Experiment 3: Acid/base Extraction and Separation of Acidic and Neutral Substances

Transcription

1 Experiment 3: Acid/base Extraction and Separation of Acidic and Neutral Substances Introduction Your task in this lab is to separate two compounds by taking advantage of differences in their acidity, and then to identify which compounds you have in your mixture. You will be given a mixture which contains two substances in equal amounts, one which is acidic and one which is neutral. Your acidic compound will either be benzoic acid or 2-naphthol and the neutral compound with either be naphthalene or biphenyl. All of these compounds are soluble in a slightly polar organic solvent such as diethyl ether but are essentially insoluble in cold water. However, benzoic acid and 2-naphthol are acidic due to their H groups and so will be converted to their ionic, conjugate base forms upon reaction with an appropriate base in water. Naphthalene and biphenyl are unaffected by aqueous bases. Benzoic acid (pka 4.2) is sufficiently acidic that it will react with sodium bicarbonate (a weak base) whereas 2-naphthol is a weaker acid (pka 9.5) and will only react with the stronger base sodium hydroxide. The sodium salts of the acids are ionic and soluble in water; therefore the uncharged, free acid form dissolved in ether can be extracted into an aqueous base if the base is strong enough. H H benzoic acid 2-naphthol naphthalene biphenyl diethyl ether pk a 4.2 pk a 9.5 Extracting a compound from a liquid using a second liquid is called liquid-liquid extraction, and requires that the two liquids be immiscible, in order to be separable. It is performed on a macroscale using a separatory funnel (or sep funnel for short). In this lab, we rely on the fact that diethyl ether and water (think water whenever using aqueous solutions such as NaH(aq) or NaHC 3 (aq)) do not dissolve well in each other and will form layers in a container, with the less dense liquid floating on top of the more dense one. Shaking the two solutions together however allows transfer of components between them ( portioning ) so that a compound will be extracted into the phase in which it is most soluble. You will do two sequential liquid-liquid extractions on an ether solution of your mixture, first using sodium bicarbonate aqueous solution, then using sodium hydroxide solution. Your acidic compound (as its ionic, conjugate base form) will be in one of the two aqueous solution extracts, depending on its acidity.

2 Use of two different bases with two different strengths allows for selective reaction of the stronger acid versus the weaker acid. The weaker base, sodium bicarbonate, is only strong enough to react with the stronger acid, benzoic acid, but not strong enough to react with the weaker acid, 2-naphthol. The sodium salt that forms is ionic, highly polarized and soluble in water. Therefore an uncharged acidic compound dissolved in ether can be extracted into an aqueous base if the base is strong enough, by turning into an ionic salt. This will leave behind whichever neutral component in you might have in your unknown: + ne Neutral Compound H NaHC 3 (aq) Na ne Neutral Compound water-soluble! moves into aqueous base layer Still soluble in ether layer Note: Even if benzoic acid AND 2-naphthol were present together, 2-naphthol wouldn t react with the weaker base and it would remain neutral and remain soluble in the ether layer. A stronger base, sodium hydroxide, is required to react with the less acidic 2-naphthol. The two-component mixture in the ether layer containing 2-naphthol can then be separated using NaH: + H NaH (aq) Na water-soluble! moves into aqueous base layer ne Neutral Compound ne Neutral Compound Still soluble in ether layer Note: NaH is strong enough to form salts from both acidic compounds. If BTH benzoic acid AND 2-naphthol were present together in the solution, BTH compounds could react with base to form ionic salts and both could be extracted into the aqueous base. It is important in this experiment that you do not use the stronger NaH(aq) base first or you will not know which compound you have isolated. 2

3 Note: Naphthalene and biphenyl do not have any acidic protons and cannot react with either base. They will remain in the upper ether layer. The free acids may then be recovered as solids by adding a strong acid to the aqueous solution of the sodium salt of the conjugate base, protonating it and causing it to precipitate from the aqueous solution. Na HCl (H + ) H water-soluble not water-soluble Na HCl H (H + ) water-soluble not water-soluble The neutral compound that remains in the ether solution can be isolated as a solid by simply allowing the ether to evaporate. Both compounds will be purified by recrystallization and their identities confirmed using melting point analysis and infrared spectroscopy. A flow chart is provided on the lab webpage and might help you follow the procedure below. Pertinent information about the chemicals you are using: MW (g/mol) MP (ºC) BP (ºC) Density (g/ml) benzoic acid N/A N/A 2-naphthol N/A N/A Naphthalene N/A N/A Biphenyl N/A N/A diethyl ether Water % sodium bicarbonate (aq) 84.01(*) N/A N/A % sodium hydroxide (aq) 40.00(*) N/A N/A 1.05 Conc. (12.1M) Hydrochloric acid 36.46(*) N/A N/A 1.20 (*) MW of solute in solution 3

4 FR YUR SAFETY 1. Concentrated hydrochloric acid is corrosive and an irritant. Avoid skin contact by wearing gloves and, as always, goggles must be worn. In case of contact with skin, wash thoroughly with running cold water immediately. 2. Sodium bicarbonate solution will exothermically produce C 2 gas when mixed with acid. Acid must be added cautiously to such solutions to control the reaction. Also, this will cause a pressure build-up in a closed container such as a stoppered separatory funnel. Frequent venting of the pressure is required to prevent uncontrolled pressure release. PRCEDURE Separating Your Compounds 1. Weigh out approximately 2 g of the solid mixture and transfer it to a 100 ml beaker. Record the exact mass used in your notebook. 2. Set up your separatory funnel Use an iron ring clamped to the metal lattice on the bench, to support your separatory funnel. Make sure the stopcock is closed and place a 125 ml Erlenmeyer flask underneath the stem, in case it leaks. 3. Add 30 ml of diethyl ether to the beaker containing your mixture, and stir gently with a glass stirring rod to dissolve the solids. nce it is all dissolved, carefully transfer the mixture to your separatory funnel (don t spill it!). Add another 5 ml of ether to dissolve any remaining solids stuck to sides of beaker and transfer the dissolved remnants to the separatory funnel also. 4. Add 10 ml of deionized (DI) water to the separatory funnel you should see two layers. (Which is the aqueous and which is the organic (ether) layer?) Then add 10 ml of the 10% sodium bicarbonate solution to the funnel. 5. CAUTIN: When the bicarbonate solution is mixed together with the ether layer, C 2 gas will be generated causing a pressure buildup in the funnel. You must vent the funnel frequently as demonstrated by your instructor to release the pressure. Place the stopper in the funnel and then, while holding the stopper tightly, carefully invert the funnel several times. Be sure to keep one hand under that stopper at all times so the stopper does not pop out and your solution winds up on you and the floor! Immediately vent the funnel while the stem is pointing upwards and away from you (and other students) by opening the stopcock. Close the stopcock and repeat the process several more times, venting the pressure each time until no more C 2 is given off. The goal is to get the two liquid layers to mix well enough so the solutes can freely move into the phase in which they are most soluble. You do need to thoroughly mix the layers to ensure the base reacts with the intended acid to form the necessary salt! 6. Place the separatory funnel back in the iron ring stand, let the two layers separate, remove the stopper, and allow the layers to fully separate. nce separated, drain the lower layer 4

5 into a 125 ml Erlenmeyer flask. Label this as Flask 1. Repeat the extraction sequence again two more times on the organic layer remaining in the separatory funnel using an additional 10 ml of 10% sodium bicarbonate solution for each extraction (no additional DI water). Drain these new aqueous extracts (bottom layers) also into Flask 1. (What organic compound(s) did you remove from the mixture with this first base extraction?) 7. To the separatory funnel containing the organic layer, now add 10 ml of 5% aqueous sodium hydroxide solution. With the stopper in place, shake the mixture thoroughly as before, venting the pressure frequently as before. Replace the funnel in the iron ring stand, let the layers separate, remove the stopper, and drain the lower layer into a clean 125 ml Erlenmeyer flask. Label this flask as Flask 2. (What organic compound(s) did you remove from the mixture with this first base extraction?) 8. Repeat the extraction sequence of Step 7 two more times on the organic layer remaining in the separatory funnel using an additional 10 ml of 5% sodium hydroxide solution for each extraction. Drain the new aqueous extract (bottom layer) also into Flask 2. Be sure to completely drain the bottom layer without draining any of the top layer on this last separation. (What organic compound(s) is/are left behind in the ether layer in the separatory funnel?) IMPRTANT: If attempting to complete the entire lab in a timely fashion, you will need to multi-task the remaining steps. Do not just wait to complete one step before moving on to the next think about how you may use your time in the most efficient manner. You also need to finish Expt 2 in this lab period! [In an effort to assist you, the following steps have been reorganized from the original procedure.] 9. Drain the ether layer that remains in the separatory funnel out the top of the separatory funnel into a clean 50 ml Erlenmeyer flask carefully. Label this as Flask 3. Add about 1 g of anhydrous magnesium sulfate (MgS 4 ) drying agent to the ether extract, swirl for a minute or so, and leave the flask to sit for at least 5 minutes. D NT LEAVE BTTLES F MAGNESIUM SULFATE PEN close and cap tightly when finished with weighing the drying agent. (Set Beaker 3 aside momentarily while you acidify Flasks 1 and 2.) Why do we need to dry the ether layer? Even though water and ether appear to be immiscible, a small amount of water is soluble in the ether and will be left behind to contaminate the solids when the ether evaporates. A solid drying agent such as magnesium sulfate absorbs the water and can be filtered off, allowing for easy removal of the water. PRCEDURE Isolating Your Compounds 10. Take a clean, empty sample vial to the hood, add 5 ml of concentrated hydrochloric acid (gloves!!) to it and return to your bench. Place a magnetic stir bar in Flask 1 and place it on your stirrer hotplate. Turn the STIRRER (not the HEAT) on to a setting of 5-6. Cautiously, and in small portions, add 5 ml of the concentrated hydrochloric acid drop-wise, to Flask 1, 5

6 using a Pasteur pipette. You will observe vigorous bubbling at the beginning of the addition so only use small amounts initially. Use ph paper to check that the solution is ph = 1 at the end of the addition and add additional hydrochloric acid if necessary until ph 1 is reached. When using ph paper, remember to always use a glass stirring rod to place a drop of aqueous solution on the test paper, rather than dipping the paper directly into the solution. Note whether you observed a solid form (Which compound reacts with NaHC 3?) and the color and form of the solid. Set Flask 1 aside on the benchtop until after Step Take a clean dry sample vial (or the one you used previously for the hydrochloric acid), return to the hood, get 5 ml again of concentrated hydrochloric acid and return to your bench. Place a magnetic stir bar in Flask 2 and put it on your stirrer hotplate, still stirring at 5-6. Add 5 ml of concentrated hydrochloric acid in small portions to Flask 2, using a Pasteur pipette. Use ph paper to check that the solution is ph = 1 at the end of the addition add additional hydrochloric acid if necessary until ph 1 is reached. Again, when using ph paper, remember to always use a glass stirring rod to place a drop of aqueous solution on the test paper, rather than dipping the paper directly into the solution. Note whether you observed a solid form (Which compound reacts with NaH?) and the color and form of the solid. (nce the acidification is completed for both Flasks 1 and 2, return to Flask 3 and gravity filter the drying agent from your solution.) You should have a significant amount of solid in either Flask 1 or Flask 2 at this stage, but not both, depending on which acidic compound is part of your mixture. If this is not the case, consult with your instructor before continuing. 12. Label a small clean, dry 50 ml beaker as Beaker 3, and include your name and lab section and record the mass of the empty beaker. Filter the ether solution from Flask 3 using a glass narrow stem funnel and fluted filter paper into this beaker to remove the magnesium sulfate drying agent. Use a 3-5 ml of ether to wash the drying agent to ensure all of the organic compound has been transferred to the filtrate. Your instructor will assist in evaporating the ether in the fume hood (after which you may continue with Step 18). You will now recrystallize the solid in either Flask 1 or 2 from an aqueous solution by adding additional water and heating until it is boiling. The solution that did not produce any solid on acidification is no longer needed and will just be discarded at the end of the experiment. 13. Place Flask 1 or 2 that contains your solid on the stirrer hotplate, again, still stirring at 5-6. Turn the HEAT on to a setting of 6 and begin heating the solid suspension. Add an additional 50 ml of deionized (DI) water to the flask. 14. Continue heating the mixture from Step 13 until the solution is boiling if there is undissolved material then add small portions of water using a Pasteur pipette until it is completely dissolved (use no more than 15 ml). You may observe small amounts of an oily substance that will not dissolve even after adding additional water. In this occurs, immediately go to Step 15 to follow the instructions for hot gravity filtration. therwise, 6

7 once all the solid has dissolved, lift the flask from the hotplate and stand it on the benchtop and allow it to cool, continue to step 16. Turn off your hotplate so that it can cool down for Step Hot Gravity Filtration only necessary if oily material that won t dissolve is observed in Step 14! Take a 250 ml Erlenmeyer flask and add about 10 ml DI water to it. Stand it on the hotplate next to your boiling solution and place a powder funnel in the neck with a fluted filter paper. Pre-wet the filter paper with 2-3 ml DI water. When the water in the bottom of this flask is boiling, carefully pour the hot contents of your other flask (whichever one you are attempting to recrystallize, Flask 1 or 2), in portions through the filter. As each portion drains through the filter, place the flask back on the hotplate to keep any remaining solution hot. Also keep the filtration flask on the hotplate until the filtration is complete. When the filtration is complete, lift the flask containing the filtrate from the hotplate and stand it on the benchtop and allow it to cool. You are now ready for Step 16. Discard the filter paper in the trash and turn off your hotplate so it can cool down for step You should observe crystals form as the solution starts to cool. nce the solution has cooled significantly prepare an ice bath in a small crystallizing dish and place the flask in the ice bath. Allow the solution to cool completely to ice bath temperature. The slower the cooling the larger the crystals that are formed so you should not hurry these steps but bear in mind that you need to complete the lab work in the time allowed. (While cooling, work on the compound from your ether layer in Beaker 3) 17. After the ether has fully evaporated from Beaker 3, scrape down the solid from the walls of the beaker using a spatula so that it collects as a powdery solid at the bottom of the beaker and reweigh the beaker to get the crude mass of the solid (weight by difference). Then add ~8 ml (4 Pasteur pipettes) of methanol to the solid and swirl gently to start dissolving the solid. 18. Stand Beaker 3 on the hotplate at a setting of 2.5 and heat to fully dissolve the compound. Place a watchglass on top of the beaker to prevent the solvent from evaporating and continue heating until it boils. Add additional methanol dropwise as necessary until all the solid just dissolves while boiling. Try to wash any solid down off the sides of the beaker when adding the additional methanol so that all solids are dissolved. 19. nce the solid in Beaker 3 has dissolved, carefully place the beaker on the benchtop to cool to room temperature (HT beaker!). You should start to observe crystals forming. nce it has cooled somewhat, place it in your ice bath and cool it to ice temperature, again remembering that slow cooling gives better crystals. Place 5-6 ml of methanol in a clean dry sample vial and cool this in the ice bath also, for later use when rinsing your crystals. (While Beaker 3 is cooling, vacuum filter the contents of Flask 1 or 2.) Procedure: Vacuum Filtrations of Recrystallized Solids 7

8 20. Set up a clean vacuum filtration apparatus and isolate the crystals of your compound from Flask 1 or 2. Wash the crystals with a small amount of ice-cold water. Pull air through the crystals to dry them for a few minutes. Label the Buchner funnel according to whether the compound was obtained from Flask 1 or Flask 2. Allow the compound to dry in the funnel in your locker drawer until next week. 21. Clean and set up a second vacuum filtration apparatus and isolate the solid formed in Beaker 3. Wash out the beaker to transfer all of the crystals with a small amount of the ice-cold methanol from Step 19 repeat if necessary until all the crystals are transferred. Pull air through the crystals for at least 5 minutes to air dry them. 22. Label a sample vial as Beaker 3. Weigh the empty vial (with cap) then transfer the crystals isolated from Beaker 3 to the sample vial and obtain the crude (still damp) mass of the compound collected. Loosely cap and store the compound until next week in your locker. [Note: the neutral compounds slowly sublime and so mass loss may occur if the compound is stored on the Buchner funnel for a prolonged period.] Procedure: Week Two 1. Weigh an empty sample vial with cap and transfer (using a powder funnel) the product from Flask 1 or 2 to it. Reweigh the sample vial/cap/product. Record both weights. 2. Reweigh the sample vial/cap that contains the neutral compound from Beaker 3. Record this weight. 3. btain melting points on your two compounds. a. Take a fast run of BTH compounds at the same time using the following Digimelt settings: START temp: 50ºC, RAMP rate: 20ºC/min, STP temp: 140ºC. Record both fast mp ranges. b. After determining the Ball-Park temperatures for each, adjust the START temp accordingly for the Beaker 3 Compound and perform its slow run (RAMP RATE: 2ºC/min, STP temp: 140ºC). c. Then raise the START temp accordingly for the Flask 1/2 Compound and perform its slow run (RAMP RATE: 2ºC/min, STP temp: 140ºC). 4. btain an IR spectrum on your acidic compound, as directed by your instructor. 5. Finishing by labeling the vials with your name, the name of the compound and the melting point range of the compound and submit to your instructor. WASTE DISPSAL: 1. Place the methanol filtrate and any unused methanol from the Beaker 3 recrystallization in the methanol/acetone waste container. 2. The aqueous filtrate from your recrystallization and the other flask that did not produce any solid can be poured down the drain. 8

9 CALCULATINS: 1. Calculate the percent recovery of the recrystallization of the neutral compound, based on the crude and final masses you obtained. 2. Calculate the overall percent recovery of the acidic component (assuming that the unknown mixture contains a 1:1 mixture by mass of the acidic and neutral compounds.) 3. Calculate the overall percent recovery of the neutral component (assuming that the unknown mixture contains a 1:1 mixture by mass of the acidic and neutral compounds.) 9