The ratio of the concentrations of a substance in the two solvents at equilibrium is called its distribution coefficient, K D :

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1 CHM 147 Advanced Chemistry II Lab Extraction: A Separation and Isolation Technique Adapted from Extraction: A Separation and isolation Technique, Hart, Harold; Craine, Leslie; Hart, David; Organic Chemistry, A Short Course, 10 th Ed., Houghton Mifflin Co., Boston, MA, The purpose of this experiment is two-fold: first you will separate a three-component mixture containing a base, an acid, and a neutral substance by extraction; second, you will identify the components and composition of your mixture. The structures of the possible components are shown on the last page of these instructions. Introduction Often when conducting a synthesis, your desired product will be formed along with other, undesired amterials. As such, your product must be separated from the by-products, excess reactants, impurities, and other substances that may be present in the reaction mixture. Similarly, substances in nature are always mixed with other substances. Extraction is the most common technique used to separate a desired organic product from a reaction mixture or to isolate an organic substance from its natural source. Extraction usually involves shaking a solution that contains the desired substance with an immiscible solvent in which the desired substance is more soluble than it is in the starting solution. Upon standing, the solvents form two layers that can be separated. Extraction may have to be repeated several times to effect complete separation. Most commonly, one of the solvents is organic and the other is aqueous. Inorganic compounds can usually be separated from organic compounds in this way: The former dissolve in the aqueous phase and the latter in the organic solvent. In such cases, a single extraction may suffice to effect a satisfactory separation. However, many organic compounds (particularly oxygen- or nitrogencontaining compounds, such as aldehydes, alcohols, esters, and amines, which can form hydrogen bonds) are partially soluble in water. They distribute themselves between the aqueous phase (w, for water) and the organic solvent (0) in proportion to their relative solubilities (S) in the two solvents. The ratio of the concentrations of a substance in the two solvents at equilibrium is called its distribution coefficient, K D : For example, suppose the solubility of compound A is 0.60 g/l00 ml in ether and 0.12 g/100 ml in water. K D is then 0.60/0.12 = 5. Knowing the distribution coefficient of a solute can be useful in determining what quantity of it you can expect to recover during an extraction. For example, if equal volumes of water and ether are used to extract compound A above, one can solve for the amount of material extracted into ether from an initial solution of A in water. By simple inspection, in this instance, since K D = 5, then 5/6 of the solute will be in the organic phase and 1/6 in the aqueous phase when the volumes of the two solvents are equal.

2 Practical Considerations An extraction solvent must readily dissolve the substance to be extracted, yet it must be only sparingly soluble in the solvent from which the desired substance is to be extracted. Also, it should extract only the desired substance or as small an amount as possible of any other substance present; it should not react chemically with the solute in an undesirable way, and it should be easily separated from the desired solute after extraction. This last requirement can be met if the solvent is low-boiling and easily removed by distillation. Common organic solvents that fulfill these requirements include many hydrocarbons and their chloro derivatives, such as benzene, petroleum ether (a mixture of low-boiling alkanes), dichloromethane, chloroform, and carbon tetrachloride. If benzene or chlorinated hydrocarbons are used, however, it is important to avoid breathing their vapors because these compounds are toxic and some are carcinogenic. They can be used safely if we carry out operations in an efficient hood and take care to avoid getting them on the skin. Diethyl ether is another common extraction solvent, but here, too, care is necessary. Diethyl ether (usually referred to simply as ether) is highly flammable and, upon standing in air, its solutions may develop dangerous concentrations of explosive peroxides. Furthermore, ether is slightly water-soluble (about 7 g/l00 ml). Nevertheless, because most organic compounds are highly soluble in it and because of its low boiling point (35 C), ether is frequently used despite its drawbacks. Sometimes we can use desirable, easily reversed chemical reactions such as acid-base reactions to effect separations by extraction. For example, dilute sodium hydroxide (an inorganic base) converts organic acids (usually carboxylic acids) to their sodium salts: RCO 2 H + Na + OH - RCO 2 - Na + + H 2 O Although a particular acid (RCO 2 H) may not be soluble in water, its more polar sodium salt (RCO 2 - Na + ) usually is. When a mixture of a neutral compound and an acidic, water-insoluble compound in an organic solvent is shaken with dilute aqueous sodium hydroxide, the acid is converted to its sodium salt, which dissolves in the aqueous layer, and the neutral compound remains in the organic layer. After the layers are separated, the acid is recovered by acidifying the aqueous layer with a strong acid. RCO 2 - Na + + HCl RCO 2 H + Na + Cl - Thus acids can easily be separated from neutral (or basic) contaminants by extraction with aqueous alkali. A similar process can be used to extract bases from a mixture. Instead of using a strong base, a dilute aqueous acid can be used to extract basic compounds, particularly amines, from neutral or acidic substances by converting them to water-soluble alkylammonium salts: RNH 2 + HCl RNH Cl - After separating the organic and aqueous layers, we can recover the amine from the aqueous layer by making the solution alkaline with a strong base. RNH 3 + Cl - + Na + OH - RNH 2 + H 2 O + NaCl In today s experiment we will apply both of these ideas in order to separate a three-component mixture of organic compounds by extraction.

3 The Separatory Funnel Extractions are usually performed with a separatory funnel, shown at right. Improperly handled, this moderately expensive piece of glassware is easily broken. Always follow proper handling technique. Support the funnel in an iron ring padded with plastic or rubber tubing as shown in the figure at right. Close the stopcock, and add the liquids to be separated. Insert the stopper and invert the funnel, being sure to hold the stopper in with one hand and the stopcock in with the other, as shown in the figure below. Then, with the barrel pointed up and away from you and from anyone else in the vicinity, slowly open the stopcock. This will relieve any pressure that may have built up inside the funnel. Pressure buildup is quite common when volatile solvents such as ether are used because the vapor pressure of the solvent adds to the atmospheric pressure already present. This situation is further aggravated when the funnel is warmed by the heat of your hands or when a gas is generated during the extraction, as happens when an ether solution of an acid is extracted with sodium bicarbonate. After the pressure is released, close the stopcock, shake the funnel gently two or three times, and again invert the funnel and release the pressure by opening the stopcock. Repeat this process until the pressure buildup is slight. Then shake the contents vigorously to complete the extraction. Replace the funnel in the iron ring and remove the stopper immediately. Allow the funnel to stand until the layers separate cleanly. Then slowly draw off the lower layer through the stopcock into a flask or beaker of appropriate size. As the boundary between the two layers approaches the stopcock, slow the flow. Close the stopcock just as the upper layer enters the stopcock bore. If the upper layer is to be transferred to another vessel, pour it out through the top of the funnel. Do not run the upper layer through the stopcock. The relative positions of the aqueous and organic layers in the separatory funnel depend on their densities. The more dense solvent forms the lower layer. Hydrocarbons and ether are less dense than water (see the figure at right), whereas chlorinated hydrocarbons (chloroform, dichloromethane, and carbon tetrachloride) are more dense than water. If you have any doubt about which is the organic layer and which the aqueous layer, withdraw a few drops of the lower layer and determine whether or not they dissolve in water. Sometimes, especially with alkaline solutions, it is difficult to obtain a sharp separation of layers because an emulsion has formed. Gentle swirling of the funnel in a near-upright position, gentle stirring with a glass rod, addition of salt to the aqueous layer, or addition of certain defoaming gents may overcome this difficulty.

4 Day 1: Procedure for the Separation of a Three-Component Mixture by Extraction Caution: Ether is highly flammable: No flames should be allowed in the laboratory when this experiment is being performed. Also, wear disposable gloves to avoid skin contact with the mixture being separated and with its components. Select an unknown mixture and weigh out approximately 1.5 g of it into a small beaker (record the exact mass you use) and dissolve the sample with about 15 ml of ether. Pour the solution into a 60- or 125-mL separatory funnel rinse your beaker with a few ml of ether and combine this portion with the rest of your sample. To extract the basic component from the mixture, add 18 ml of 1.5-M hydrochloric acid to the separatory funnel water and shake thoroughly, using the technique described above. Draw off the lower (aqueous) layer into a 125-mL Erlenmeyer flask and repeat the extraction two more times. Finally, extract with 5 ml of water to remove excess hydrochloric acid that may be dissolved in the ether layer (this is actually a rinse ). Combine the three acid extracts with the water extract and set them aside. To separate the acidic component from the mixture, extract the remaining ether solution three times with 18 ml of 10% aqueous sodium hydroxide and once with 5 ml of water. Combine the alkaline and water layers and set them aside. Pour out the remaining ether solution (which should contain only the neutral component) through the top of the separatory funnel into a small Erlenmeyer flask. Add enough anhydrous magnesium sulfate to cover the bottom of the small Erlenmeyer flask, and swirl the mixture occasionally for 15 min. Then decant the ether into a small beaker of known weight (record the beaker mass inyour notebook!) this is a technique in which you carefully pour the ether out of the flask but leave behind the magnesium sulfate. Rinse the flask contents with a small amount (3 ml) of ether, and add the rinsing to the beaker. Then place the beaker in the appropriate storage area with a label (name and which component you have in it) to allow the ether to evaporate. To recover and identify the basic component you must first neutralize the combined acidic extracts by adding 10% aqueous sodium hydroxide until the solution is alkaline to litmus paper. Next, extract the now-alkaline solution twice with 18-mL portions of ether, collecting and combining both ether extracts. Add anhydrous magnesium sulfate, and swirl as described above. Then decant the ether solution into a small beaker of known weight, rinse as described above, and place this beaker in the storage area (again, labeled) to allow the ether to evaporate. Lastly, to recover the acidic component of the mixture, neutralize the combined alkaline extracts by adding concentrated hydrochloric acid drop by drop until the solution is acid to litmus paper. The solution may be kept cool with ice during neutralization. During this process the acidic component will crystallize out of the solution precipitate out of the water because it is no longer soluble in it. Recover the precipitated acid by vacuum filtration using a Buchner funnel. To do so, set-up a vaccum filtration apparatus, and pre-weigh a piece of filter paper. Place the paper intot he funnel and then pour your acid crystals and solution onto the filter paper in the funnel. Rinse the crystals with a small amount, ~2-4 ml, of cold water (this can be prepared ahead by placing a small beaker of water in an ice bath). Allow them to air-dry in the funnel for a few minutes before transferring them (with the filter paper) to a beaker of known weight. Place this beaker in the storage area (again, labeled) to allow the sample to fully dry.

5 Waste Disposal Neutralize the basic and acidic aqueous filtrates with dilute hydrochloric acid and 10% aqueous sodium hydroxide, respectively. When the aqueous solutions are neutral to litmus, you may discard them in the sink. Day 2: Identification of Components of the Mixture For each component, first weigh the beaker containing the compound this will allow you to determine the recovered mass of compound. Determine the melting point of each compound. Make a tentative identification based on the structures on the last page be sure you select an appropriate compound, i.e., only pick an acid for what you know to be the acidic component from your mixture. Confirm the identity of each component by completing a mixed melting point of your unknown with your preliminary guess of its identity. (If the mixed melting point does not confirm your preliminary identification, then you must select another compound.) Report: Electronically complete the report sheet available in the Dr. Steel s facdata folder see the CHM 147 folder for it.

6 Possible Structures of the Three-Component Mixtures Bases H 2 N H 2 N O 2 N NH 2 NO 2 NO 2 p-nitroaniline mp 147 o C 3-nitroaniline mp 111 o C 2-methyl-3-nitroaniline mp 89 o C Acids O OH HO HO HO O benzoic acid mp 122 o C trans-cinnamic acid mp 132 o C O salicylic acid mp 156 o C Neutral Compounds naphthalene mp 80 o C biphenyl mp 70 o C phenanthrene mp 100 o C