In the Laboratory
Bromination, Elimination, and Polymerization: A 3-Step Sequence for the Preparation of Polystyrene from Ethylbenzene
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Elizabeth M. Sanford* and Heather L. Hermann Department of Chemistry, Hope College, Holland, MI 49422-9000; *
[email protected] Adding new topics to the organic lecture and laboratory curriculum to improve the course is a desirable goal. Making room for these new topics, however, is difficult because we are often justifiably unwilling to sacrifice components of the current course content. With this in mind, we developed a polymer chemistry lab that introduces students to the importance of polymer chemistry, links organic and polymer chemistry, produces a polymer that can be characterized by gel permeation chromatography (GPC), and still exposes students to three classic reactions covered in first-semester organic chemistry. Although many polymer experiments have been reported, none met our criteria. To link the synthetic concepts of organic chemistry to the preparation of polymers and make the actual experiment technically nontrivial, we wanted our students to prepare their own monomer. We also desired a THF-soluble polymer, so GPC analysis would be possible. This made the preparation of polystyrene a natural choice. The majority of published polystyrene labs, however, start with commercial styrene, which did not meet our pedagogical criteria. The only polystyrene lab that we felt was suitable is an excellent one developed by Wilen, Kremer, and Waltcher that involves the preparation and polymerization of styrene using benzene as a starting material and covers acetylation, reduction, elimination, and radical polymerization in a 4-step reaction sequence that takes 4 weeks of lab (1). Unwilling to consume that much course time, we developed a 3-step, 2-week experiment in which students prepare and polymerize styrene and characterize polystyrene.
nation to produce alkenes is an important first-semester organic reaction sequence that existed in our curriculum and that we wanted to maintain. The NBS bromination of ethylbenzene gave us the opportunity to introduce a radical reaction. The bromination and subsequent elimination reaction are particularly useful in pointing out the special reactivity of the benzylic position. The elimination reaction of (1-bromoethyl)benzene with quinoline is useful in helping students understand the relationship between substitution and elimination reactions and between E1 and E2 elimination reactions in a discussion of the use of quinoline as the base and solvent. The radical polymerization of styrene was used both to introduce the mechanism of the reaction and the concept of synthetic macromolecules and as an opportunity to give a brief history of and introduction to polymer chemistry. We also used this experiment to introduce students to precipitation, molecular weight determination (GPC) (6 ), and casting films to record IR spectra. Those without access to GPC equipment can do molecular weight analysis with thin-layer chromatography (TLC) (7).
The Experiment
This experiment was successfully implemented during the later part of the first-semester sophomore-level organic sequence. Two weeks of lab time (10 hours total) was devoted to the experiment, although students needed only about 7 hours to do the entire experiment including characterization. Approximately 100 students completed the experiment under the direction of 5 different instructors. The Mw of the polystyrene obtain ranged from 3,000 to 230,000, while Mn ranged from 2000 to 110,000. Polydispersities ranged from 1.1 to 2.7. Students were allowed to choose the amount of initiator within a range given for their polymerization. Yields of polystyrene ranged from trace amounts to 56%. Although these yields are low, even students with trace amounts of material were able to characterize it. Pedagogically, we feel that the experiment met our goals. Students also responded to the experiment very positively, rating it the favorite synthetic lab of the semester (identification of an organic unknown was the overall favorite). Reasons why included such comments as “I enjoyed doing a multistep synthesis which applied many lecture covered reactions”; “It was interesting to make a practical product”; and “Polymers are used in everyday life and we made them. GPC is cool!”
Students complete a radical bromination of ethylbenzene (2, 3) and the subsequent elimination reaction of (1-bromoethyl)benzene (4) to produce styrene; this is followed by radical polymerization (1, 5) to produce polystyrene as shown in the following scheme. CH2CH3
CHBrCH3
NBS, (C6H5CO)2O2 hexane
e
lin
ino
qu
CHCH2 CHCH2
AIBN toluene
n
After purification by precipitation, the polystyrene was characterized by IR and GPC. Halogenation followed by elimi-
Equipment A GPC system including a sample changer was used in this experiment. Relative molecular weights can be determined simply and inexpensively using thin layer chromatography (7). Results
JChemEd.chem.wisc.edu • Vol. 77 No. 10 October 2000 • Journal of Chemical Education
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In the Laboratory
Procedure
Week 1: The Preparation of (1-Bromoethyl)benzene Add ethylbenzene (5.0 mL, 0.041 mol) and 50 mL of hexane to a 100-mL round-bottom flask containing a magnetic stir-bar. Add N-bromosuccinimide (NBS) (7.5 g, 0.042 mol) and benzoyl peroxide (0.25 g, 0.0010 mol). C AUTION : Benzoyl peroxide is explosive and may detonate upon heating or with friction. Weigh out on glassine paper, do not use a metal spatula, and avoid contact with ground-glass joints).
Equip the round-bottom flask with a water-cooled reflux condenser and bring to reflux for 1 hour. Cool the flask to room temperature. Remove the solid succinimide using vacuum filtration, making sure to rinse the solids with a few milliliters of hexane. Add the filtrate (mother liquor) to a roundbottom flask and distill off the hexane. Transfer the crude product to a tared vial and cap tightly for week 2’s experiment. Determine a crude yield. Using a few drops of ethyl benzene and a few drops of the crude product, determine the density of each relative to water. Confirm halogen content with an ethanolic silver nitrate test.1,2
Week 2: The Preparation of Styrene and Polystyrene Add all of the crude (1-bromoethyl)benzene and 10 mL of quinoline to a 25-mL small-scale distillation flask equipped with a thermometer and a sidearm test tube. Upon heating with a heating mantle, elimination will occur, producing styrene. The styrene will distill out of the reaction mixture. Discard any low-boiling material and start collecting the product fraction at 120 °C. Discontinue distillation if the head temperature exceeds 160 °C. While the distillation flask is still hot, add toluene to the flask and dispose of the reaction residue in the waste container. C AUTION : The toluene may splatter.
Transfer the styrene distillate to a tared 25-mL round-bottom flask. Determine the weight of styrene. Dilute the styrene with 10 mL of toluene and add between 20 and 60 mg of 2,2′azobisisobutyronitrile (AIBN). Attach a water-cooled reflux condenser to the round-bottom flask and heat the solution to reflux for 1 h with a heating mantel. Cool to room temperature and pour the reaction mixture into 150 mL of rapidly stirred methanol. Isolate the precipitated polysytrene by vacuum filtration. Record an IR spectrum and determine the molecular weight by GPC or TLC.2–6 Acknowledgments This material is based upon work supported by the National Science Foundation under grant CHE-9701800, with additional support from a Howard Hughes Medical Institutes grant to Hope College. Thanks are extended to
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the Hope chemistry students who provided the data for this publication. W
Supplemental Material
Additional background information and a detailed procedure are available in this issue of JCE Online. Notes 1. If students have not distilled the majority of the hexane from the (1-bromoethyl)benzene, their product will not be more dense than ethylbenzene. 2. Students qualitatively characterized (1-bromoethyl)benzene with a density comparison and a chemical test and styrene by boiling point in this experiment. A more complete characterization of the intermediates can be conveniently performed by GC, GC–MS, or NMR. Impurities detected by a more rigorous characterization can then be discussed in the context of chain transfer.6 3. Technical grade quinoline was used for the elimination reaction. 4. If toluene is not added to the quinoline residue while it is hot and disposed of immediately, an intractable material that cannot be removed from the flask forms. 5. The most common student mistake in this lab was to add the wrong amount of AIBN by orders of magnitude because students are unfamiliar in working with milligram quantities. Students who made this mistake did not get a precipitate when the reaction mixture was added to methanol because such low molecular weight material was produced. 6. The purpose in having students add different amounts of AIBN was to make the experiment discovery based. However, a good correlation between amount of initiator used and molecular weight was not found. This was most likely due to inaccurate styrene yields. Many students did not separate lower-boiling impurities such as hexane from their styrene, and if any unreacted ethylbenzene remains in the reaction mixture, it codistills with the styrene. The lack of correlation between molecular weight and initiator/monomer ratio can be used to discuss the concepts of chain transfer due to monomer impurities as well as oxygen and conversion—two important concepts in polymer chemistry.
Literature Cited 1. Wilen, S. H.; Kremer, C. B.; Waltcher, I. J. Chem. Educ. 1961, 38, 304. 2. Djerassi, C. Chem. Rev. 1948, 43, 271. 3. Abhyankar, S. B.; Strickland, D. W. J. Chem. Educ. 1991, 68, 253. 4. Emerson, W. S. Chem. Rev. 1948, 45, 347. 5. Roberts, R. M.; Gilbert, J. C. Martin, S. F. Experimental Organic Chemistry; Saunders College Publishing: Fort Worth, TX, 1994; p 627. 6. Moore, G. R.; Kline, D. E. Properties and Processing of Polymers for Engineers; Prentice-Hall: Englewood Cliffs, NJ, 1984; p 39. 7. Slough, G. A. J. Chem. Educ. 1995, 72, 1031.
Journal of Chemical Education • Vol. 77 No. 10 October 2000 • JChemEd.chem.wisc.edu
