CHEM 230L _ exp 2. Recrystallization

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Jan 9, 2024

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CHEM 230L: Organic Chemistry I Lab Chapman University Experiment 2: Purification of Organic Compounds Using Recrystallization and ID by Melting Point Intended Learning Outcomes By completing this lab, we will:  Understand the relationship between compound structure, solubility, and temperature and the role they play in recrystallization.  Learn how to purify impure solid organic compounds by the method recrystallization.  Be able to determine the purity of organic compounds and the identity of unknown organic compounds by using melting point. Introduction Organic compounds that we use every day such as medicines, materials, food products, and fuels are either synthesized in a lab or isolated from nature. When synthesized in the lab, desired organic compounds are often mixed with impurities from side reactions or unreacted starting material. Similarly, when organic compounds are isolated from nature they are often found in complex mixtures. To make optimal use of organic compounds, we need to be able to isolate and purify them There are many methods for purifying organic compounds and one of the best ways to purify solid organic compounds is by using recrystallization. Recrystallization utilizes solubility, which you explored in some detail during our first experiment. In addition, recrystallization employs the fact that different compounds have different degrees of solubility in the same solvent at different temperatures. An impure solid can be dissolved in a solvent at high temperature while also dissolving the impurities present. To successfully recrystallize an impure solid, the recrystallization solvent needs to be chosen carefully. The desired organic compound must be insoluble at low temperatures and soluble at high temperatures. If the desired compound is too soluble at room or low temperature, recrystallization may be very difficult and percent recovery will be low. Ideally, most impurities will dissolve and stay in solution even at low temperatures. If the proper solvent is chosen, the desired organic compound can be crystallized and recovered while leaving the impurity behind in solution upon cooling down. Pre-Lab Reading The reading below needs to be completed before the start of lab. 1. Lab textbook (Pavia, 6 th edition): Technique 11 – Crystallization: Purification of Solids Part A (Theory) - page 696-699 Part B (Semimicroscale Crystallization) - page 699-704 2. Lab textbook (Pavia, 6 th edition): Technique 9 – Physical Constants of Solids: The Melting Point Section 9.1 to 9.4 - page 678-680

CHEM 230L: Organic Chemistry I Lab Chapman University Pre-Lab Assignment (15 points) Answer the following questions: 1. If the solubility of a solid in acetone is 125 mg/mL, how many mL of acetone is needed to dissolve 300 mg of the solid? (3 pts.) 300 mg 1 mL = 2.4 mL 125 mg 2. If 250 mg of impure solid is purified by recrystallization and 100 mg of the pure solid crystals are recovered after cooling in an ice bath, what is the % recovery for this recrystallization if the impure sample was 15% impurity by mass? (4 pts.) 100 - 15% impurity = 85% pure solid of 250 mg  0.85*250 mg = 212.5 mg  100mg/212.5mg = 0.471*100 = 47.1% 47.1% 3. A student dissolves 124 mg of sulfanilamide in 95% ethanol/water by directly adding 25 mL of 95% ethanol/water to the sulfanilamide. The student then boils the solution until all solid dissolves, cools the solution to room temperature, and then places the flask in an ice bath for 20 minutes. Upon removing the flask from the ice bath, they notice that no crystals have formed. What did the student do that resulted in no crystals forming? Explain how this mistake caused the recrystallization to fail and describe how they might be able to correct this mistake? (HINT: check the solubility of sulfanilamide at 0 o C in 95% ethanol/water). (4pts.) At 0 C, the solubility of sulfanilamide in 95% ethanol/water is 14mg/mL. Given that the student dissolves 124 mg of sulfanilamide, this means that he only needed about 9 mL of 95% ethanol/water (124 mg/14 mg = 8.86 mL). Since the student added an excessive amount of 95% ethanol/water of 25 mL, it may have made it difficult for crystals to form. To correct this mistake, they can restart the crystallization by adding a minimal initial amount of solvent (1 mL) and adding minimal amounts throughout the dissolving process until the solute has completely dissolved. 4. Explain why the slow cooling of the hot solution with all of the sample dissolved is necessary to increase the purity of the final crystals that are isolated. (4 pts.) The slow cooling of the hot solution with all of the sample dissolved is necessary because it disables the impurities from being included with the resulting crystals and gives us a higher chance of obtaining pure crystals. It also gives the crystals more time to collide with each other to give a higher yield of larger crystals and a greater purity. Procedure Purification of Organic Compounds Using Recrystallization

CHEM 230L: Organic Chemistry I Lab Chapman University A. Recrystallization of an impure organic compound. 1. Choose an impure sulfanilamide sample and add 300 mg of the impure sulfanilamide sample into a 10 mL Erlenmeyer flask. It is if you cannot measure out exactly 300 mg so long as you are close. Record the actual mass used for the recrystallization as well as a description of the appearance of the impure sample (color, granule shape, etc.) 2. Add 1.00 mL of 95% ethanol to the Erlenmeyer flask and begin to heat the Erlenmeyer flask slowly on a hot plate while swirling the flask periodically until the solution begins to boil gently. 3. Check to see if all the sulfanilamide has dissolved. If solid sulfanilamide is still present, slowly add more 95% ethanol (5-10 drops at time) to the solution while still swirling and heating until all of it dissolves. Make sure to swirl and wait 1-2 minutes before adding subsequent aliquots of solvent (using minimal solvent will increase recovery). 4. Once all the sulfanilamide has dissolved, move the flask off the hotplate and onto the benchtop and let it cool down to room temperature. Cover the flask to prevent particles in the air from falling into the flask. While waiting, prepare an ice bath in a small beaker. 5. After the flask has cooled to room temperature, add it to the ice bath carefully. Secure the flask with a clamp to ensure that it does not fall into the bath. Leave the flask in the ice bath for a minimum of 20 minutes (leaving the flask for longer will slightly increase the amount of crystals formed). B. Recovery of purified crystals. 1. Connect a vacuum flask to the vacuum in the fume hood using a thick vacuum hose. Add a Hirsch funnel fitted with a gray rubber filter adapter to the vacuum flask and turn on the vacuum line. Pour the contents of the Erlenmeyer flask onto the Hirsch funnel and wash the crystals with 2 small aliquots of cold 95% ethanol in water. Note the color of the solution that is now in the vacuum flask. 2. Cover the Hirsch funnel with a clean beaker to prevent particles in the air from getting onto the crystals. Keep the crystals under vacuum for a minimum of 15 minutes to ensure that all solvent has been removed from the crystals. 3. After the crystals are sufficiently dried, tare an empty weigh boat, and transfer all the crystals from the Hirsch funnel onto the weigh boat. Record the mass of the recovered crystals and a description of the crystals (color, shape, etc.). C. Determining crystal purity by melting point. 1. Pack some of the isolated crystals into a small capillary tube by poking the open end of the capillary tube into the crystals. Drop the capillary tube through a hollow plastic pipette with the closed end down to pack the precipitate tightly. 2. Follow the MelTemp procedure posted near the MelTemp machine to obtain the melting point of your crystals. Record the melting point of your crystals.

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CHEM 230L: Organic Chemistry I Lab Chapman University Identification of an Unknown Organic Compound by Melting Point 1. Obtain an unknown sample and determine the melting point of this pure sample by using the MelTemp machine. Record your unknown sample number and the observed melting point. 2. The unknown compound is either Benzoic Acid, Bezoin, Acetylsalicylic Acid, Succinimide, or o-Toluic Acid. Compare the melting point of your unknown compound with the literature melting points of these five compounds on page 27 of the lab textbook (you may also look these values up online). 3. Mix 25 mg of your unknown sample with 25 mg of the compound that has the closest melting point to the experimentally determined melting point of your unknown. 4.Determine the melting point of the mixture and record this value . If the melting point of this mixture and that of your unknown is the same, then the unknown compound is the same as the compound it was mixed with. If the melting point of the mixture decreases significantly (more than 5 o C) then try another compound to mix with your unknown with until the melting point remains the same. Record the melting point of all tested mixtures . Post-Lab Assignment (40 points) Data Analysis - Recrystallization 1. Enter the mass of your impure Sulfanilamide sample used (1pt.) __________ 2. Describe the physical appearance of the impure sulfanilamide sample (2 pts.) 3. What was the color of the solution that ended up in the vacuum flask (1 pt.) 4. Enter the mass of your purified Sulfanilamide crystals (1 pt.) ________ 5. Describe the physical appearance of your purified sulfanilamide crystals (2 pts.)

CHEM 230L: Organic Chemistry I Lab Chapman University 6. Calculate the % recovery of pure Sulfanilamide from the impure sample that you used. Assume that the impure sample had 10% impurity by mass. Show your calculations. (5 pts.) % recovery ____________ 7. Enter the melting point range of your purified Sulfanilamide crystals (2 pt.) ________ 8. Based off your experimental melting point and the literature value for the melting point of sulfanilamide, how pure do you think your Sulfanilamide crystals are? (3 pts.) 9. Comment on the overall success of your recrystallization (did you have a good % recovery, high purity, etc.). (3 pts.) Data Analysis – Melting Points of an Unknown Compound 1. Enter the ID of your unknown sample used. (1pt.) __________ 2. What is the melting point of your unknown sample? (2 pts.) __________

CHEM 230L: Organic Chemistry I Lab Chapman University 3. What was the first compound that you mixed unknown with to check for its identity and why did you choose this compound? (3pts.) 4. Enter the melting point of the mixture? (2 pts.) _________ 5. Did you mix your unknown with any other compounds? If so, what did you mix it with and what was the melting point? (2 pts.) 6. What was the identity of your unknown? Explain how you came to this determination. (3 pts.) Additional Conceptual Question: 1. Suppose you wanted to recrystallize an impure sample of anthracene (shown below). If you could choose water, 95% ethanol (CH 3 CH 2 OH), or acetone as a recrystallization solvent, which would you choose? Explain your answer. (4 pts.) Anthracene 2. A student is attempting to recrystallize caffeine, but accidently add too much solvent. How can the student remedy this situation if adding more caffeine is not an option? (3 pts)

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