Recrystallization and Melting Point Determination PDF

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Running Head: RECRYSTALLIZATION AND MELTING POINT DETERMINATION

Recrystallization and Melting Point Determination of N-bromosuccinimide Isra Rashid Nova Southeastern University

RECRYSTALLIZATION AND MELTING POINT DETERMINATION 1. Introduction a. This experiment will be done to use experimental techniques to measure the physical properties of organic substances. These properties include the boiling point, density, melting point for solids, refractive index for liquids, and absorption spectra of organic substances. The melting point is defined as the temperature where a solid becomes a liquid. Melting points are different for differing compounds as there are varying intermolecular forces that are present. The more intermolecular forces, the higher the melting point will be. Recrystallization is a technique done to separate the impurities to have a pure compound separated from the impurities. The purpose of this experiment is to separate the impurities of one impure organic compound, N-bromosuccinimide by crystallization and then assess the purity of the compounds by comparing their melting point ranges to those that are hypothetical ranges found in the text. Afterwards, gravity filtration will be used to isolate the crystals and find their mass. Hypothesis: The impure N-bromosuccinimide acid will have a lower melting point than the purified substance as the impurities decrease the amount of intermolecular forces present. 2. Chemical Table

Structure of Chemical

Name of Chemical

Boiling Point

RECRYSTALLIZATION AND MELTING POINT DETERMINATION

Ethyl Acetate

77.1 °C

Methanol

64.7 °C

Water

100 °C

RECRYSTALLIZATION AND MELTING POINT DETERMINATION N-bromosuccinimide

221.4 °C

Toluene

110.6 °C

Hexane

68 °C

RECRYSTALLIZATION AND MELTING POINT DETERMINATION Methyl Chloride

-24.2 °C

Ethanol

78.37 °C

Hexane

68 °C

3. Glassware and Setup a. Glassware i. 125-mL or 250-mL glass filter flask ii. 600 mL glass beaker iii. 250 mL glass beaker iv. 5 to 10-mL glass graduated cylinder v. 10-mL glass graduated cylinder vi. 125-mL Erlenmeyer flask vii. 25-mL Erlenmeyer flask

RECRYSTALLIZATION AND MELTING POINT DETERMINATION viii. Glass stirring rod ix. Glass funnel with small stem x. 50-mL glass beaker xi. 50-mL glass bottle with solvent-resistant cap xii. 50-mL glass graduated clinder xiii. Glass funnel b. Setup i. Lab stand ii. 3-prong clamp iii. Flask tongs iv. Vacuum Pump with tubing v. Buchner funnel vi. Filter adaptor vii. Medium stir bar viii. Metal tweezers ix. Pasteur pipette bulb x. Melting point capillary tube xi. Boiling chips xii. N-bromosuccinimide (>99% purity) xiii. Ethanol, absolute xiv. Various solvents xv. Pasteur pipettes xvi. 70-mm filter paper

RECRYSTALLIZATION AND MELTING POINT DETERMINATION xvii. 110-mm filter paper xviii. Analytical balance xix. Weighing boats xx. Melting point analyzer xxi. Deionized water xxii. Crushed ice xxiii. Ice container xxiv. Ice scoop xxv. Lab wipes 4. Procedure and Observations a. Before beginning lab, a lab coat, safety glasses and nitrile gloves are required to begin. Perform all steps within a lab hood to prevent exposure to fumes. b. Bring a 125-mL Erlenmeyer flask to the analytical balance and obtain 50 mg of N-bromosuccinimide and bring it back to the fume hood. c. Measure 0.5 mL of deionized water and pour it into the obtained Nbromosuccinimide. d. If the sample does not dissolve, heat the test tube until it boils. If it still does not dissolve, add more boiling water drop wise. . e. If this is not a good solvent, try another solvent to dissolve the sample. Make sure the solvent has at least a boiling point of 40 °C and a boiling point below 120 °C. f. To do a hot filtration, put 15 mL of deionized water in and a boiling chip in a 50mL beaker and heat on a hotplate, then clamp another 125-mL Erlenmeyer flask

RECRYSTALLIZATION AND MELTING POINT DETERMINATION on a hotplate and place a wide stem or funnel into the flask. Fold a filter paper in quarters, put it pointed down in the funnel, and open it up in the cone. g. Once water boils, wet the filter paper with the hot water. Pick the flask of Nbromosuccinimide with tongs and pour the hot liquid carefully so as to not let it overflow. h. Rinse the first flask with hot water and pour the contents into the funnel. i. Once the funnel is empty, move it to the first flask and unclamp the second flask. j. When there are no visible solids in the N-bromosuccinimide solution, turn off the heat and stir and use the to place the flask in the hood table area. Leave the solution for 15 minutes to allow it to cool to room temperature. k. While waiting, measure a small amount of N-bromosuccinimide in a weighing boat and bring it to the melting point apparatus. Pack a capillary tube with the Nbromosuccinimide and measure and record the melting point of it. Once the solution cools, look for crystals, once a seed crystal is found, drop it into the room-temp solution on N-bromosuccinimide and watch the crystal growth. l. Once there are several crystals, prepare two ice-baths in 600-mL beakers, one that occupies all of the beaker and the other only half of the beaker. Measure 5 mL of deionized water and put it in the full ice bath. Make sure the flask is cool to touch and that crystals have formed and place it in half-full ice bath undisturbed. m. Check the flask every 15 minutes and once most of the N-bromosuccinimide is crystalized, set up a vacuum filtration using a 125 mL or a 250 mL flask.

RECRYSTALLIZATION AND MELTING POINT DETERMINATION n. Wet filter with the 1 mL of cold ice water, and vacuum and pour out the contents of flask in the Buchner funnel of vacuum filtration. Scrape crystals into the funnel with 1-mL of cold deionized water. o. Break the seal of the vacuum to stop it, and pour the rest of the deionized water into the funnel, gently stir crystals and wash them carefully so as to not tear the filter paper. p. Turn on vacuum to drain the funnel, break the seal and filtrate into the water. beaker, turn the vacuum on again and leave washed crystals in the funnel as the vacuum dries them. Drying them completely takes about half an hour. q. Measure the mass of crystals and find the melting point. r. Dispose of the crystals in a designated solid waste container and flush out filtrate down the sink with leftover ice and water. s. Clean glassware and put away all other materials. 5. Data and Calculations (% recovery) a. Data Compound

Mass (g)

N-bromosuccinimide

0.5 g

Recrystallized N-bromosuccinimide

0.36 g

Percent recovery

72%

b. Calculations:

RECRYSTALLIZATION AND MELTING POINT DETERMINATION

Percent recovery:

Material recovered ∗100 Starting recovered

Percent recovery:

0.36 g * 100 = 72% 0.5 g

6. Conclusion a. This experiment was done in order to determine the purity of a substance through its melting point and to eliminate impurities of a compound through filtration and recrystallization. b. It was found that after dissolving the compound in its solvent and recrystallizing it, there was a 72% recovery for the pure substance of N-bromosuccinimide. 7. Post-lab Questions a. What is the ideal solvent for crystallization of a particular compound? What is the primary consideration in choosing a solvent for crystallizing a compound? i. The ideal solvent for crystallization of a compound is one that is used minimally. When choosing a solvent, it should also be considered that “like dissolves like,” meaning a nonpolar compound is going to dissolve well in a nonpolar solvent. Other considerations include: the solvent having a lower boiling point, b. Impure benzoic acid was dissolved in hot water. The container of solution was placed in an ice-water bath instead of being allowed cooling slowly. What will be the result of cooling the solution in this manner? i. Cooling quickly will result in more nuclei, resulting in smaller sized, less pure crystals. c. What are the purposes of the following in recrystallization of solids?

RECRYSTALLIZATION AND MELTING POINT DETERMINATION i. Boiling stones 1. Boiling stones are sometimes used to provide a nucleation site so that there is no violent flash boiling. ii. Seed crystals 1. Seed crystals provide a surface for further growth of the crystal. d. Give some reasons why Suction filtration (vacuum) is to be preferred to gravity filtration. i. Suction filtration provides for a much dryer specimen and suction

filtration is also a faster method than gravity filtration....