EXPERIMENT 2: Recrystallization and Melting Point

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1 Recrystallization (or Crystallization) is a technique used to purify solids. This procedure relies on the fact that solubility increases as temperature increases (you can dissolve more sugar in hot water than in cold water). As a hot, saturated solution cools, it becomes supersaturated and the solute precipitates (crystallizes) out. In a recrystallization procedure, an impure (crude) solid is dissolved in a hot solvent. As this solution is cooled, the pure product crystallizes out and the impurities stay dissolved. General Recrystallization Procedure: 1) Choose an appropriate solvent(s) product is very soluble in it at high temperatures product is not soluble in it at low temperatures impurities are either soluble at all temps or insoluble at all temps (can be filtered off) 2) Dissolve impure solid weigh out crude solid and record its mass (also take a melting point for reference) add a boiling chip or boiling stick (otherwise, it may "bump" and spill) use a minimum amount of hot solvent (Add a bit, heat/swirl. Not dissolved? Add more!) NOTE: IF YOU USE TOO MUCH SOLVENT, YOU WILL GET NO CRYSTALS! 3) ONLY IF NEEDED: Decolorize most pure compounds are white and give colorless solutions (looks like water) to remove color (really trace contaminants), add activated charcoal (adsorbs the impurities) 4) ONLY IF NEEDED: Gravity filter to remove insoluble materials (including charcoal!) use fluted filter paper and a hot, stemless funnel NOTE: IF THE SOLUTION COOLS, PRODUCT WILL CRYSTALLIZE OUT & BE LOST! use a small amount of hot solvent to rinse flask, filter 5) Crystallize solute (Finally! This is the RECRYSTALLIZATION part!) cool the solution slowly: hot (boiling) room temperature 0 C (put in ice water bath) NOTE: THIS GIVES LARGE, PURE CRYSTALS & LEAVES IMPURITIES IN SOLUTION may need to scratch glass with a stirring rod or add a seed crystal to start crystallization 6) Collect the pure crystals quickest method is vacuum filtration (Büchner funnel, water aspirator and trap) the impurities will stay dissolved in the solvent that is being removed (hopefully) rinse the pure crystals with a small amount of cold solvent (don't redissolve the crystals!) OPTIONAL: a second crop of crystals can be obtained from the filtrate (mother liquor) 7) Analyze product let crystals dry thoroughly (ideally, this means overnight at room temperature or under vacuum) record mass (how much of your original solid did you recover? % recovery?) record melting point range (did you succeed in PURIFYING your solid?) 9

2 Melting Point is: a constant physical property of a solid (the temperature at which it turns to a liquid) reported in the lab as a melting point range (usually 1 2 C wide), not as a single temperature used for: 1) characterization of a compound (also IR, NMR, mass spec.) 2) identification of an unknown (compare with known mp's) 3) determination of purity If a sample of a compound contains impurities, its mp is usually depressed (lowered) and the range is broadened. Therefore, a narrow mp range (1 2 ) suggests the sample is a pure compound. How can this fact help us in the lab? 1) take a "mixed melting point" of an unknown (if you mix two samples - typically a known and an unknown - and the mp doesn't change, then the two samples are the same compound) 2) take a mp before and after a purification technique (like recrystallization, this week's lab) Method for measuring a Melting Point range: LOAD sample in a capillary (mp) tube (pack the sample down into the CLOSED end of tube) SLOWLY HEAT the sample and closely WATCH both the crystals and the temperature at the first sign of melting (crystals look wet), RECORD the temperature (first number of reported range) when the last crystal melts, RECORD the temperature (second number of reported range) for best results: use a small, well-packed sample and heat slowly to ensure even heating Using the Melting Point Apparatus: (Mel-Temp or Thomas Hoover UniMelt) uses a heating coil in oil (oil is stirred automatically) or a hot plate has a light and a magnifying glass (easy to see crystals) rate of heating is easily adjusted (voltage control) Instructions for use: check to make sure temperature is low before inserting mp tube turn unit on: various switches for light, heat and stirring adjust the voltage control to begin SLOWLY heating your sample (higher number = faster heating) turn all switches off when done; discard mp tube in glass waste container 10

3 EXPERIMENTAL PROCEDURE: (refer to Zubrick text for additional details) Recrystallization: Using a hot plate, dissolve approximately 1.0 g of impure benzoic acid in ml of hot water (water at or near its b.p.) in a 125 ml Erlenmeyer flask. If there is a residual amount of material that does not dissolve upon adding a small amount of additional solvent (H 2 O), do not continue to add more solvent; it is important to use a MINIMUM amount of solvent in a recrystallization. This material is probably insoluble in the hot solvent and will be separated from the hot solution by gravity filtration. If there is any such insoluble solid residue, gravity filter this hot solution through a fluted filter paper using the apparatus shown in your laboratory textbook. Since the glass funnel you have used is much cooler than the hot solution, cooling of the solution will occur, resulting in some crystallization of the benzoic acid in the stem of the funnel and in the filter paper. Preheating the funnel by running a SMALL AMOUNT of hot solvent (H 2 O) through the funnel will reduce the loss due to this crystallization. To further reduce the amount of loss of material, do not fill the funnel with solution but rather add small portions of the hot solution so a minimum amount of solution is in the funnel at once. Allow the gravity-filtered hot solution to gradually cool to room temperature. As this occurs, much of the benzoic acid will crystallize from the solvent. When the cooled solution is at or near room temperature, cool it further in an ice-water bath. Collect the resulting crystals by vacuum filtration, using a water aspirator and the apparatus shown in your laboratory textbook. Transfer the crystals from the filter paper to a tared watch glass (note weight on Data sheet) and let dry thoroughly before weighing. Never weigh filter paper along with the crystals; the filter paper will be wet. Determine the amount of benzoic acid recovered and, based upon the initial amount you started with, determine the percent recovery. Melting Point: Determine the melting point ranges of the impure benzoic acid and the crystallized benzoic acid (after it is dry). Be sure to grind each sample well before introducing it into the melting point tube (you may use a glass rod and a watch glass). Scoop a small amount of the powder into the opening of the melting point tube and gently tap the tube on the benchtop to move the sample down to the bottom (dropping the melting point tube through a long glass tube can also help you pack the sample). If you do not have at least 1 mm of sample, scoop another small portion and repeat. If the column of sample is over 3 mm, discard the melting point tube and start again. Insert the packed melting point tubes into a melting point apparatus. SLOWLY heat the sample (~ 1 C per minute) and record the temperature at the very first sign of melting. Continue to watch the sample and when the sample has melted completely, record the temperature again. EVERY MELTING POINT IS REPORTED AS A RANGE. Waste disposal: Please discard your purified benzoic acid into the container in the hood labeled: Benzoic Acid - Student Prep. Melting point tubes should be placed in the glass waste container. Your complete report for this experiment should include the data page, answers to the following questions, and a conclusion (1 page maximum. Discuss your results yield and mp. Consider: Did the recrystallization work? How can you tell? How could you have improved your yield?). These pages must be stapled. 11

4 Data Sheet Record data in INK. Name: Day/Time of Lab: Instructor: Benzoic Acid Data sample physical appearance melting point range impure pure literature value (cite source) Amount of benzoic acid used initially Tare weight of watch glass Weight of watch glass + pure crystals Amount of benzoic acid recovered Percent recovery of benzoic acid (show method; include units) Your complete report for this experiment should include the data page, answers to the following questions, and a conclusion (1 page maximum. Discuss your results yield and mp. Consider: Did the recrystallization work? How can you tell? How could you have improved your yield?). These pages must be stapled. 12

5 Questions Attach answers to these questions on a separate sheet along with Experiment 2 Report. 1. What are the important solubility characteristics for a solvent for crystallization of an organic solid which is contaminated with a small amount of impurity? Remember you want to separate the desired solid from the contaminating impurities. 2. There are many opportunities during a recrystallization to lose crystals. How could you improve your percent recovery? Explain. 3. Why is it necessary to cool a hot, supersaturated solution SLOWLY during a recrystallization? 4. A 20 g mixture containing 95% (by weight) of A and 5% of B is recrystallized in toluene (b.p. 110 C). Solubilities in g/100 ml are given below. What amount of solvent is needed to obtain pure A and how much pure A will be recovered? Show all calculations and explain your answers. Hint: the goal of a recrystallization is to keep ALL of the impurity dissolved at the low temperature. Knowing that, how much solvent is needed for this example? Temperature Solubility A 1.5 g 10.0 g (g/100 ml) B 0.5 g 8.0 g 5 Why do you need to heat the sample SLOWLY to record its melting point? Give at least two reasons. 6. How can a sample s melting point indicate the presence of an organic impurity? 13