In the Laboratory
The Synthesis of Copper(II) Carboxylates Revisited
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Kevin Kushner, Robert E. Spangler, Ralph A. Salazar, Jr., and J. J. Lagowski* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX 78712-0165; *
[email protected] The growing interest in introducing undergraduate chemistry students into a research ambience (1) has piqued our interest in establishing student activities—primarily laboratory-oriented—that would provide the basis for helping students to begin to learn many of the intellectual and cognitive skills associated with a research environment. Recently, we have shown (2) that the Cognitive Apprenticeship Theory (3) forms the basis for a good model for the situated learning paradigm that exists in many graduate chemistry education environments. It follows, in our view, that the Cognitive Apprenticeship model contains the elements of the environment that should be created if undergraduates are to begin to learn how to “do” chemical research. One of the characteristics of a research group is that most members are working on “several” related problems, which encourages many of the elements of the Cognitive Apprenticeship Theory (2) to come into play. From this point of view, only a few of the published undergraduate laboratory experiments—let alone experiences—are suitable to establish a research-oriented environment in a formal course; in many educational environments found at state-supported researchoriented institutions, formal courses of instruction are the preferred route of education, probably because this environment is easier to manage with the large number of students. Perhaps the earliest example of experiments with a researchoriented focus is Project ACAC, a modest 135 page booklet by Gray, Swanson, and Crawford (4), but that volume is now out of print. Of the experiments published in this Journal, the synthesis and characterization of copper(II) carboxylates (5) has the potential to provide students with a research experience that exhibits the elements of the Cognitive Apprenticeship Theory that define a research environment. The synthesis of a number of different, but related, compounds using several synthetic methods can be the focus for a group of students “all doing the same different” things. In this environment, it is possible to play out the four components of the Cognitive Apprenticeship Theory (2)—domain knowledge, heuristic strategies, control strategies, and learning strategies, but, as is the case with many multiple-section teaching laboratories, the training of the graduate student assistants involved is among the critical issues—if not the issue—for the success of any new program of instruction. Yoder et al. (5) have discussed the general pedagogic usefulness of the copper(II) carboxylates as a suite of compounds that can be used as the focus for undergraduate experiments illustrating two of the important pillars of chemistry—synthesis and characterization. In their article, Yoder et al. stressed the use of conventional chemical methods of synthesis, namely, the reaction of basic copper(II) carbonate with an aqueous solution of an appropriate carboxylic acid; the reaction of the sodium salt of a carboxylic acid, which can be prepared in situ from the acid, with copper sulfate; and the reaction of copper acetate with the carboxylic acid in 1042
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ethanol兾water mixtures. Here, we propose to add electrochemical methods of synthesis to that suite of reactions. Many curricula incorporate discussions of Faraday’s Laws and redox chemistry with, essentially, no indication of the synthetic utility of these subjects; the experiments described here can serve as the basis to remedy that defect. Much of electrochemistry involves the use of inert electrodes, which effectively act as the source of, or sink for, electrons in the redox behavior of solute species, as well as undergoing reactions with one or more species in solution. Electrodes can also be the source of reactive species that are generated in solution as the electrolysis proceeds. The electrochemical synthesis of metal salts of weak organic acids was pioneered by Tuck and his students (6). The synthetic method described here is derived from that work as well as from a more recent report by Banart and Pahil (7). Weak acids such as acetylacetone, acetylpyrrole, and cyclopentadiene as well as carboxylic acids have been employed to produce the corresponding transition-metal “salts” electrochemically using a variety of nonaqueous solvents such as acetone, acetonitrile, methanol, and dimethylsulfoxide (DMSO). We chose acetone as a solvent for the procedure described here because it is readily available in a pure state, it is relatively easy to handle, and it exhibits a low Lewis basicity that prevents the formation of solvates (acetonates) of the desired products. The small quantities of water in the pure commercial product do not seem to adversely affect the results. With most of the weakly coordinated solvents, the presence of water often yields products that are hydrates, which does not affect the general educational value of this kind of reaction. Experimental In a typical experiment, strips of copper metal, 1 mm × 1 cm × 4 cm (approximately 4 g), were used as electrodes— both anode and cathode. The electrolyte solution consisted of 50 mL of acetone, the carboxylic acid, and a supporting electrolyte that carries most of the current. A 100-mL, 3-neck round-bottom flask acted as the electrolysis cell. The two outside necks of the flask were used to support the electrodes using septa through which wire conductors carrying alligator clips on the inside of the flask could be inserted. The central neck of the flask was used to introduce solvents and solutes into the flask. The electrodes were weighed and attached by alligator clips to the copper leads that had been passed through the septa covering the two outer necks of the three-neck flask used as the electrolysis cell. The electrodes were positioned in the electrolyte solution so that only the copper strips were submerged to about two-thirds of their length. Care was taken to position the electrodes to prevent them from touching and to avoid submerging the alligator clips. The electrical connections were then made to the constant-potential, constant-current power supply, again using
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In the Laboratory
alligator clips. The electrolytic solution was stirred continuously during electrolysis with a magnetic stirrer. The power source was turned on and the electrolysis was begun using a potential in the range of 25–50 V. The current through the cell was maintained at about 40 mA with the power source switched to current regulation. As the product forms, the current increases significantly if the potential is held constant. The electrolysis was continued for about 1 to 1.5 h. The electrolytic solution generally turns light blue (indicating the generation of Cu2+ ion) within minutes of starting the electrolysis and becomes darker blue or blue– green as more of the product forms. Depending on the carboxylic acid, the carboxylate product will precipitate during the course of the electrolysis. Some of the carboxylates are soluble in the electrolytic solution and remained in solution during the entire experiment. The variability of the solubility of the copper carboxylate product and the corresponding student accommodation to the method of isolating the product helps create an individual research-like focus for students’ activities. Two classes of products are formed. The less soluble products were allowed to settle, filtered, washed with cold acetone and methanol, air dried, and weighed. For reactions that produced the more soluble products, the solution was removed from the reaction flask and the acetone was allowed to evaporate to dryness. The solid mixture obtained from the synthesis of the more soluble carboxylates, which consisted of electrolyte, carboxylic acid, and the target copper(II) carboxylate was washed with sufficient 4-mL portions of cold tetrahydrofuran to remove the white crystalline mixture of unreacted acid and the supporting electrolyte. Some of the copper carboxylates were also slightly soluble in the tetrahydrofuran wash, which decreased the yield for these compounds. The colored copper(II) carboxylate crystals obtained for both the soluble and insoluble products were then washed with cold methanol and allowed to air dry. After drying several days in a desiccator over an appropriate desiccant, the product was weighed. The cathode and anode, in all cases, were cleaned and weighed. The yield of the product was calculated on the basis of the mass loss of the anode, as corrected by the mass gained by the cathode in cases where the solubility of the products was high. In such instances, copper was deposited on the cathode, which was revealed in the mass measurements of the cathode. Analysis
Water of Hydration A porcelain crucible is heated to redness in a Bunsen flame, allowed to cool in a desiccator, and weighed. Approximately 0.5 g of the compound, weighed to four significant figures, is added to the crucible. The crucible is placed in a crystallizing dish in a 110 ⬚C oven for 1 h and, after cooling in a desiccator, is reweighed. The crucible is then returned to the oven for another hour, reweighed, and this process continued until constant weight is achieved. Copper Copper in the carboxylate product could be determined gravimetrically as copper(II) oxide—CuO. For this method, a sample of the dry carboxylate (∼500 mg) was weighed into www.JCE.DivCHED.org
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a porcelain crucible that previously had been heated to red heat for 15 minutes with a Meeker burner and allowed to cool in a desiccator. The crucible containing the carboxylate was then heated carefully over a 10-minute period to red heat, which was then maintained for 30 minutes; the crucible containing the black copper oxide (CuO) was then allowed to cool in a desiccator, after which time it was weighed and the mass of CuO was obtained by difference. Care was taken to ensure that the carboxylate moiety present in the product was converted to the oxide. This was accomplished by passing a gentle jet of air into the crucible while the carboxylate product was being heated to red heat. In cases where the carboxylate was particularly difficult to oxidize, the crucible containing the combustion product was cooled and 0.5 mL of 30% H2O2 added to the crucible. After 10 minutes, the crucible containing H2O2 reaction mixture was gently heated to evaporate the liquid present and then heated strongly to red heat for 10 minutes, cooled in a desiccator, and weighed. The analysis of copper as the amine complex in these complexes also could be accomplished spectrophotometrically using the classical method as described in the literature (for example, see ref 5 ). Hazards Two kinds of hazards exist in this experiment: the usual chemical-related hazards and electrical hazards. With respect to the latter class, the students focus should be on the power supply and its connection to the house current as well as connections to the electrolysis flask. Electrical leads should be carefully inspected for abrasions or breaks in the insulation because these can be the source of electrical shock and shorts, especially when the leads are handled or may become coated, unknowingly, with water or aqueous solutions. Whenever a power supply or the leads associated with it are handled, the line to the house current should be unplugged. The carboxylic acids employed in this experiment are weak and, accordingly, not corrosive. However, as with all chemical substances in a laboratory setting, they should be handled with care and not ingested. The solvent, acetone, is flammable and should be handled accordingly. Hydrogen peroxide (30%) is a strong oxidizing agent and must be handled accordingly. A more detailed description of the potential hazards associated with this experiment is given in the Supplemental Material.W Results and Discussion The data obtained for the electrochemical synthesis of 18 copper carboxylates are given in Table 1. For the most part, the analytical data (% Cu) correspond to the theoretical expectations for the formulas indicated. The data in the table represent the results of multiple determinations involving three students; these results reported here are consistent with those obtained with ∼75 students in a formal first-year laboratory courses for chemistry majors. A spectrophotometric method could also be employed for the analysis of the carboxylates (5), but we chose to analyze for copper as CuO because of the simplicity of the method and because the method represents a process (decomposition) that is in the
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In the Laboratory
Table 1. Results for the Synthesis and Analysis of Copper Carboxylates Solubility in Acetone
Cu (%)
Acid
Formula
Color
Acetic
Cu(C2H3O2)2⭈H2O
partial
dark blue
31.80
31.53
28.42
Benzoic
Cu(C7H5O2)2⭈2H2O
soluble
blue
18.60
17.37
16.66
Butyric
Cu(C4H7O2)2⭈2H2O
soluble
dark blue
23.48
23.83
23.96
Citric
2CuC6H4O7⭈5H2O
insoluble
light blue
35.30
29.99
36.58
Crotonic
Cu(C4H5O2)2
partial
blue–green
27.65
30.23
33.21
Formic
Cu(CHO2)2⭈4H2O
soluble
light blue
28.20
30.60
40.31
Glutaric
Cu(C5H6O4)⭈2H2O
insoluble
blue–green
27.98
28.53
26.40
Glycolic
Cu(C2H3O3)2
insoluble
light blue
30.03
26.80
31.46
Isobutyric
Cu(C4H7O2)2⭈H2O
partial
blue
25.13
26.36
25.12
L-Tartaric
Cu(C4H4O6)⭈3H2O
insoluble
light blue
24.20
23.16
26.45
Lactic
Cu(C3H5O3)2⭈2H2O
insoluble
llight blue
23.16
24.71
23.23
Lauric
Cu(C12H23O2)2
insoluble
blue–green
13.93
—
12.85
Malonic
Cu(C3H2O4)⭈2H2O
insoluble
light blue
31.80
32.42
—
Oxalic
Cu(C2O4)⭈H2O
insoluble
gray–blue
37.47
42.21
—
Theory
Spec.a
Grav.b
Phthalic
Cu(C8H4O4)⭈H2O
insoluble
blue
26.17
25.32
26.55
Propionic
Cu(C3H5O2)2⭈2H2O
soluble
blue–green
27.91
28.07
27.29
Salicylic
Cu(C7H5O3)2⭈H2O
soluble
green
17.86
14.05
12.25
Succinic
Cu(C4H4O4)⭈H2O
insoluble
blue
32.49
32.93
35.28
a
Spectrophotometric method (5).
b
Gravimetric method.
common experience of most novice chemistry students. We found that the combustion of the copper carboxylates sometimes yielded poor results indicating an incomplete combustion process. Yoder et al. (5) have described the difficulties in establishing the correct formulation for some of the copper carboxylates and we concur with their observations. This experiment has the same pedagogical advantage as the Yoder suite of syntheses in the sense that each student can be assigned a different system (carboxylate) to synthesize. Indeed, the general approach can be used, as we done have in the past in other laboratory courses for science and chemistry majors, as a focus for interested students to develop the synthesis of other carboxylate salts not listed in the table. Adaptation to Local Conditions The experience described here can be manipulated on several levels, depending on local needs, which could include time constraints and the developmental stage of the students, that is, their ability to deal with ambiguity. For example, the experience can be truncated by choosing to do only electrolyses that produce insoluble products (see Table 1), which eliminates the problems of isolation, which might be consonant with the time available for the class and the students’ capabilities of handling ambiguities. There are always the interesting challenges available in this suite of compounds of coaxing a soluble product and a known impurity (the supporting electrolyte) out of solution and separating the impurity. Some might see this as a barrier that slows students down; others will see those problems are reflecting the reality found 1044
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in a relatively large fraction of synthesis experiments. Clearly, this kind of decision is driven by local conditions, philosophical and pedagogical views of laboratory instruction, and perceived student needs. The electrochemical synthesis described here can be accommodated to the “standard three-hour” laboratory period using a variety of approaches, for example, eliminating one or both of the analytical procedures associated with the characterization; however, in our view, a start in establishing a more authentic research environment begins with the realization by students that interesting chemical experiments are not quantized in neat, three-hour blocks. Interesting challenges occur when the rhythm of nature must be accommodated to by the rhythm of human activities. This tension provides the focus for true creativity on the students’ parts. Thus, although this experiment can be shortened, our preference is not to do so, but to maintain the integrity of one of the key activities of chemists—synthesis and characterization. At the other extreme, the experience described here can be enriched—extended—by considerations of the structure of the products obtained. The infrared spectra of the products obtained from the electrolysis and the spectra of the corresponding dehydrated products (obtained after the water analysis) produce data that, together with the spectra of copper compounds of known structure, can be used to advantage in a discussion of the nature of the anion present, for example, in the case of the citrate product, or, perhaps, to decide whether the carboxylate anion is mono- or bidentate. Again, this kind of extension is a function of the perceived local educational needs.
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Acknowledgment We are grateful to the Robert A. Welch Foundation for support of this project in the form of undergraduate scholarships. W
Supplemental Material
2. 3.
4.
Instructions for the students and notes for the instructor are available in this issue of JCE Online. 5.
Literature Cited 1. (a) Moore, J. W. J. Chem. Educ. 2001, 78, 431. (b) Wink, D. J. J. Chem. Educ. 2000, 77, 1549. (c) Doyle, M. P. J. Chem. Educ. 2002, 79, 1038. (d) Garrison, L. J. Chem. Educ. 2003,
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6. 7.
80, 866. (e) Werner, T. C.; Lichter, R. L.; Kruch, T. R. J. Chem. Educ. 2001, 78, 691. Stewart, K. K.; Lagowski, J. J. J. Chem. Educ. 2003, 80, 1362. Collins, A.; Brown, J. S.; Newman, S. Cognitive Apprenticeship: Teaching the Craft of Reading, Writing, and Mathematics. In Knowing, Learning and Instruction: Essays in Honor of Robert Glaser; Resnick, L. B., Ed.; Erlbaum: Hillsdale, NJ, 1989. Gray, Harry B.; Swanson, John; Crawford, Thomas. Project ACAC: An Experimental Investigation in Synthesis and Structure; Bogden and Quigley; Tarrytown-on-Hudson, NY, 1972. Yoder, C. H.; Smith, W. D.; Katolik, V. L.; Hess, K. R.; Thomsen, M. W.; Yoder, C. S.; Bullock, E. R. J. Chem. Educ. 1995, 72, 267. For a comprehensive review, see Tuck, D. G. Pure Appl. Chem. 1979, 51, 2005; and references therein. Banart, J. S.; Pahil, P. K. Polyhedron 1985, 4, 1031.
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