water, it will comprise the top layer. A novice might simply evaporate the water and ether to get rid of them. The problem is that the inorganic by-product, MgCl2, would not evaporate and the crystals of it would be mixed with crystals of the organic product triphenylmethanol. Your knowledge of chemistry and application of the principle like-dissolves- like should help you to figure out that in the 2-phase ether-water reaction mixture, the ionic inorganic salts of magnesium should want to be completely in the aqueous phase or layer, and the water-insoluble organic product, triphenyl methanol should want to be in the organic ether phase or layer. Extraction uses the solubility differences of these molecules to selectively draw the product into the organic layer. Although the two layers are immiscible, they work together to separate and select the compounds you are attempting to isolate. By simply separating these two layers, we can separate the inorganic salts from the organic materials. In almost all cases, extraction can be used to separate or “partition” ionic or polar low-molecular-weight substances into an aqueous phase and less polar water-insoluble substances into an immiscible liquid organic phase. This phenomenon is governed by the distribution coefficient. Distribution Coefficient In the typical example of liquid/liquid extraction described here, the product was a fairly large organic molecule which you would predict to be not very soluble in water. On the other hand, if the product were a lower molecular weight or “small” molecule, you should predict that it might be at least partially water-soluble. Therefore, it might not completely “move” into the organic layer, but also partially dissolve in the aqueous layer. For water-soluble organic materials, such K = distribution as acetic acid or sugar, most of the solute will reside in the water phase. A quantitative measure coefficient of the how an organic compound will distribute between aqueous and organic phases is called the 74 distribution or partition coefficient. It is the ratio, K, of the solubility of solute dissolved in the organic layer to the solubility of material dissolved in the aqueous layer. (Note that K is independent of the actual amounts of the two solvents mixed.) K= distribution coefficient solubility of organic (g/100 mL) K = solubility of water (g/100 mL) The constant K, is essentially the ratio of the concentrations of the solute in the two different solvents once the system reaches equilibrium. At equilibrium the molecules naturally distribute themselves in the solvent where they are more soluble. Inorganic and water soluble materials will stay in the water layer and more organic molecules will remain in the organic layer. By using the correct solvent system, a molecule can be specifically selected and extracted from another solvent. Since the distribution coefficient is a ratio, unless K is very large, not all of a solute will reside in the organic layer in a single extraction. Usually two, three, or four extractions of the aqueous layer with an organic solvent are carried out in sequence in order to remove as much of the desired product from the aqueous layer as possible. The effectiveness of multiple small volume extractions versus one large volume extraction can be demonstrated by a simple calculation. Imagine that one extraction can recover 90% of the compound. A second extraction with the same solvent may be able to pull out 90% of the remaining material. Effectively 99% of the compound was recovered with two extractions. One large extraction would have only obtained the initial 90%. Many smaller extractions are more efficient than one large extraction. This phenomenon can be proved mathematically, but in short follows the equation: 1 n fraction extracted into B = 1 + VB VAn K This equation provides the fraction of material extracted by solvent B where n is the number of extractions performed, K is the distribution coefficient, VA is the volume of solvent A and VB is the volume of solvent B. There is a problem at the end of this chapter to demonstrate that more extractions are better than one larger extraction. Give it a try! Distribution coefficients play a large role in the efficacy of a drug. In order for a drug to be absorbed into a brain cell, it must pass through what is called the blood-brain barrier, into the brain cell. The drug must have enough water solubility to dissolve in the blood and be carried to the brain. However, to pass through the cell wall which consists largely of water insoluble fatty lipids Cell membranes with solubility properties similar to an organic solvent, the drug must have a reasonable organic use the same solvent solubility too. Cell membranes use the same fundamental solubility principles as the solubility principles extraction process. The cell membrane shown in Figure 6.4 consists of an ionic head and a very as extraction. nonpolar or hydrophobic center. Ionic Head Nonpolar interior Ionic Head Figure 6.4: Simple schematic of a cell membrane. 75 The ionic head of the lipid orients itself in aqueous environments creating a very nonpolar interior. Ions such as K+ and Ca+ can not traverse the interior of the cell readily because the interior is very nonpolar and will not support these ions. Extraction uses this same partitioning effect to isolate organic compounds. Just like this biological example, in the extraction process organic compounds will choose where to dwell according to the distribution coefficient. Synthetic drug design must take into account the importance of having a distribution coefficient that will allow transport in both aqueous blood and through organic membranes. Choosing a Solvent System Ether-water is a One important aspect when choosing a solvent system for extraction is to pick two immiscible good choice for a solvents. Some common liquid/liquid extraction solvent pairs are water-dichloromethane, water- solvent system. ether, water-hexane.
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