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Extraction - Chemistry 251 3rd Lab Period Extraction...

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Unformatted text preview: Chemistry 251 3rd Lab Period Extraction Reading Assignment : F&F, pp. 49 - 61 Of all the techniques for dealing with mixtures of compounds, the process of solvent extraction represents perhaps the most powerful tool for removing impurities in a sample, for separating components from mixtures, or for isolating individual classes of compounds from the complex combinations of organic substances which make up living systems. The type of extraction we will encounter most frequently in this course consists of the partitioning of compounds between two immiscible liquids based on the differences in their solubilities in these liquids. A variant of this involves a series of specific acid-base reactions. This is termed chemically active extraction. This week’s experiment will be in two parts. In the first procedure, you will be given a mixture of acetanilide (a neutral organic compound) and benzoic acid (an organic acid) dissolved in methylene chloride (CHzclg) and asked to separate them by means of their solubility properties. You should work as carefully as possible, because both of these materials will be used in experiments to come. In the latter portion of the period, you will carry out some very simple experiments illustrating the benifits of multiple extractions and of extraction techniques useful for small scale samples. Chemically Active Extraction Extraction works most effectively when the materials you are trying to separate have very different solubilities in each of the immiscible liquids used. The immiscible liquids used in our extractions will always consist of an aqueous layer (water) and an organic layer (usually methylene chloride or diethyl ether). Ideally, if you are trying to separate a mixture of two different compounds, one compound will be very soluble in one of the immiscible liquids (say water) and nearly insoluble in the other (organic) liquid layer. The other compound should exhibit exactly the opposite solubility behavior. When this is the case, a simple extraction of these compounds using an aqueous and organic layer will be sufficient to separate them most effectively. In cases where the compounds to be separated both have very similar solubility behaviors, something must be done (ie. chemistry) to reverse the solubility preferences of one of them in order for extraction to be a useful separation strategy. This type of extraction is termed a chemically active extraction. It is most often employed in the separation of organic acids and/or organic bases from neutral organic compounds because, as we will see, the solubility behavior of organic acids and organic bases can be controlled by reaction with an appropriate extraction solvent. Dilute aqueous (inorganic) bases such as sodium hydroxide, sodium carbonate or sodium bicarbonate are actually ions in water and can be used to convert a normally water insoluble organic acid into a water soluble compound which can then be extracted into an aqueous layer. This transfer between phases is possible because the aqueous base reacts with the organic acid to form the sodium salt of the acid. NaOH + R-COzH ---> RCOz'Na+ + H20 The salt formed is ionic and thus will now dissolve readily and completely in the water layer. Sufficient shaking of the two immiscible liquids is necessary to get the hydroxide base and the organic acid into contact with one another and eventually draws the organic acid into the aqueous base (water layer) as the salt. The two immiscible liquids can now be separated and the organic acid will essentially be completely removed from the organic solvent. The organic acid, now found as a salt in the aqueous layer, can then be fully recovered, by reversing the above reaction. This can be done by the addition of a strong acid, like concentrated HCI, to the aqueous solution. In a similar fashion, organic bases (amines) can be extracted from an organic solvent by the proper pH adjustment to produce a water soluble salt. RNH2+ HCl ---> RNH3+C|' The most difficult aspect of this lab is determining which chemicals belong in each layer. The organic layer contains organic solvents (like methylene chloride or ether) and is immiscible in the aqueous layer, which contains water and salts, and aqueous solutions such as sodium hydroxide and hydrochloric acid. Procedure (A) Separation of Benzoic Acid and Acetanilide Obtain from your TA 40 mL of a solution containing 2 g of benzoic acid and 2 g of acetanilide dissolved in methylene chloride. Using the general techniques for laboratory extractions given in lecture and in the book (F&F section 3.5), you will follow the general scheme outlined on p. 56. Note that in this case the "hydrocarbon” mentioned in the flowchart is the acetanilide and the carboxylic acid is benzoic acid. Also, your organic layer will be methylene chloride and not ether. 15 M NaOH will be used as the inorganic base. During the course of the extractions, one should always monitor the pH of the aqueous wash solutions with pH paper when appropriate. CAUTION: As with all organic solvents, work in a well ventilated area when using methylene chloride. Calculate the volume of 1.5 M NaOH necessary to react completely with the 2.0 gms of benzoic acid that is present in your original mixture. It may be a good idea to check your answer with other students, or your TA, before proceeding. Place the 40 mL of methylene chloride obtained from your TA into a separatory funnel supported in a ring clamped to a ringstand. Add to this layer the volume of 1.5 M NaOH you calculated above, then add an additional 10 mL of water to the funnel. Place a lightly greased glass stopper into the top of the funnel and shake vigorously - always remembering to vent the separatory funnel at appropriate intervals. After shaking, return the funnel to the ringstand and allow the two liquid layers to separate completely. One or both layers may be somewhat cloudy, but you should see a clear and sharp boundary between the two layers before proceeding. lf sufficient time has passed and the interface between the two liquids is still not sharp and distinct - consult your TA as to how to proceed. Drain the bottom (organic) layer from the separatory funnel into a clean erlynmeyer flask and then pour the top (aqueous) layer out from the top of the funnel into a beaker. Return the organic layer to the separatory funnel and repeat the extraction with another portion of NaOH and water for a second time. Stopper, shake and return the funnel to the ringstand as before. Again, drain the organic layer from the funnel into the flask and pour the aqueous layer into the same beaker combining it with the aqueous layer from the first extraction. Be sure to test the pH of this second aqueous layer with pH paper to determine if the removal of all of the acid is complete. (What should the pH be ?) If the extraction of all of the acid has not yet been accomplished, repeat the extractions with NaOH until it is ! Return the organic layer to the separatory funnel and repeat the extraction with a final plain water wash (~ 20 mL). Shake, separate and combine the aqueous water wash with the other aqueous extracts in the beaker. You now have two containers of liquid, each of which should contain one of the two original compounds in them. Dissolved in the methylene chloride (organic) layer in the erlynmeyer flask is the acetanilide and dissolved in the aqueous solution in the beaker is the salt of the original benzoic acid, sodium benzoate. All that is left now is to isolate these two solid compounds from their respective solvents. Isolation of Acetanilide: To recover the solid acetanilide, you must first dry, (i.e. remove all traces of water from), the organic layer. This is done by using an inorganic drying agent to complex with and adsorb any traces of water present. (See the procedure in F&F sections 4.2-4.3, pp 71-73). While the book suggests 1-5 grams of drying agent - this is extreme overkill for our experimental scale. Rather, you should add the drying agent (magnesium sulfate) to your organic liquid slowly, a small spatula full at a time, corking and swirling the flask between each addition. As it absorbs water, the drying agent will form large clumps in the bottom of the flask. When some of the added powder does NOT clump, but remains instead as a loose powder flowing around the bottom of the flask as you swirl it, you have added enough drying agent. Let the flask sit, with occasional swirling, for about 10 minutes to completely absorb all of the water. With the water removed, the organic layer should be clear (not cloudy). Transfer the dried methylene chloride solution into a clean, dry, 100 mL beaker by passing it through a funnel containing fluted filter paper (F&F p. 28) to remove the drying agent (M9804). Add two or three boiling stones to the beaker and evaporate the filtrate to dryness on a steam bath in a hood. Put the isolated solid between two pieces of filter paper "sandwich“ and store it in your desk drawer to dry for next week. At the beginning of the next laboratory period, you will , determine the weight of the recovered acetanilide and record its melting point range. Isolation of Benzoic Acid: To recover the original sample of benzoic acid from the aqueous solution you must first convert it back from its salt, sodium benzoate, to free benzoic acid. As we mentioned before, we can do this by adding concentrated acid (HCl) to the solution to drive the reaction back to free, undissociated, benzoic acid. Once it is returned to its original form, it will no longer be soluble in water and thus, should precipitate out of the aqueous solution. ‘ Since the dilution or reaction of concentrated acids is always exothermic, we will always carry out these reactions with appropriate cooling. Place the beaker containing the aqueous solution of sodium benzoate in an ice bath and allow enough time for it to cool. Slowly, and with constant stirring, add concentrated HCI to the beaker, a pipet full at a time. Continue adding HCl until the solution permanently turns a milky white. At this point, test the pH of the solution with pH paper. If it is not very acidic (pH paper turns a cherry red) continue adding HCI until it is. Once the solution is sufficiently acidic, allow it to cool for a few additional minutes and then separate the solid benzoic acid from the solution by vacuum filtration. (see diagram in F&F p. 30) After the filtration is complete, sandwich the solid between two pieces of filter paper and store it in your desk drawer to completely dry out for next week. At the beginning of the next laboratory period, you will determine the weight of the recovered benzoic acid and record its melting point range. (B) Small Sample Technique (Squirting Technique) In the second portion of this weeks lab, we will demonstrate how one carries out extractions on a small scale as well as demonstrate the advantages of multiple extraction. Half the class will be assigned to use test tubes and the small scale squirting technique introduced in F&F (pp. 62-63), and the other half will use the graduated screw top extraction vials found in the common glassware drawers in the lab. Place two mL of aqueous crystal violet solution into a test tube (or extraction vial) and add to this, two mL of methylene chloride from the squeeze bottle on the solvent shelf. Mix the liquids using either the squirting technique (test tube people) or by capping and shaking vigorously the extraction vial. Some of the purple color (crystal violet compound) will have left the water layer and transferred into the organic layer as a result of this mixing. Using a pipet, remove, and discard the organic (methylene chloride) layer (which layer is this ?) and save the aqueous layer for later comparison. Now, repeat the procedure, but this time perform two extractions on the aqueous layer instead of just one, each time using only 1 mL of methylene chloride instead of two. Again, discard the methylene chloride layers and save the aqueous layer. Now compare the aqueous layer from the double (multiple) extraction to the aqueous layer from the single extraction. Is one a lighter color than the other ? What does a lighter color indicate ? In both cases, a TOTAL volume of 2 mL of methylene chloride was used to remove crystal violet from the water solution. According to your results, what can you conclude about the relative merits of multiple v.s. single extractions? Waste Disposal - Aqueous extracts and washes from extraction mixture-—Place in Organic Waste bottle. (While we normally would not put aqueous (water) solutions into the organic waste bottles we make an exception for any water solution that has come in direct contact with methylene chloride and hence is saturated with methylene chloride) - Acids and Bases, especially concentrated ones, should NEVER be placed in the organic waste bottle. They can react with the organic compounds and release enough heat to erupt and possibly break the bottles. They should always be flushed down the drain with sufficient water. - Magnesium Sulfate (drying agent) --Rinse down drain - Filter Paper-Rinse and place in trash - Aqueous and methylene chloride extracts from crystal violet--Place in Organic Waste bottles Chemistry 251 Due 3rd Lab Period Name TA, Day, Time, Location Preliminary Report for Extraction 1. Draw structural formulas for benzoic acid, sodium benzoate, acetanilide, and aniline. 2. Which solvent has the higher density, dichloromethane or water? Which should form the upper layer in your separatory funnel? How could you tell experimentally if you were uncertain? 3. How will you be able to tell when you have extracted with sufficient 1.5 M NaOH to remove all of the benzoic acid from the methylene chloride layer? 4. Suppose you are given a 1:1 mixture of benzoic acid and napthalene. Which technique would you apply first to separate them: Recrystallization, Fractional Distillation, or Extraction? Briefly explain your choice. 5. Indicate where the following waste products are to be disposed: a) extracts from crystal violet b) filter papers c) organic extracts d) aqueous layers e) melting point capillaries f) left over HCI (conc) 9) magnesium sulfate h) 1.5 M NaOH Chemistry 251 Name TA, Day, Time, Location Report on Extraction 1. Weight of crude, dry benzoic acid Percent Recovery Melting point range of sample Calculations: 2. Weight of crude acetanilide Percent Recovery Calculations: Answer the following questions: 3. Given 100 mL of an aqueous solution containing 5 g of substance A, how many grams of A can be removed in a single extraction with 50 mL of ether? How many total grams of A can be removed with two successive extractions of 25 mL each? Assume the distribution coefficient for A is: [ amt of A in ether layer] [amt of A in aqueous layer] 4. In the extraction experiment, you separated a neutral organic compound from an uncharged organic acid. How might you separate a mixture of a neutral organic compound and an uncharged organic base (e.g. RNHz)? 5. Describe the results of the crystal violet extraction. What did each look like, and what can you conclude? 6. You (or another student) may have seen a “precipitate” form at the interface of the aqueous and organic layers which makes it difficult to see where one layer stops and the other begins. What is this called? Why is it formed, and how can it be dispersed? ...
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