Laboratory Experiment pubs.acs.org/jchemeduc
A Colorful Solubility Exercise for Organic Chemistry Christopher R. Shugrue, Hans H. Mentzen, II,† and Brian R. Linton* Department of Chemistry, College of the Holy Cross, Worcester, Massachusetts 01610, United States S Supporting Information *
ABSTRACT: A discovery chemistry laboratory has been developed for the introductory organic chemistry student to investigate the concepts of polarity, miscibility, solubility, and density. The simple procedure takes advantage of the solubility of two colored dyes in a series of solvents or solvent mixtures, and the diffusion of colors can be easily visualized. The clear and obvious results help promote an immediate understanding of the relevant concepts, as well as providing a memorable foundation to draw upon in future experiments.
KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Solvents, Solutions, Dyes, Discovery Learning, Laboratory Instruction, Hands-On Learning/Manipulatives
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EXPERIMENTAL PROCEDURE Before the laboratory session, students are asked to think about the polarity of both water and more nonpolar organic solvents. Most are familiar with the phrases that “oil and water do not mix” and “like dissolves like” and use these thoughts to make predictions about the solubility of two dyes in water (polar) or in oil (nonpolar). Methylene blue (Figure 1) is employed as a
olubilities of molecules in organic solvents play a significant role in chemical reactions, as well as being crucial to purification techniques such as extraction. For students beginning their study of organic chemistry, these concepts often seem straightforward in a lecture format but can be more of a challenge to appreciate fully in the laboratory. Practical experience can be the best tool to understand what factors are important to solubility, including which solvents are miscible with each other, and which solvent is going to be on top of the other when solvents do form two layers. A simple discovery chemistry experiment has been designed to guide students through the process of organic solubility in the most visually striking method possible using colored dyes. The obvious and memorable results make it easy to refer back to this series of experiments throughout the semester to reinforce the concepts of polarity, solubility, density, and chemical separations.1 This experiment takes about 30−40 min and has been performed several times in a lab of 30 students during the first laboratory session along with check-in of a second-year undergraduate organic chemistry course. Students have a beginning understanding of the concepts of Lewis structures and polarity, but have not put significant thought into organic structures. Additionally, students are quite familiar with water from their previous chemistry coursework, but have not yet gained a sense that water is not the only colorless, transparent solvent, and that organic solvents are very different from water. The actual experiment requires no other laboratory skills than pouring solvents, transferring small amounts of a premade solid dye mixture with a spatula, and recording observations. © XXXX American Chemical Society and Division of Chemical Education, Inc.
Figure 1. Chemical structures of dyes.
charged, water-soluble dye and disperse red 1 is used as a neutral, organic-soluble dye. Despite a limited exposure to lineangle representations, most students are able to see immediately that one dye is charged and therefore likely to be water-soluble. Each student performs a series of seven experiments where a series of test tubes is prepared containing either pure solvents
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Figure 2. Student results from experiments 1−7: (left) test tubes immediately after addition of the dye mixture and (right) test tubes a few minutes after gentle swirling (1 = water, 2 = ethanol, 3 = ethyl acetate, 4 = water/ethyl acetate, 5 = water/dichloromethane, 6 = water/ethanol, 7 = ethanol/ ethyl acetate).
solvents were indistinguishable as clear liquids but appeared drastically different with the addition of the dye mixture. The addition of the dye mixture to water resulted in the blue color slowly diffusing throughout the water and the red particles settling to the bottom of the test tube. The second solvent was ethanol, which resulted in a deep green color. The last pure solvent was ethyl acetate, where the solution adopted the red color and the blue particles settled to the bottom of the test tube. Most students were easily able to record these observations and come to accurate conclusions about the solubility of methylene blue in water and disperse red 1 in ethyl acetate. Since only blue and red dyes were employed, the most common explanation for the green color in ethanol was that both dyes were soluble. With careful observation of the pure solvents in the next four test tubes, some students were able to discern the two layers in the fourth and fifth test tubes that contained immiscible solvent combinations. Since both layers were clear, many students did not note these observations, but this became remarkably clear when the dye mixture was added. For the test tube containing water and ethyl acetate, the addition of the dye mixture led to a burst of the red color in the top layer, with the blue color settling to the interface between layers. Within moments the blue color started to diffuse downward through the lower layer, as well. Based on their results from the pure solvents, students were able to identify each solvent and some even commented on the density of the two layers. When the dye mixture was added to a combination of water and dichloromethane, the results were reversed, with a blue top layer and red bottom layer. Students were usually able to refer to the bottom layer as the “oil” layer and identify it as dichloromethane, although a few invoked changes in the dye molecules themselves. This provided a nice initial foray into the problematic issue of which layer was the organic layer: it may be the less dense or the more dense layer. The combination of water and ethanol produced a solution that was light blue throughout, clearly showing the miscibility of ethanol and water. A similar experiment with
or solvent combinations, and observations are recorded. The experiments include (1) water, (2) ethanol, (3) ethyl acetate, (4) water/ethyl acetate, (5) water/dichloromethane, (6) water/ ethanol, and (7) ethanol/ethyl acetate. Students add a premade solid dye mixture to each test tube and give it a few seconds to settle, which permits a striking visualization of molecular diffusion as the dyes dissolve and spread through the solution. Students record their observations for each experiment and provide answers to a series of questions that follow the principles of discovery chemistry.2 Each hood also contains two additional premade test tubes, one containing acidic water and ethyl acetate, and the second containing a commercial salad dressing. The last experiment uses a perfluorinated solvent,3 but cost makes it prohibitive for each student to perform this experiment. Each student looks at one vial prepared ahead of time that contains water, ethyl acetate, perfluorooctane, and the dye mixture. The complete procedure is available in the Supporting Information.
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HAZARDS This exercise uses traditional organic solvents, so it is best performed in a hood or ventilated space, using appropriate safety glasses and gloves. Ethanol and ethyl acetate are both highly flammable, while dichloromethane is a skin irritant and suspected carcinogen. Hydrochloric acid is corrosive and can cause burns when in contact with skin or eyes. There are no documented hazards for perfluorooctane. All liquids should be disposed of in a suitable chemical waste container. The dyes are used in small quantities, but care should be taken during cleanup to avoid staining. Methylene blue is an irritant, and disperse red 1 is a skin sensitizer, so gloves are recommended for all manipulation of the dye mixture.
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RESULTS Figure 2 shows the results both immediately after addition of the dye and a few minutes later after a gentle swirling led to the colors being uniform throughout the solutions. The three pure B
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ethanol and ethyl acetate led to one green layer, suggesting that again these solvent pairings were miscible. The color differences were largely due to the red dye being sparsely soluble in aqueous ethanol and more soluble in ethanol/ethyl acetate. The next experiment used acidic water and ethyl acetate. In this case two layers were formed, but the top layer was clear and the bottom layer was a purple color (Figure 3). Students
At the completion of the observations, students answered a series of questions. The objective was to get them to consider the raw data they observed and to start to develop models that would serve them throughout their organic chemistry course work. The follow-up questions are in the Supporting Information.
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CONCLUSIONS When introduced in a lecture format, the concepts of polarity and solubility are not very difficult, but also not particularly memorable. The greatest benefit of this experiment was to build a foundation for a variety of fundamental concepts of organic chemistry. By doing the experiments, students became more invested in the data and relevant conclusions. The striking use of colors provided results that were more likely to serve as a foundation for future learning. Several anecdotal results illustrate the usefulness of this approach. Following a traditional approach, students often referred to every clear liquid as water, but the colors of the dyes made it obvious which layer was water and more importantly which solvent was not. While it was useful to think about many organic solvents being immiscible with water and forming layers, it was also useful to be challenged with some solvents, such as ethanol, that were soluble in both water and organic solvents. Since organic solvents, such as ethyl acetate and dichloromethane, were both used in the organic laboratory sequence, there was often confusion of which solvent was which when both are clear liquids. This experiment demanded that students think about the fact that sometimes the organic layer was on top of water and sometimes it was the other way around. At one point toward the end of the semester during a conversation about extraction, a student remarked “Yeah, blue layer on top”, suggesting that the first experiment of the semester had made an impact. Overall, students had a better command of density as a concept than in previous semesters. While in previous years students were more likely to add water to an extraction to determine which layer was water, following this experiment students were more likely to consider density directly when thinking about the identity of layers. Lastly, the dyes served as indicators of the molecular nature of clearly visible events. How a solute dissolves in a solvent can be discussed in a classroom context, but when these dyes are added to the solvents the colors clearly diffused downward like fireworks. This was fascinating to the majority of students and undoubtedly helped to increase engagement in the experiments, and for some it led to a more detailed discussion of how a collection of molecules separates into individual molecules as the color slowly spreads through the solvents. These experiments produced clear and obvious results, and left a lasting impression on students. This approach encouraged students to engage actively in the discovery of how polarity affects solubility, which is fundamental to organic chemistry laboratory technique. The experiments were easy to perform, and the visual results could be revisited later in the semester to remind students how these concepts were interconnected with other topics throughout organic chemistry.
Figure 3. Experiments with (left) ethyl acetate (top: clear) and acidic water (bottom: purple); (middle) salad dressing; (right) ethyl acetate, water, and perfluorooctane.
were able to make the observations, but were largely at a loss for why the organic layer had little color. It was most helpful to facilitate a discussion from data to conclusions. A clear top layer and purple bottom layer suggested that both dyes were in the lower layer, determined from an earlier experiment to be water. The difference between this experiment and experiment #4 was that the red dye was now soluble in the water. Some students remained fixated on the solvent changing, but many came to the accurate conclusion that the properties of the red dye changed to make it water-soluble. In the laboratory sequence, the following week covered extraction in a more traditional format and students revisited the concept of solubilities changing with chemical reactivity. A commercial salad dressing was included to draw parallels to the chemistry of daily life. Students looked at the salad dressing, then shook it up, and looked again. What were two separate layers became one emulsion with shaking and then settled into two layers again. When students added the dye mixture, after some time the top layer was clearly orange and the bottom layer clearly blue-green. With these results they were easily able to conclude that the water layer was on the bottom and the “oil” layer was on top. The last experiment containing ethyl acetate, water, perfluorooctane, and the dye mixture generated the most student fascination. There were three clearly visible layers, with the top layer being red, the middle being blue, and the bottom remaining clear. Students were quite comfortable with the top two layers, but were typically confounded about why there was a third layer. While this was not a crucial experiment, it was nice to introduce the concept of a fluorous layer and how perfluorinated substances, such as Teflon, do not interact well with either aqueous or organic substances. This result sparked significant student curiosity and was revisited later in the semester during the discussion of polymers and Teflon. Of course, instructors can also choose to point out other seminal examples of multiple immiscible layers.4
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ASSOCIATED CONTENT
S Supporting Information *
Procedures for instructors, handouts, and guided inquiry questions for students along with characteristic answers. This material is available via the Internet at http://pubs.acs.org. C
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Department of Chemistry, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States Notes
The authors declare no competing financial interest.
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REFERENCES
(1) For other recent uses of color in the laboratory see the following: (a) McCain, D. F.; Allgood, O. E.; Cox, J. T.; Falconi, A. E.; Kim, M. J.; Shih, W.-Y. A Colorful Laboratory Investigation of Hydrophobic Interactions, the Partition Coefficient, Gibbs Energy of Transfer, and the Effect of Hofmeister Salts. J. Chem. Educ. 2012, 89, 1074−1077. (b) Stoddard, R. L.; McIndoe, J. S. The Color-Changing Sports Drink: An Ingestible Demonstration. J. Chem. Educ. 2013, 90, 1032−1034. (c) Garber, K. C. A.; Odendaal, A. Y.; Carlson, E. E. Plant Pigment Identification: A Classroom and Outreach Activity. J. Chem. Educ. 2013, 90, 755−759. (2) For a few characteristic examples see the following: (a) Ricci, R. W.; Ditzler, M. A.; Jarret, R.; McMaster, P.; Herrick, R. The Holy Cross Discovery Chemistry Program. J. Chem. Educ. 1994, 71, 404− 405. (b) Jarret, R. M.; McMaster, P. D. Teaching Organic Chemistry with Student-Generated Information. J. Chem. Educ. 1994, 71, 1029− 1031. (c) Horowitz, G. A Discovery Approach to Three Organic Laboratory Techniques: Extraction, Recrystallization, and Distillation. J. Chem. Educ. 2003, 80, 1039−1041. (d) Vittimberga, B. M.; Ruekberg, B. A Discovery-Learning 2,4-Dinitrophenylhydrazone Experiment. J. Chem. Educ. 2006, 83, 1661−1662. (3) (a) Ubeda, M. A.; Dembinski, R. Fluorous Compounds and Their Role in Separation Chemistry. J. Chem. Educ. 2006, 83, 84−92. (b) Zhang, W. Fluorous Linker-Facilitated Chemical Synthesis. Chem. Rev. 2009, 109, 749−795. (4) (a) Hildebrand, J. H. Seven Liquid Phases in Equilibrium. J. Phys. Colloid Chem. 1949, 53, 944−947. (b) Pattle, R. E. Systems of Mutually Immiscible Liquid Layers. Nature 1950, 165, 203−204. (c) Arce, A.; Earle, M. J.; Katdare, S. P.; Rodriquez, H.; Seddon, K. R. Mutually Immiscible Ionic Liquids. Chem. Commun. 2006, 2548− 2550.
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