In the Laboratory
The Comparative Nucleophilicity of Naphthoxide Derivatives in Reactions with a Fast-Red TR Dye A Discovery-Oriented Capstone Project for the Second-Year Organic Laboratory Cheryl M. Mascarenhas Department of Chemistry, Benedictine University, Lisle, IL 60532;
[email protected] The second semester of an organic chemistry lab course at this university focuses on organic synthesis. We have observed that towards the end of the course, students have mastered the requisite synthetic techniques and are consequently less challenged. In an attempt to overcome this ennui, a new discoveryoriented student project was designed with the aim of serving as a capstone experience for our chemistry and biochemistry organic majors. The goals for the project were (i) to design a multi-step synthetic experiment that would be easy to perform with discovery-oriented results; (ii) to incorporate concepts from a variety of second-semester organic topics; (iii) to include a variety of instrumentation; and (iv) to assess the impact of the project on student learning. R2 1 6
R1
O 0.1 M NaOH
O
4
Fast-Red TR CH3CN
Theory
1a R1 = H, R2 = H 1b R1 = Br, R2 = H 1c R1 = Br, R2 = Br R2 OH CH3 R1
N
N
Cl
3a–c orange–red color
Scheme I. Competitive reaction of naphthyl acetates with a dye. Note substitution occurs at C1 for 1a and 1b and at the C3 position for 1c. H(Br) OH
O
O Et3N
O
The competitive reaction of naphthyl acetates (1a–c) with a Fast-Red TR dye (4-chloro-2-methylbenzenediazonium chloride) under basic conditions is an ideal capstone project (Scheme I). The project is best suited to an honors chemistry lab section and should ideally be performed at the end of the first-year organic curriculum. The concepts incorporated in the project include (i) reactions of anhydrides with alcohols; (ii) base hydrolysis of esters; (iii) reactions of naphthoxides with diazonium salts; and (iv) the effects of electron-withdrawing substituents on the nucleophilicity and stability of aromatic compounds. We believe that this is the first comparison of reactions between naphthoxide derivatives and diazonium salts in an undergraduate lab course (1). This project was inspired in part by recent reports of the incorporation of a Fast-Red TR diazonium salt into colorimetric solution- and solid-phase combinatorial assays to screen for naphthoxide formation (2, 3). Naphthyl acetates 1b and 1c are not commercially available and were synthesized by the students as part of the project (Scheme II).
CH2Cl2
Br H(Br) O O
Br 1b–c
range of yields obtained by students: 30–61%
Scheme II. Synthesis of naphthyl acetates that were not commercially available.
The diazonium salt Fast-Red TR reacts with naphthoxide derivatives 2a–c to form diazo dyes 3a–c whose colors are orange to red (Scheme III). The reaction is monitored for the formation of orange–red dye over the course of time either visually or through UV–vis spectroscopy. The structure of the diazo dye formed (3a–c) is dependent on that of the substrate and according to the literature (4, 5), substitution favors the C1 position rather than the C3 position in the case of naphthoxides 2a and 2b. This experiment could have been designed to only look at the rate of electrophilic aromatic substitution on naphthoxide derivatives (step 2, Scheme III). However there is greater educational value in including ester hydrolysis in the study (steps 1 and 2, Scheme III) as we were interested in determining whether students can effectively rationalize reaction trends over the course of a two-step, one-pot reaction. The rate of ester hydrolysis is expected to increase in the presence of electron-withdrawing groups (1c >1b > 1a, step 1, Scheme III) (6, 7). However, when students actually perform the hydrolysis reaction coupled with the diazotization reaction (steps 1 and 2), they discover the opposite trend (reaction rate of 1a >1b >1c). This should allow them to conclude that ester hydrolysis is not rate-limiting and that the decrease in nucleophilicity from 2a to 2c dictates the trend observed. Experimental Procedure Students in the class were divided into two groups and each group was divided into two subgroups, with each subgroup assigned the synthesis of either compound 1b or 1c. Thus each group synthesized both compounds 1b and 1c. The project was
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 9 September 2008 • Journal of Chemical Education
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In the Laboratory
performed over a two-week period according to the following schedule: Week 1: Part A—The assigned compound, 1b or 1c, was synthesized and purified. NMRs for compounds 1b and 1c were obtained. Week 2: IRs for compounds 1b and 1c were obtained. Two different concentrations of stock solutions of 1b and 1c were made for the visual color tests and the UV–vis tests. Part B (visual comparative diazo formation) and Part C (UV–vis comparative diazo formation) for compounds 1a, 1b, and 1c were performed.
The syntheses and purification of compounds 1b and 1c were easily accomplished (Scheme II). Complete product conversion was achieved in 30 minutes when the corresponding naphthol was stirred with acetic anhydride in the presence of triethylamine. Compounds 1b (average yield of 45%) and 1c (average yield of 48%) were purified through recrystallization from isopropanol (long white crystals were formed in the case of product 1c). 1H NMRs were obtained in deuterated dichloromethane and provide definitive structural identification. The NMRs are obtained in deuterated dichloromethane rather than deuterated chloroform to avoid overlap of the aromatic region of the esters with the residual chloroform peak. The IR complements the 1H NMR and is an optional step. Students then prepared stock solutions of the newly-synthesized compounds 1b and 1c in acetonitrile. Step 1
R2 O 0.1 M NaOH
O
R1 1a R1 = H, R2 = H 1b R1 = Br, R2 = H 1c R1 = Br, R2 = Br
R2 ź
á
O Na
R1 Step 2
2a–c R2 ź
á
O Na
Fast-Red TR CH3CN
R1 2a–c
R2 OH CH3 R1
N
N
3a–c orange–red color
Cl
Scheme III. Reaction of the diazonium salt Fast-Red TR with naphthoxide derivatives 2a–c to form diazo dyes 3a–c. Note substitution occurs at C1 for 1a and 1b and at the C3 position for 1c.
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The visual comparative diazo formation (Part B) was performed by the addition of stock solutions of compounds 1a, 1b, and 1c to three separate test tubes followed by a solution of Fast-Red TR in acetonitrile and a simultaneous addition of 0.1 M aqueous solution of sodium hydroxide to each test tube. The reactions were monitored over a period of 15 minutes for the formation of an orange–red color. Students were also asked to observe the solutions in the test tubes after about an hour, at which time the orange–red dye begins to precipitate out of solution and is visible to the naked eye. Part B provided students with preliminary qualitative information that would be predictive of the quantitative results obtained in Part C. Part C required the use of a UV–vis spectrophotometer. Stock solutions of ester 1a, 1b, or 1c were mixed with the FastRed TR, acetonitrile, and 0.1 M aqueous sodium hydroxide solutions in a cuvette and the absorbance at 300 nm recorded as a function of time. Hazards All experiments should be performed in fume hoods with suitable eye protection, gloves, and lab coats. Dichloromethane and deuterated dichloromethane are suspected cancer agents. 6-Bromo-2-naphthol and 1, 6-dibromo-2-naphthol are irritants. Fast-Red TR salt may be harmful if absorbed through the skin or swallowed. Triethylamine is flammable, corrosive, harmful by inhalation, and a lachrymator; it is readily absorbed through the skin. Acetic anhydride is combustible, flammable, corrosive, and a lachrymator. Magnesium sulfate anhydrous may be harmful if absorbed through the skin or swallowed. 2-Propanol and hexanes are flammable and irritating to eyes and skin. Ethyl acetate is flammable and irritating to eyes, respiratory system, and skin. 2-Naphthyl acetate may cause skin or eye irritation. Acetonitrile is flammable, harmful by inhalation, in contact with skin, and if swallowed. Sodium hydroxide solution is caustic and may cause burns. Potassium bromide is an irritant to the eyes, respiratory system, and skin. Results and Discussion This experiment illustrates that electrophilic aromatic substitution rather than ester hydrolysis is rate-limiting and that the presence of electron-withdrawing groups decreases the nucleophilicity of naphthoxides in reactions with Fast-Red TR. The UV–vis and the visual studies complement each other and serve as effective teaching models for the project. Figure 1 depicts pictures taken of the compounds during Part B. Figure 2 represents the UV–vis curves of esters 1a, 1b, and 1c. The data show that the reaction of compound 1b is similar to 1a, albeit slower in its reactivity towards Fast-Red TR, whereas 1c is significantly slower than 1a or 1b and only reaches completion after 2 hours (not shown in Figure 2). A substituent at the C6 position on the aromatic ring (1b) decreases the nucleophilicity of a naphthoxide intermediate towards Fast-Red TR, although the effect is more significant with a dibrominated naphthoxide (1c). It should be emphasized that the marked decrease in reactivity in the case of substrate 1c is due to both the decrease in nucleophilicity of the dibrominated naphthoxide 2c and the fact that diazotization must now occur at the less reactive C3 position of the naphthol since the C1 position is blocked by the bromine substituent. There are thus two factors (decreased nucleophilicity and regiochemistry) that contribute towards the lower rate of reaction of substrate 1c.
Journal of Chemical Education • Vol. 85 No. 9 September 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory A
B
C
D
Figure 1. Part B depicts the change in color of substrates 1a (left vial), 1b (center vial) and 1c (right vial) over the course of time. The photographs were taken after (A) 1 min; (B) 2 min; (C) 4 min; and (D) 12 min. (Figure shown in color on p 1159.)
In the UV–vis spectrum, the downward slope at about 20 minutes for compound 1a indicates the precipitation of dye out of solution after a period of time. This precipitation can also be observed in the test tubes after about an hour. The students performed Parts A and B without any difficulty: esters 1b and 1c were synthesized and purified within 2.5 hours and the visual color test (Part B) was completed within 30 minutes. The first time this project was performed, the students were hesitant about the operation of the UV–vis spectrophotometer mainly because they had minimal prior hands-on exposure to UV–vis spectroscopy. Poor technique resulted in their having to repeat some of their UV–vis runs, causing the lab to run overtime. The second time around, the instructor demonstrated the UV–vis spectroscopy for the class and that proved to be more time-efficient.
Students were also surveyed to determine their level of preparedness for the experiment and their perception of the success of the experiment (see the online material). In general students believed that the experiment was a positive learning experience, although there was a higher level of anxiety associated with the operation of the UV–vis spectrophotometer the first year, when the students were required to do the spectroscopy themselves. The students were less anxious about the UV–vis spectroscopy the following year because the instructor assisted with that step.
Assessment of Student Understanding and the Merit of the Experiment Student comprehension was assessed through a formal lab report, a test, and final examination. In general, a student’s grades for this experiment mirrored his or her performance in the course. It should also be noted that a majority of the students invoked the mechanism of diazo dye formation to rationalize the results they obtained and thus recognized that the trends they observed related directly to the nucleophilicity of the naphthoxide rather than ester hydrolysis. It was heartening to observe that the students had accurately comprehended and rationalized their experimental data in this two-step, one-pot reaction.
Literature Cited
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Absorbance at 300 nm
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1a 1b
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Acknowledgments I would like to thank my colleague Edward Ferroni for the use of his UV–vis spectrophotometer and for valuable input on this project. Jeff Bjorklund and North Central College are acknowledged for their assistance with the use of their NMR spectrometer. 1. For an undergraduate experiment using azo dyes, see: Gung, B. W.; Taylor, R. T. J. Chem. Educ. 2004, 81, 1630–1632. 2. Lavastre, O.; Morken, J. P. Angew. Chem., Int. Ed. 1999, 38, 3163–3165. 3. Townes, J. A.; Evans, M. A.; Queffelec, J.; Taylor, S. J.; Morken, J. P. Organic Lett. 2002, 4, 2537–2540. 4. (a) Tanaka, F.; Kerwin, L.; Kubitz, D.; Lerner, R. A.; Barbas C. F., III. Bioorg. Med. Chem. Lett. 2001, 11, 2983–2986. (b) Boga, C.; Degani, J.; Del Vecchio, E.; Fochi, R.; Forlani, L.; Todesco, P. E. Eur. J. Org. Chem. 2002, 3837–3843. 5. For a discussion of hydrogen-bonding contributing to ortho diazotization of naphthol derivatives, see: Zollinger, H. Diazo Chemistry I: Aromatic and Heteroaromatic Compounds; VCH: Weinheim, Germany, 1994; pp 307–308. 6. For general acetate hydrolyses under basic conditions, see: (a) Paredes, R.; Gil, J.; Ocampo, P. J. Chem. Educ. 1988, 65, 1109–1110. (b) Bender, M. L.; Turnquest, B. W. J. Am. Chem. Soc. 1957, 79, 1656–1662. 7. For examples of ester hydrolyses in the chemical educational literature, see: (a) Lombardo, A. J. Chem. Educ. 1982, 59, 887–888. (b) Hadd, A. G.; Lehmpuhl, D. W.; Kuck, L. R.; Birks, J. W. J. Chem. Educ. 1999, 76, 1237–1240. (c) Bugg, T. D. H.; Lewin, A. M.; Catlin, E. R. J. Chem. Educ. 1997, 74, 105–107.
Supporting JCE Online Material
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http://www.jce.divched.org/Journal/Issues/2008/Sep/abs1271.html Abstract and keywords
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Full text (PDF) with links to cited JCE articles; Figure 1 in color 0.0 0
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Time / min Figure 2. UV–vis data of the reaction of substrate 1a, 1b, or 1c with Fast-Red TR.
Supplement Detailed experimental procedures for the students
Notes for instructors
NMR, IR, and UV–vis spectra and TLC data
Assessment information
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 9 September 2008 • Journal of Chemical Education
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