1-8-13
Chem 2123 Experiment 2 Separation and Purification Techniques:
Acid/Base Extraction, Distillation, and Recrystallization
Purpose : To separate, purify and identify components of an unknown mixture of compounds using extraction, distillation and recrystallization techniques. Chapters in Mohring to read – 4, 5, 6, 7, 10, 11, 12, 13, 15
Equipment: Büchner funnel, separatory funnel, stemless funnel, small filter paper, large filter paper.
Website link to videos of interest!! - http://bcs.whfreeman.com/mohrig3e/ under standard taper and
Williamson resources look at extraction,
Chemicals: 10% NaHCO3, 1.25 M NaOH, 6 M HCl, activated carbon, MgSO4
Mixture Components:
Liquids / Boiling Point (°C) / Melting Point (°C)Methyl tert-butyl ether (MTBE) / 54-56
Toluene / 110-111
Unknown Solids / Boiling Point (°C) / Melting Point (°C)
Benzoin/Benzil / - / 132-137/94-96
Dimethylterephthalate / 141-142
Benzoic Acid / - / 122-123
4-t-Butylphenol / - / 98-101
2-Chlorobenzoic acid / - / 141-143
2-Naphthol / - / 122-123
o-Toluic acid / - / 104-105
Triphenylmethane / - / 92-94
Triphenylmethanol / - / 162-163
Notes: 1. Do not leave containers of MTBE or MgSO4 open.
2. Work in the hood!!!
3. Cover the MTBE when walking your beaker or graduated cylinder to your hood
Procedure:
Week One
Obtain an unknown mixture from the TA and determine the total mass, ~1.6 grams. The mixture will contain 0.8 g each of two unknown solids. Dissolve the solid in a mixture starting with 15 mL of toluene. Then add ~10 mL MTBE. Note which seemed to dissolve more of the unknown. Use additional amounts of whichever solvent seems to dissolve your sample to complete transfer and dissolution. Don’t use more than 50 mL of total solvent. It shouldn’t take ~5 minutes to dissolve the sample. Record the amount of each solvent you added.
Transfer the mixture to a separatory funnel. Add about 10 mL of water and note which is the organic layer and which is aqueous. Add 10 mL of 10% aqueous sodium bicarbonate (NaHCO3), stopper the funnel and mix the contents taking care to vent the solution frequently*. Once you’ve vented a couple times, shake the funnel more vigorously and allow the layers to separate. Collect the aqueous layer in a labeled Erlenmeyer flask Repeat the process using a second 10 mL of water and 10 mL of sodium bicarbonate solution and collect the aqueous layer in the same Erlenmeyer.
*When sodium bicarbonate reacts with acid, it forms carbonic acid, which readily forms CO2 gas. Just as a shaken can of soda sprays liquid all over the place, the carbon dioxide in your separatory funnel will take any liquid that gets in its way along for the ride. Make sure there’s no liquid trapped at the neck of the stopcock when you vent, and vent carefully.
Add 10 mL of 1.25 M aqueous sodium hydroxide (NaOH) to the funnel, stopper, and carefully mix the contents as before. Remember to vent the solution in the funnel during mixing. Draw off the lower aqueous layer into a new Erlenmeyer flask. Repeat with a second 10 mL of NaOH solution, and add this to the same flask.
Add 10 mL of distilled water to the organic layer in the funnel, stopper and carefully mix as before. Drain the water layer into a waste beaker and work up the organic layers as described in next section.
Collect the organic layer from the funnel and add a small amount (size of half a grape) of anhydrous MgSO4. Swirl the flask and, if the MgSO4 doesn’t flow freely, add a little more. After drying for ~ 5-10 min, filter the solution through a gravity filter into a clean, dry 50-mL roundbottom flask. Cap the flask, label it with your name and date, and store it in the flammable cabinet to use next week.
Calculate the approximate amount of acid required using M1V1 = M2V2 and review number with TA before proceeding. Do this calculation before lab!!! We want you to do this calculation so you know about how much acid you need so that when you go to get the acid out of the container, you take an appropriate amount and don’t take too much.
Acidify the contents of the other flasks (labeled #1 and #2) by addition of 6 M hydrochloric acid (HCl), swirling the solution gently and test the pH with litmus paper. Slow addition is particularly important for the NaHCO3…dropwise is recommended.
Note which flask produced solids in your notebook, this is key information for interpreting the results.
Cool any flasks with precipitates in an ice water bath. Isolate the solids by vacuum filtration with a Büchner funnel. Wash the solid with a small amount of ice water and disconnect the vacuum.
Check the filtrate solution to see if any more solids form by adding a little 6M HCl, to see if any more precipitate forms. If so, you should also collect this by vacuum filtration.
Collect any solid on a pre-weighed watch glass and air dry for at least a week. If the solid is clumpy, you should break it up into small pieces with a spatula after drying for a few days, and then let it dry a few days more.
Extraction Flow Chart: What do I do with each flask?
Week 2
Determine the mass of the recovered solid from week 1 and calculate the percent recovery. Also determine the melting point of the dry solid. You may want to do a mixed melting point with the known compound to confirm identity. Or you could do an IR to confirm identity.
Add a stir bar to the 50 mL round bottom containing your filtered organic layer. Assemble a simple distillation apparatus (dry glassware) as shown below, using this flask as your still pot. Pay particular attention to the location of the thermometer bulb, because if it’s too high or too low, your temperature readings may be significantly off. When complete, ask a TA or the instructor to check your setup. (It’s a good idea to pre-weigh the receiving flask!)
Heat the round bottom with a heating mantle and use a stir plate to stir. The exact heat setting will depend on your particular mantle and heating controller, but ~3 is probably a good place to start. If nothing’s distilling after 20 min, gradually adjust the setting to a higher level. As the material distills, more heat will be required to keep it going. Therefore, you will have to increase the heat setting to 5-6 or more. Remember, there is a lag time for the electrical heat!!
When liquid starts to drop in your collection flask, observe the thermometer and take an initial reading once the temperature levels off. Continue collecting distillate until the temperature rises to about 100 °C. At that point, switch the collection flask to a pre-weighed,100-mL round bottom or Erlenmeyer flask and continue collecting until there’s less than ~1 mL of liquid left in the still pot or the temperature rises by another 15 °C.
Record the temperature range and amount of each liquid, mL or grams, collected in your notebook.
Determine the mass of MTBE and toluene that you distilled and using its density, calculate the volume and percent recovery based on the original amounts you added in week 1 to dissolve the solids.
Week 2 Prelab Questions (Due on the day Week 2 is performed in the lab)
1. In your Extraction from the previous week, you had two layers in the separatory funnel. One of these layers was an aqueous layer while the other was an organic layer. What property determines which layer is which in the separatory funnel?
2. If you do not know which layer is organic and which is aqueous in the separatory funnel, what test can you perform to figure this out?
3. Sodium bicarbonate and sodium hydroxide are two different types of bases. Explain what type of base they are and what type of acids they would be associated with in an acid/base extraction.
4. For a distillation, explain the importance of the thermometer placement.
Week 3 – Recrystallization
After allowing the 50-mL round bottom to cool, a solid should form. Determine an appropriate solvent for recrystallization of the impure solid provided. The three solvents to choose from are methanol/xylenes, ethanol, and heptane.
Add a small amount of the solid (~10-20 mg) to three test tubes. To each test tube add a different solvent (~ 1-2 mL) and note the solid’s solubility at room temperature using a stirring rod for mixing. Heat any samples that do not dissolve using a hot water bath and note whether the solid dissolves. If the solid dissolves on heating, cool the test tube in an ice water bath and note whether solids reform. All of the solid may not dissolve, but that is not important at this stage. The key question is which solvent provides crystals after cooling in ice bath. Be cautious, the water is hot enough to melt some of the samples and they will then solidify on top of water not form crystals in the water phase.
Once you have picked a recrystallization solvent, add ~10 mL of hot recrystallization solvent to the round bottom flask. Use this liquid to transfer the solid in the rb flask to an Erlenmeyer flask and boil.
If some of the solids haven’t dissolved after 5 min of boiling, add additional solvent by pipet in ~2-10 mL increments. Once the mixture returns to a boil, note whether more solids dissolve. If not much dissolves with 2 mL add more!!!! Repeat as necessary until solids dissolve. Since you will be performing a hot filtration, you will need to add a small excess of solvent so that recrystallization doesn’t occur during filtration.
Add an extra 30% of the total volume (these solvents evaporate easily, so premature crystallization is likely…you can always boil off the excess solvent after the hot filtration). This is an excellent time to add your activated carbon while it is cool!
You will need to use activated carbon to adsorb any colored impurities. Remove the flask from the heat and let it cool to just below boiling. Add a small amount of activated carbon (no more than 0.1 g, size of 2 peas) and return the flask to the heat ing mantle. DO NOT add activated carbon to a boiling solution. It WILL boil over!! Have some hot solvent ready, ~5-10 mL, to rinse your filter paper to recover any solids that drop out.
Place a 125-mL Erlenmeyer and a stemless funnel in the oven for ~5 min and prepare a fluted filter. Perform a hot, gravity filtration using the fluted filter, preheated funnel and Erlenmeyer flask. If needed, boil off the excess solvent you added.
Allow the solution to cool to near room temperature. Complete the crystallization by cooling the flask in an ice water bath for 10-15 min. You should also cool some of your recrystallization solvent in an ice bath during this time for use later.
Collect the crystals on a Büchner funnel by vacuum filtration and wash the filter cake with small portions of the cold recrystallization solvent. Spread the crystals on a pre-weighed watchglass using a spatula and allow them to dry for one week.
Determine the weight and melting point (or you could take an IR) of the purified product. Calculate the percent recovery.
Questions to answer for lab report! (Make a section for these in your lab report and answer them. They will be graded as a homework assignment.)
1. What is the solubility of most organic compounds in water?
2. What effect does reaction with NaOH or NaHCO3 have on organic compounds to affect their solubility in water. What form are they in the water phase?
3. What does the addition of HCl do to cause the organic compound to become less soluble in water.
4. Is the MTBE you distilled pure MTBE or a mixture of MTBE/toluene, explain your answer.
5. How does the extreme volatility of MTBE during the extractions affect your recovery?
6. Describe how temperature affects solubility of a compound in a good recrystallization solvent versus poor recrystallization solvent?
7. Why do the impurities stay in solution?
8. Where is your major yield loss in recrystallization, refer to answer to question 6 above?
Lab Report Outline
Include theory of extraction, distillation, and recrystallization. (1/2 page for each).
Correctly identity both compounds that were in your unknown mixture
% recovery of both compounds
% recovery of your MTBE and toluene from distillation
It would probably be a good idea to start working on your lab report throughout this three week lab instead of waiting for the last week to start it.