In the Laboratory
Parallel Combinatorial Synthesis of Azo Dyes
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A Combinatorial Experiment Suitable for Undergraduate Laboratories Benjamin W. Gung* and Richard T. Taylor Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056; *
[email protected] Combinatorial chemistry has become an increasingly important tool in the search for compounds with desired properties, with broad applications in science and engineering (1–6). Its introduction into our classroom and teaching activities at the undergraduate level is important for several reasons. The paradigm of combinatorial chemistry is a powerful research technique that also readily accommodates other desirable educational outcomes. A suitably designed laboratory experience in combinatorial chemistry emphasizes the relationship of structure to molecular properties. It reinforces the concept that data acquisition must often precede a theoretical framework. Finally, it allows each student to work independently, yet leads them to share data and interact collaboratively to reach conclusions. We have been engaged in a program to design laboratory experiments that are compatible with a wide variety of student populations and equipment inventories, yet retain the flavor of the combinatorial approach to doing science. The first combinatorial experiment suitable for secondsemester organic laboratory was reported several years ago (7). A combinatorial synthesis of esters using the traditional
NH2
ⴙ
N
NCl ⴚ
NaNO2, HCl, H2O
X
X
diazonium ion
Fischer esterification experiment was employed, which creates diversity by using different combinations of alcohols and carboxylic acids. The distinct smell of each ester produced served as a biochemical assay for the experiment (7). More recently, a combinatorial synthesis of hydrazones was reported to be suitable for high school and undergraduate laboratories (8). The principle of deconvolution of libraries of mixtures was demonstrated by screening for antibiotic activity against Escherichia coli (8). We have developed an experiment in the parallel synthesis of azo dyes that illustrates the concepts of structure– activity relationships and chemical diversity with vivid colors. Since the compounds are obtained in relatively pure form and the screening of “activity” is visual, the experiment can be readily transported to most laboratories. Both chemistry majors (16 students per lab) and nonmajors (100 students in a one-semester organic chemistry course) have carried out this experiment and both groups were able to become acquainted with combinatorial chemistry. The principle of combinatorial chemistry is illustrated by generating a relatively large number of colorful dyes using only one common reaction, the diazo coupling, and two common reactants with small variations. Each student station is turned into an individual “well” in terms of combinatorial chemistry. At the conclusion of this experiment, students were asked to discuss the relationship between structure and function when comparing the dye structures and the multifiber strips dyed with their own azo dyes. The pedagogical value of this experiment lies in that the structure–function relationship is demonstrated in vivid colors. Instructor-led discussions can be expanded to combinatorial reactions where structure variations lead to improvement in other functions of an organic compound, such as antimicrobial activity or catalytic capacity, et cetera. Combinatorial Synthesis of Azo Dyes
an electron-rich aromatic compound
azo coupling
HO
X N
N
R
an azo dye X = SO3 or NO2
Scheme I. Generalized synthesis of an azo dye.
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Various dyes were considered for a combinatorial experiment that would show a spectrum of colors. Azo dyes are prepared from the coupling of aryl diazonium ions with an activated aromatic compound (Scheme I). Diazotization reactions are discussed in second-semester organic chemistry courses. The preparation of dyes is commonly performed in organic chemistry laboratories (9, 10). Azo dyes can be prepared easily in one laboratory period. This includes the dyeing of the multifiber strip at the end of the lab period. Azo dye preparation turns out to be an excellent reaction to illustrate diversity oriented synthesis. Diversification can be derived from both arenes (Scheme I). Both the substitution pattern (ortho, meta, or para) and the substituents can be varied to produce different coupling products. Furthermore, the starting aromatic compounds are available commercially and are inexpensive.
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In the Laboratory
In the laboratory, the positions of the students are divided into columns headed with letters (A–D, in Table 1) and rows labeled with numbers (1–4, in Table 1). Different columns of the lab bench positions are given different aromatic amines while each row is assigned a unique aromatic compound to couple with the diazonium ion generated from the amine. Each student produces a unique azo dye, whose structure is coded according to his or her lab bench position (A1–D4, Table 1). Some of the azo dyes are known certified colors in the United States. The unknown combinations provide opportunities for “discovery” of new azo dyes with different shades of colors. At the end of the experiment, a multifiber strip is dyed using the student’s own synthetic dye. A sample collection of the dyed strips from this experiment is shown in Figure 1. A total of ca. 60 students from two laboratory classes and a NSF-sponsored workshop for college teachers have performed this experiment. This combinatorial experiment uses a color assay, unlike a previously reported undergraduate laboratory experiment, which uses
Figure 1. A collection of dyed multifiber strips from the combinatorial experiment. (This image appears in color on page 1539.)
odor of the products (7). It is a much safer process using color assay for obvious reasons. This experiment is best suited for second-semester organic chemistry laboratory since the diazo coupling reaction is commonly covered in the second semester. However, the simple operation of the experiment and the colorful end results allow this experiment to be used for a one-semester organic laboratory as well.
Table 1. Illustration of the Parallel Combinatorial Synthesis of Azo Dyes A
B
C
D
SO3H
NO2 SO3H
SO3H
NH2
NH2
NH2
NH2
OH
1
A1 (orange II)a
B1
C1
D1 (American flag red)
A2
B2
C2
D2 (magneson II)
A3
B3
C3
D3 (solochrome orange M)
A4
B4
C4
D4 (Easter purple)
OH
2
OH CO2H
3
PhNH
SO3NH4
4
a
Colors in parentheses are certified colors in the United States.
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In the Laboratory
Hazards
Literature Cited
4-Nitroaniline is a highly toxic compound. Students should be instructed to avoid skin contact with arylamines. Diazonium salts are explosive in the solid state and should be kept in solution and used immediately after preparation. Azo dyes are skin irritant. Wear gloves when dyeing fabrics. Sodium hydroxide is caustic. Avoid skin contact. Hydrochloric acid is highly corrosive. Handle it with care. Sodium nitrite is a toxic oxidizer. Naphthol derivatives are irritants. 1-Naphthol is toxic.
1. Liu, D. R.; Schultz, P. G. Angew. Chem., Int. Ed. Engl. 1999, 38, 36–54. 2. Schreiber, S. L. Science 2000, 287, 1964–1969. 3. Truran, G. A.; Aiken, K. S.; Fleming, T. R.; Webb, P. J.; Markgraf, J. H. J. Chem. Educ. 2002, 79, 85–86. 4. Hof, F.; Nuckolls, C.; Rebek, J. J. Am. Chem. Soc. 2000, 122, 4251–4252. 5. Jandeleit, B.; Schaefer, D. J.; Powers, T. S.; Turner, H. W.; Weinberg, W. H. Angew. Chem., Int. Ed. Engl. 1999, 38, 2495–2532. 6. Kuntz, K. W.; Snapper, M. L.; Hoveyda, A. H. Curr. Opin. Chem. Biol. 1999, 3, 313–319. 7. Birney, D. M.; Starnes, S. D. J. Chem. Educ. 1999, 76, 1560– 1561. 8. Wolkenberg, S. E.; Su, A. I. J. Chem. Educ. 2001, 78, 784– 785. 9. Palleros, D. R. Experimental Organic Chemistry, 1st ed.; John Wiley & Sons: New York, 2000. 10. Lehman, J. W. Operational Organic Chemistry, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 2002.
Acknowledgment RTT thanks the National Science Foundation for a grant (0127205) supporting undergraduate instruction in combinatorial chemistry. We also thank graduate teaching assistants, Lizhi Zhu, Godwin Kumi, and the CHM254 (class 2002) and CHM 255 (class 2003) for their enthusiastic participation in this experiment. Supplemental Material Instructions for the students, notes for the instructor, and a sample lab report are available in this issue of JCE Online. W
The structures of a number of the molecules discussed in this article are available in fully manipulable Chime format as JCE Featured Molecules in JCE Online (see page 1680).
Featured Molecules
an interactive modeling feature, Only@JCE Online
http://www.JCE.DivCHED.org/JCEWWW/Features/MonthlyMolecules
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