In the Laboratory
Investigation of Unexpected Reaction Intermediates in the Alkaline Hydrolysis of Methyl 3,5-Dinitrobenzoate Clésia C. Silva, Ricardo O. Silva, Daniela M. A. F. Navarro, and Marcelo Navarro* Departamento de Química Fundamental - CCEN, Universidade Federal de Pernambuco, Cidade Universitária 50740-901, Recife (PE), Brazil; *
[email protected] The majority of organic reactions pass through intermediate species in which a carbon atom at the reaction center may have two or three valences. The most common intermediates, such as carbocations (I), free radicals (II), carbanions (III), and carbenes (IV), +
R
C
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C Rb
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C Rb
Rbb
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are typically short-lived species that are rapidly converted into more stable molecules (1). Such intermediates are, therefore, difficult to identify by spectroscopic (UV–vis, IR, and NMR) or spectrometric (MS) techniques. In the relatively rare cases in which the intermediate species are stable, UV–vis spectroscopy offers the advantage that it is a sensitive technique and is by far the most commonly used procedure for determining the kinetic parameters of a chemical reaction system. Unfortunately, however, UV–vis spectroscopy presents a disadvantage in that it is a relatively non-diagnostic method. Hence, the identities of many observed species depend entirely on accurate spectral assignments and may remain uncertain, a situation that can directly affect the interpretation of the kinetic data. While IR spectroscopy is somewhat more diagnostic, depending on the presence of specific and often identifiable groupings of atoms in the molecule, it is less sensitive than UV– vis spectroscopy. NMR spectroscopy, on the other hand, is far less sensitive than UV–vis spectroscopy (often by several orders of magnitude) but has an enormous advantage as a diagnostic tool in that it can provide a great deal of information concerning the detailed molecular structure of the species under investigaR H Nu ź
R
Nuź O2N
O2N
á
NO2 cmp: V VI VII
NO2
R
R: H NO2 CO2CH3 or CN
ź
O2N
NO2 H Nu
Scheme I. Formation of highly colored intermediates as a result of nucleophilic attack on substituted dinitro-aromatic compounds.
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tion. For this reason, NMR has become one of the most widely used spectroscopic techniques for the identification of equilibrium components in chemical reactions that occur slowly in a static system (2). This article describes a laboratory project that introduces UV–vis and 1H NMR spectroscopic techniques to identify stable reaction intermediates and to explore the kinetic and thermodynamic parameters of reactions. One of the most spectacular demonstrations of the existence of reaction intermediates is provided by the formation of stable Meisenheimer complexes (σ-adducts) (3, 4). In the experiment described herein, the intermediacy of this type of carbanion complex is introduced through an unexpected reaction (aromatic nucleophilic attack, ANAr) that occurs in parallel with the alkaline hydrolysis (carbonyl attack) of methyl 3,5-dinitrobenzoate. The Meisenheimer complexes are often intensely colored, especially those obtained from dinitrobenzene (V), trinitrobenzene (VI), and m-dinitrobenzene derivates (VII) (2), the structures of which have been confirmed by X-ray and 1H NMR analysis (3–10). The presence of nitro groups on the aromatic rings of V–VII (Scheme I) give rise to an electronwithdrawing effect that positively activates the ortho and para positions for aromatic nucleophilic attack (A NAr). Several nucleophiles (Nu‒), including methoxide and hydroxide ions as well as primary and secondary amines (10), promote ANAr at these free positions leading to the formation of intermediates (Scheme I) that are unusually stable by virtue of the delocalization of the negative charge over the aromatic ring (the resonance effect). The described project forms part of our experimental organic chemistry course offered to fourth-year undergraduate students reading for a degree in chemistry. Typically, 10 students (working in pairs) would take part in the project, which requires an 8 hour laboratory period for its completion. The UV–vis experiments, which typically require 4 hours to complete, are performed first followed by the NMR analyses. The set of results obtained should permit and encourage students to discuss and propose a mechanism for the reaction sequence observed, which they then describe in the format of a research paper. The project is most effective at introducing the importance of spectroscopic techniques when students are permitted to operate both the UV–vis and the NMR instruments themselves. If this is not possible, the NMR spectra should at least be obtained by a technician in the presence of the students and with their active participation. The experiment may also be applied as a practical test following instructions to students on the manipulation of NMR equipment. Discussion When an ester and hydroxide ions are mixed together in an organic solvent a simple hydrolysis reaction would be expected to occur yielding, as reaction products, the corresponding alco-
Journal of Chemical Education • Vol. 86 No. 4 April 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory A
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Wavelength / nm Figure 1. UV–vis spectra showing (A) the in situ formation of the Meisenheimer complexes by successive addition of 1.0 µL aliquots of aqueous sodium hydroxide (8.0 × 10−2 M) to 2.5 mL of methyl 3,5-dinitrobenzoate (1.0 × 10−4 M) in dimethyl sulfoxide (DMSO), and (B) the disappearance with respect to time of the Meisenheimer complexes formed in situ by the direct addition of 7.0 µL of aqueous sodium hydroxide (8.0 × 10−2 M) to 2.5 mL of methyl 3,5-dinitrobenzoate (1.0 × 10−4 M) in DMSO.
Figure 2. 1H NMR spectra of (A) 0.21 M methyl 3,5-dinitrobenzoate in DMSO-d6 (H2O at 3.35 ppm and DMSO at 2.49 ppm), and (B) the Meisenheimer complexes prepared in situ by the addition of 10.0 µL of 5.3 M sodium hydroxide in D2O to 0.21 M methyl 3,5-dinitrobenzoate in DMSO-d6.
hol and the carboxylic acid salt. In the described experiment, however, an intense red color is observed immediately following the addition of a small quantity of sodium hydroxide to the ester but, after further addition of alkali, the reaction mixture becomes colorless as would be expected. The purpose of the experiment is to stimulate the student’s curiosity for an explanation of this unexpected phenomenon and to illustrate the normal process of scientific research, namely, observation, construction of a hypothesis, and design of further experiments to test the hypothesis. In the experiment described, the Meisenheimer complex is formed instantaneously upon mixing sodium hydroxide and methyl 3,5-dinitrobenzoate. The initial nucleophilic attack of the hydroxide group on the electron-deficient carbons of the aromatic ring gives rise to a color change of the reaction mixture to intense red, and this can be followed by UV–vis spectroscopy. The evolution of the nucleophilic attack following the sequential addition of 1.0 μL aliquots of aqueous sodium hydroxide (8.0 × 10‒2 M) to 2.5 mL of methyl 3,5-dinitrobenzoate (1.0 × 10‒4 M) in dimethyl sulfoxide (DMSO) is shown in Figure 1A. The gradual appearance of new absorption bands in the visible region can be observed at 523 and 555 nm,
corresponding to the ortho and para Meisenheimer complex isomers (5–8), respectively, and at 386 nm, which is common to both isomers. Additional bands below 300 nm (Figure 1A) correspond to π–π* electronic transitions of aromatic species, with the absorption at 261 nm being associated with the dinitro-aromatic ester and that at 294 nm with the σ-adduct. The disappearance of the Meisenheimer intermediate, as indicated by the gradual decrease of the associated bands in the visible region, may be demonstrated by the addition of 7.0 μL of aqueous sodium hydroxide (8.0 × 10‒2 M) to 2.5 mL of methyl 3,5-dinitrobenzoate (1.0 × 10‒4 M) in DMSO and acquisition of spectra at 20 minute intervals (Figure 1B). It is important to note that the band at 555 nm diminishes faster than the one at 523 nm and, over time, disappears completely confirming that the ortho Meisenheimer complex is the thermodynamically more stable intermediate (5–7). 1H NMR analyses are required for characterization of all of the species present in the reaction. The 1H NMR spectrum of a 0.21 M solution of methyl 3,5-dinitrobenzoate in DMSO-d6 is presented in Figure 2A, while Figure 2B shows the spectrum recorded immediately after the addition of a 10.0 μL aliquot of a 5.3 M solution of sodium hydroxide in D2O. The protons of the dinitro-aromatic ring of
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© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 4 April 2009 • Journal of Chemical Education
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In the Laboratory CO2CH3
CO2CH3
k3 ź
O2N
NO2
O2N
H OH
OH
kź1
k1
CO2ź
CO2CH3 OHź
NO2
k5
á O2N
NO2
k2
O2N
NO2
kź2 CO2CH3 H OH
CO2CH3 OH
k4
ź
O2N
NO2
O2N
NO2
Scheme II. The reaction of hydroxide ion with methyl 3,5-dinitrobenzoate showing the intermediate formation of ortho and para Meisenheimer complexes and the end product of hydrolysis, namely, 3,5-dinitrobenzoic acid (and methanol, not shown).
the substrate, initially at 9.04 and 8.91 ppm, are displaced to high field following nucleophilic attack. Moreover, five new aromatic peaks (from 8.22 to 5.70 ppm) are observed in the 1H NMR spectrum of the Meisenheimer intermediates that are formed in situ. In addition, three new methyl peaks are observed at 3.68, 3.65, and 3.22 ppm, corresponding to the ortho and para intermediates and methanol, respectively. Methanol is produced in the hydrolysis process. At this point, it is important that the student be asked to consider the possible intermediate structures that might be formed following aromatic nucleophilic attack. ANAr attack may occur at either the ortho or para positions of the aromatic ring (see the online material), and thus it is possible that two intermediates might be formed. The 1H NMR spectrum of the para intermediate exhibits two spin-coupled peaks at 7.97 and 6.14 ppm with an integration ratio of 2:1, while the ring protons of the ortho intermediate presents three spin coupled peaks at 8.22, 7.78, and 5.70 ppm, each of equal area. Over time, the areas of the peaks associated with the para isomer decrease with respect to those of the ortho intermediate, and after a period of 1 hour a further spectrum shows the almost complete disappearance of the Meisenheimer intermediates (see the online material). A further addition of 10.0 μL of 5.3 M sodium hydroxide in D2O to the reaction mixture ensures the total consumption of the methyl ester (see additional NMR spectra provided in the online material), and three new peaks at 8.90, 8.79, and 3.22 ppm corresponding to the carboxylic acid salt and methanol can be observed in the spectrum, indicating the concurrent hydrolysis reaction. 486
The observed behavior should be discussed within the general framework of the reaction as shown in Scheme II (5–7). Thus the formation of the kinetic mixture is fast in comparison with the establishment of the thermodynamically controlled equilibrium, indicating that the rate constants k1 and k2 are roughly similar and that both are larger than k‒1 and k‒2. After the thermodynamic equilibrium has been established, with k‒1 > k‒2 in this case, the ratio of the Meisenheimer complex isomers (with ortho > para) should remain constant, assuming that k3 and k4 are very small such that the corresponding products of aromatic nucleophilic substitution (S NAr) are not formed (8). After noting the kinetic and thermodynamic phenomena concerning the formation of the Meisenheimer intermediates, the final reaction products can be identified by 1H NMR as the corresponding alcohol and carboxylic acid salt (see the online material) as expected from the hydrolysis of the aromatic ester (8). Hazards Always use safety glasses and disposable gloves when handling organic solvents and reagents and be careful to avoid skin contact with irritant compounds such as methyl 3,5-dinitrobenzoate, sodium hydroxide, and dimethyl sulfoxide. Sodium hydroxide is caustic; it may cause irritation of the eyes and, following prolonged exposure, can give rise to burns that may result in the permanent impairment of vision and even blindness. Dimethyl sulfoxide, which can also cause mild eye irritation, readily penetrates the skin and may significantly enhance the absorption of other reagents. Never inhale the vapor of any organic solvent, and be especially aware that methanol and dichloromethane are toxic. Dichloromethane is a carcinogen. Concentrated sulfuric acid is a strong oxidizing agent and is highly corrosive: it should be handled with extreme caution in the fume-hood using both protective gloves and appropriate eye protection. Summary This experiment offers many opportunities for undergraduate students to apply concepts that have been covered in their course, including NMR and UV–vis spectroscopic analyses, reaction kinetics, mechanisms and intermediates, and aspects of thermodynamics applied to chemical reactions. The challenge of investigating an unknown reaction and of elucidating its mechanism demands the association of all of these concepts. The laboratory instructor should discuss the experiment, using the basic set of questions available in the online material, with groups of students individually and should assess the extent of knowledge building during the project. Students may also be evaluated in terms of the degree of interest in the experiment and the level of participation in the elucidation of the final mechanism. At the end of the project, each student should deliver a mechanistic proposal that would explain the results obtained, and should present a report (including NMR and UV–vis spectra and the mechanistic proposal) written in the format of a research article, that is, with an abstract and introduction, methods, results, discussion, conclusions, and reference sections. The implementation of this project at our institution has promoted the learning of analytical techniques and has stimulated interest in further research. Students have acquired the ability to analyze experimental data and to apply theoreti-
Journal of Chemical Education • Vol. 86 No. 4 April 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
cal concepts in solving a practical problem by deduction. Our experience has demonstrated that investigative experimental methodology is much better explored through the application of a project of the type described in this article than by the more typical cookbook verification experiment (11).
The authors wish to thank Rajendra Mohan Srivastava for fruitful discussions and for correction of the manuscript. CNPq and CAPES are gratefully acknowledged for the provision of financial support.
5. Crampton, M. R.; Khan, H. A. J. Chem. Soc., Perkin Trans. 2 1972, 733–736. 6. Crampton, M. R.; Khan, H. A. J. Chem. Soc., Perkin Trans. 2 1973, 710–715. 7. Fyfe C. A.; Cocivera, M.; Damji, S. W. H. J. Am. Chem. Soc. 1975, 97, 5707–5713. 8. Crampton, M. R.; Greenhalgh, C. J. Chem. Soc., Perkin Trans. 2 1986, 873–877. 9. Terrier, F.; Millot, F.; Simonnin, M. P. Tetrahedron Lett. 1971, 31, 2933–2936. 10. Terrier, F.; Millot, F. Bull. Soc. Chim. Fr. 1974, 1823–1826. 11. Mohrig, J. R. J. Chem. Educ. 2004, 81, 1083–1085.
Literature Cited
Supporting JCE Online Material
Acknowledgments
1. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley-Interscience: New York, 1992; pp 165–204. 2. Dávila, R. M.; Widenner, R. K. J. Chem. Educ. 2002, 79, 997– 999. 3. Meisenheimer, J. Liebigs Ann. Chem. 1902, 323, 205–226. 4. Artamkina, G. A.; Egorov, M. P.; Beletzkaya, I. P. Chem. Rev. 1982, 82, 427–459.
http://www.jce.divched.org/Journal/Issues/2009/Apr/abs484.html Abstract and keywords Full text (PDF) with links to cited JCE articles Supplement Student handouts
Instructor notes with questions and answers Additional NMR spectra
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