Experiment 1: Recrystallization and Identification of an Unknown Solid
I. Recrystallization
Procedure: You will be given an unknown solid compound to purify by recrystallization using water as the recrystallizing solvent. The unknown will either be benzamide or 2- ethoxybenzamide and will contain a water-soluble organic impurity and a black carbon insoluble impurity. Note that the melting point ranges of benzamide and 2- ethoxybenzamide are quite close, so good technique will be required to distinguish between them. O O
NH NH2 2
OCH2CH3
benzamide 2-ethoxybenzamide
Pertinent information about the chemicals you are using: Reagents MW (g/mol) MP (ºC) BP (ºC) Density Benzamide 128-130 2-ethoxybenzamide 132-134 Water 18 0 100 1
Throughout this experiment, clean all glassware and equipment before using it. To clean it, wash with soap and water, rinse with water, and shake as much water as possible from it. If soap and water does not completely clean it (i.e. there is still some visible residue in it), wash with a little acetone then with water. The residue is likely organic material that the acetone will remove. This cleaning technique can be used in this experiment because water is being used as the solvent; a trace of water in the glassware will do no harm.
Recrystallization of a Solid Compound: Dissolving the Solid: 1. Place a clean 250-mL Erlenmeyer flask containing approximately 100-mL water and 3- 4 boiling stones on a hot plate set to about 6; this will be used as your source of hot water when dissolving your compound for the experiment. If necessary, decrease the hot plate setting a little once the water is boiling.
• Do we have to dry all the glassware when we clean it? Because we are going to put water into it, this flask does not have to be dry. A few more drops of water won’t make any difference. In this and other experimental procedures in our organic chemistry courses, the following distinction should be noted regarding glassware and other apparatus. “Clean” means to wash with soap and water and then with plain water, followed by a gentle shaking to remove as much water as possible. “Clean, dry” means to clean as mentioned above in the previous sentence, followed by rinsing with a couple “squirts” of acetone to remove the water, followed by drying on the laboratory dryer.
2. Record the identification number of your unknown compound in your notebook. Weigh approximately 1 gram of the unknown compound using a weighing boat and place it in a 125-mL Erlenmeyer flask containing 3-4 boiling stones. Record the precise mass of your compound in your notebook. Observe what the unknown looks like, prior to recrystallization.
• How accurately do I have to make the measurements of weight and volume in all experiments? If a volume is specified, such as “5 mL”, this may be measured with a graduated cylinder – that is, the volume is somewhat approximate. On the other hand, if you were asked to add 5.0 mL, you must measure it more accurately and the use of a syringe or other calibrated device would be required.
In this experiment, the exact mass of the compound used is not critical (hence the instruction “about 1 gram”) but you must weigh and record precisely (to 3 decimal places) how much you used. In other experiments the exact mass will be more critical and will be reflected in instructions such as “weigh out 1.00 g of compound X”. The mass of a compound that is required (i.e. 1-2 grams) means you must weigh it and get as close to the specified amount as possible. A weighing boat, or a piece of weighing paper is placed on the balance and then the balance is zeroed (this is also called “taring”). The weighing boat is then removed and some compound is placed on the paper and the paper is then placed on the balance again to see how much has been added. This process is repeated until the correct mass has been reached. In most cases, we try to avoid weighing compounds directly on the balance. Students very often “miss” the weighing paper and get organic compounds on the balance pan, which degrades the balance itself. It’s less messy to not weigh on the balance directly, especially when you are new to using balances.
When weighing liquids, it is preferable to use a disposable glass vial or a small flask instead of a weighing paper or boat. Care must be taken to keep chemicals from getting on the balance and any spills must be cleaned up immediately.
Last comment – You must always know how much of each component you are using, so make sure to record this data. This is necessary so that work can be duplicated and so you can calculate the theoretical yield of your product in a reaction.
3. Using a 2 mL Pasteur pipette (which can suck up about 2 mL using a standard rubber bulb, when the bulb is entirely deflated), add about 10-mL of boiling hot water to the flask with the unknown solid and place the flask containing the sample on the hot plate. Using a Pasteur pipette, add hot water in small portions until the unknown solid just dissolves; use a minimum of hot water. Make sure the solution maintains boiling during the addition of these additional portions of water. Remember, the unknown contains traces of black carbon that will NOT dissolve. Add only enough hot water to dissolve the whiter crystalline solid. Then add 2-3 milliliters extra to prevent premature crystallization during the hot gravity filtration.
• The water isn’t boiling but it is warm – can I get started now? Recrystallization involves the use of hot solvents, in this case water is the solvent. The hotter the solvent, the more soluble your compound will be in the solvent. So, the hotter the solvent, the less solvent you need to dissolve the compound. You must have a saturated solution in order to get back a maximum amount of crystals from your process. The more solvent, the less saturated. Keep the water boiling hot the entire time and leave the Erlenmeyer flasks on the hot plate. Some students tend to remove them and set them on the bench top, not realizing that they are now cooling off!
The extra water added at the end, to prevent premature crystallization, is important for the hot gravity filtration step. You will be using a piece of filter paper – a DRY piece of filter paper. If you pour a saturated solution through the dry filter paper, the filter paper will absorb some of your water and your solution will become super-saturated! This could lead to premature crystal formation in the powder funnel of your filtration set-up.
Filter the Insoluble Impurities using the hot, gravity filtration technique.
1. In a clean 125 mL Erlenmeyer flask place 5 mL of distilled water, and place a plastic powder funnel in the top with an appropriate-sized piece of “fluted” filter paper. Stand the assembled apparatus on the hotplate and wait until the water in this flask is boiling. Pre-wet the filter paper with a few milliliters of water (only 2-3 mL).
• Why gravity filtration with fluted filter paper? Gravity filtration using filter paper is required to remove the finely divided charcoal impurity in your sample. Other types of filtration (such as vacuum) would result in the product being contaminated with the charcoal, resulting in the formation of grayish- colored crystals. Fluted filter paper (as opposed to the “cone”) is commonly used because it provides more surface area through which the solvent can flow, allowing the filtration to occur quicker.
• Why do we heat the funnel/filter paper prior to gravity filtration? Hot gravity filtrations prevent premature crystallization of the product in the funnel. It is important that the entire apparatus be hot to avoid this – the steam generated by the boiling water will warm the flask, the funnel and the filter paper. The small amount of hot water already present in the filtration flask will not prevent the compound from recrystallizing on cooling (or lower the percent recovery) significantly.
• I’m filtering a solution with water in it – why do I need to wet the filter paper? Pre-wetting the filter paper will help prevent premature crystallization from occurring. It will help keep the saturation level of your solution uniform, as the filter paper won’t be removing any of your solvent.
2. Pour the solution containing your dissolved unknown through the filter, allowing the liquid to drain into the Erlenmeyer flask. Do not overfill the funnel - if the entire solution does not fit, place the flask back on the hotplate and add the remaining solution to the filter in portions. Remember to keep the solution hot the entire time.
Crystallizing and Isolating the Compound: 1. Once all the solution has filtered, allow the filtrate to cool by removing the flask from the hotplate and placing it on the benchtop for at least 5 minutes. Then place the flask in a 400 mL beaker of ice and water (called an “ice-water bath”) until the solution has reached ice-bath temperature and crystallization is complete.
2. While the flask is cooling in the ice-water bath, prepare a clean filter flask fitted with a rubber cone adapter and a clean Büchner funnel fitted with the appropriately sized filter paper for use in the next step. Attach the vacuum hose (heavy walled tubing attached to the vacuum control) and clamp the flask to ensure it does not tip over.
• What if I don’t see any crystals form? Sometimes the crystals have a difficult time forming a lattice and crystallization can be slow. One option is adding a “seed crystal”, a small crystal of the compound you are trying to crystallize. The dissolved compound now has a surface on which to crystallize, and the compound will rapidly “crash out” of solution. Since we don’t know what the unknown compound is yet, this isn’t an option for us. Alternatively, we can “scratch” the bottom of the flask with a clean glass stirring rod. To do so, hold the glass rod firmly and stir the solution, moving the glass rod around the bottom surface of the flask. It should feel like you are scraping the surface of the glass flask. The small scratches can provide a roughen surface on which the dissolved compound can crystallize.
• Why clean the filter flask? Filter flasks should always be cleaned before using in the event that product gets into the filtrate. If this happens, the contents of the filter flask can be re-filtered without contaminating the product.
• How cold is cold? To maximize the amount of crystals to be recovered, the colder the better. Solid compounds are generally less soluble in colder solvents, so a colder solution will typically result in a higher yield of crystals. Unfortunately, it takes a long time to cool to ice temperature (0ºC). A good rule of thumb is to cool the solution to 5-10ºC. You won’t get much more product by waiting longer.
3. When the solution is cold and crystallization is complete, turn on the vacuum to the filter flask, “seat” the filter paper on the Büchner funnel by pouring a couple mL of water on it and press the funnel down on the filter flask so a seal is made between the rubber cone adapter, the flask and the funnel. Swirl the contents of the crystallizing flask to suspend the crystals, and then pour the mixture as quickly as possible onto the filter funnel.
• Why vacuum filtration? Vacuum filtration is used to remove a solid compound from a solvent. On the other hand, hot gravity filtration is used to remove traces of insoluble impurities from a solution.
4. Rinse the product remaining in the crystallization flask with 1-2 mL of ice-cold water from the ice bath and pour it onto the product on the Büchner funnel. Repeat 3-4 times to complete the transfer of the material. Solid stuck on the sides of the flask may be loosened with a clean, glass stirring rod.
• Why rinse the flask? This will get as much solid as we can and will help to maximize the recovery. The solvent (water) must be cold to avoid re-dissolving the product.
5. Allow the material to dry somewhat by pulling air through the product on the Büchner funnel for a few minutes. In the meantime, wash all glassware and clean your work area. Carefully place the top of the Büchner funnel containing the solid in your drawer to dry until the next lab period. Do not cover the funnel - leave open to the air to facilitate the drying process.
Waste disposal and Clean Up: a. Filtrate from the filter flask – wash down the drain b. Filter paper – place in trash can c. Clean and put all glassware and equipment in proper locker. Wash and dry your work area.
End Day One, Lab 1.
Day Two: 1. During the next lab period, obtain a weight on your purified crystals. Pre-weigh a clean dry sample vial and record this weight in your notebook. Using your plastic powder funnel, transfer the crystals from the Büchner funnel to the pre-weighed sample vial and reweigh. Observe what your purified crystals look like. 2. Then determine the melting point of the solid and identify the unknown using the mixed melting point technique, as described in Part II below.
II. Melting Points
Read Zubrick, Chapter 12 for background information on the technique of melting point analysis
Since the melting point ranges of the two compounds, benzamide and 2- ethoxybenzamide, are so close to each other in value, a simple melting point analysis will not be sufficient to unambiguously determine the identity of your unknown. Remember that even a small amount of impurity can cause a drop in temperature and confuse your results.
You will do four melting point analyses. Your first analysis will be a quick, fast run (sometimes called a “ballpark run”) that will be taken intentionally too fast to get an estimate of the melting point temperature. Your second analysis will be a slower, more accurate run to assess purity of your compound, and your last two analyses will be mixed melting point runs to determine the identify of your unknown.
Procedure: A. Determine the melting point (purity) of your recrystallized compound. To first determine purity of your recrystallized product, load two capillary tubes with small amount (1-2 mm of material only) and pack as shown in prelab lecture. Perform one fast run and one slow run, as described by your instructor. Set-up your Digimelt with the following parameters for each run:
Fast Run Parameters for Digimelt apparatus: START TEMP: 60ºC RAMP RATE: 20ºC/min STOP TEMP: 140ºC.
Slow Run Parameters for Digimelt apparatus: START TEMP: (Begin 10ºC below where your Fast Run began) RAMP RATE: 2ºC/min STOP TEMP: 140ºC.
For set up and operation of the Digimelt instrument used for melting point analysis: https://tigerweb.towson.edu/jdiscord/www/331_lab_info/digitalmeltempdirections.pdf
Your Slow Run will give you information on the PURITY of your compound. When you discuss your results, do not include any discussion from the fast run as it is just an inaccurate estimate. Remember the “purity” of the compound is determined not only by having a small range but also by having a melting point that falls within the known melting point range.
B. Determine the identity of your unknown compound
In order to confirm the identity of your unknown compound, you will perform two mixed- melting point runs. A porcelain tray has been provided for you; in one of the three wells, mix a small portion of your unknown compound with an equally small portion of benzamide. In a second well, you will mix a small portion of your unknown compound with an equally small portion of 2-ethoxybenzamide. Be sure not to cross contaminate; clean your spatula between uses. Use the end of your glass, stirring rod to thoroughly mix the mixtures together – a mixed melting point analysis only works if you are truly taking the melting point of a mixture! Use one end of the stirring rod for one mixture and then flip it over and grind up the other mixture with the other end. Again – do not contaminate one mixture with the other. Just as you did for your other runs, load one capillary tube with the smallest amount you can still see of one mixture and then repeat for a second capillary tube and the second mixture. You may wish to LABEL the tubes so you don’t get them confused. Pack the capillary tubes correctly and run these analyses SIMULTANEOUSLY, using the same parameters for the Digimelt as you used for your slow run. 1. Your unknown mixed with benzamide – one run 2. Your unknown mixed with 2-ethoxybenzamide – one run
Your instructor will demonstrate how to use the Digimelt melting point apparatus. On the basis of your mixed melting point data, identify your unknown compound.
Calculations: Determine by difference the weight of the pure solid in grams (g) and the percent recovery, which is defined as:
Percent Recovery = [(mass of pure sample) / (mass of impure sample)] x 100