LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Synthesis of Two Local Anesthetics from Toluene: An Organic Multistep Synthesis in a Project-Oriented Laboratory Course Patricia Demare and Ignacio Regla* Facultad de Estudios Superiores-Zaragoza, Universidad Nacional Autonoma de Mexico, Batalla del 5 de Mayo esq. Fuerte de Loreto, Ejercito de Oriente, 09230 Mexico, D.F., Mexico
bS Supporting Information ABSTRACT: This article describes one of the projects in the advanced undergraduate organic chemistry laboratory course concerning the synthesis of two local anesthetic drugs, prilocaine and benzocaine, with a common three-step sequence starting from toluene. Students undertake, in a several-week independent project, the multistep synthesis of a pharmaceutical drug, comprising instructor-guided tasks such as literature search, planning, critical discussion, experimental design, observation, and results interpretation. In this project, in addition to searching and using information found in primary and secondary sources, students learn to design the methodology for several of the steps in the reaction sequence, bearing in mind safety and environmental concerns. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Problem Solving/Decision Making, Amines/Ammonium Compounds, Chromatography, Drugs/Pharmaceuticals, NMR Spectroscopy, Synthesis
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ur academic approach is based on the premise that learning is facilitated in a research environment, which involves independent study, decision making, and problem solving through research projects.1 3 To this end, a nontraditional advanced undergraduate organic chemistry laboratory course was developed. The project-oriented system consists of halfsemester research projects (about eight 4-h lab sessions) that involve the multistep synthesis of a variety of classical pharmaceutical drugs, with methodologies usually adapted from primary literature. One of these projects is described; the multistep synthesis of two local anesthetic drugs, benzocaine and prilocaine, from a common three-step sequence starting from toluene that requires the student to formulate a hypothesis and to design experiments to test it.
lab work and report. In this preliminary evaluation, the instructor inquires about the theoretical background of the experiment (reaction type, mechanism, stoichiometry, etc.) and the procedure, frequently advising students against the idea that they must “follow instructions step-by-step” to achieve success. This interaction is intended to promote a critical process that allows students to design their own work plan and gives them selfconfidence. The dialogue between the students and instructor may be time-consuming, but the instructors must be aware of the work the students are carrying out, making sure that the students know, before initiating each step, how they are going to do it, why, and what to expect. At the beginning of the course, pharmaceuticals are assigned to the students, who do a time-constrained bibliographic search in Chemical Abstracts (CA), either in printed form or through the SciFinder database, seeking information on syntheses for the assigned drug. This search (typically 1950 through 1980) frequently renders several nonrecoverable papers or patents, so it is sometimes necessary to discuss alternatives, and then plan the project and design experimental work based on nondetailed procedures outlined in CA, turning the process into authentic laboratory research. Laboratory limitations (time, materials, and equipment) are taken into account in choosing a synthetic strategy, as well as in designing each experiment; nonetheless, additional support from a research laboratory is sometimes necessary for reagents and facilities, which facilitates the teaching research link. Students learn that, even for the reproduction of methodology from a formal paper, adaptations or modifications may be necessary, so they are encouraged to initially explore
’ LABORATORY DYNAMICS Students work singly or in pairs, with different projects. Each instructor is in charge of up to 10 students. The semester work starts with three to five simple experiments, presented in a nonconventional laboratory manual (more accurately, a laboratory guide) that includes: (i) an introduction describing the work system; (ii) 40 experimental proposals (each featuring a reaction scheme, main objective, procedure references, a study guide, prelab questions, and notes, as well as IR and NMR spectra); and (iii) 10 appendixes addressing topics such as the lab notebook, experimental design, microscale, waste disposal, bibliographic research, and laboratory rules (the original version of this manual, in Spanish, is included in the Supporting Information). Before conducting each experiment, students search for information, discuss it with the instructor, and outline a laboratory work plan, generating a grade that will be averaged with the Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Published: October 13, 2011 147
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Journal of Chemical Education
LABORATORY EXPERIMENT
Scheme 1. Divergent Synthesis of Prilocaine Hydrochloride (5) and Benzocaine (8)
uncertain procedures on a microscale. From a rough calculation of the expected global yield, the quantity of starting material is calculated to obtain about 1 g of final product. Students typically encounter chemical transformations that have not been covered in lecture, highlighting the importance of qualified laboratory instructors, as well as good communication with lecture instructors. More than the successful culmination of the synthesis, the learning process and the interest the students demonstrate in understanding their experiments are valued. For each experiment, a grade is assigned on the basis of (i) preliminary oral evaluation (correlation between theory and practice, planning of the experiment), (ii) performance in the lab (attention to lab guidelines, time management, responsibility, ethics, etc.), and (iii) proper documentation in a lab notebook. A final-term report is assigned on the synthesis and other features (bibliographic research, pharmacology, analysis) of the target drug to encourage the students to have a comprehensive view of the prepared compound. Oral reports to the class are given by the students who carried out the most interesting or successful projects.
This project, undertaken by two students as a team, illustrates several classic reaction types and involves many common laboratory techniques. Most importantly, it requires students to design a methodology for the conversion of the nitrotoluene isomer mixture obtained in the first step (part A, Scheme 1) into the starting material for each local anesthetic. The methodology is based on a hypothesis, which can easily be verified, concerning the different reactivity of the corresponding amines. Nitration of toluene is a classical example of an electrophilic aromatic substitution on an activated benzene ring, which affords a mixture of nitrotoluene isomers in varying ratios depending on the specific conditions. In this project, students have employed the classical sulfuric/nitric method by adapting a laboratory procedure published in this Journal,7 leading to a mixture of o-, m-, and p-nitrotoluene (MNT) isomers (1) in a typical ratio of about 59:4:36, respectively, with yields of 62 80%. An alternative nitration method, which students have reproduced, employs in situ generated acetyl nitrate.8 Because the separation of the MNT isomers is too difficult to accomplish in the laboratory,9 the isomeric ratio is established by GC or HPLC analysis and the mixture is used as the starting material for the synthesis of target products. Reduction of the MNT mixture by catalytic hydrogen transfer (ammonium formate, Pd/C, AcOEt)10 affords a mixture of toluidine isomers, from which the p-toluidine may be selectively acetylated, owing to its reduced steric demand. After discussing the steric properties of the toluidines with the instructor, the students in charge of the project must propose a hypothesis regarding which of the isomers will be the most reactive toward an acylating agent, develop a work plan to verify it, and a strategy that allows them to isolate both main products, o-toluidine and p-methylacetanilide. To achieve this, students treat the dried solution of the toluidine isomer mixture, at 0 °C, with one molar equivalent (as to the amount of p-isomer theoretically present in the mixture) of acetic anhydride. TLC analysis of the reaction mixture allows a clear differentiation of all compounds involved (see the Supporting Information). Following dilute HCl extraction, p-methylacetanilide (3) is isolated and purified by one of the students as the starting material for benzocaine synthesis.
’ PROJECT DESCRIPTION The multistep divergent synthesis of two local anesthetics, prilocaine hydrochloride (5) and benzocaine (8), with a common initial three-step reaction sequence starting from toluene is described as an example project (Scheme 1). Local anesthetics, which reversibly block nerve impulses, can be divided into two main groups, esters (e.g., cocaine, benzocaine, procaine, tetracaine) and amides (e.g., lidocaine, prilocaine, bupivacaine), by the functional group that connects the hydrophobic group (generally an aromatic ring) and the hydrophilic group (frequently a secondary or tertiary amine group).4 Side effects of local anesthetics are common; a metabolite of benzocaine is p-amino benzoic acid, which is associated with allergic reactions, and a breakdown product of prilocaine, o-toluidine, can produce methemoglobinemia. Also, prilocaine has been reported to induce apoptosis in osteoblastic cells.5 Prilocaine is used as a racemate, although isomers differ in potency and in toxicity.6 148
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Journal of Chemical Education The o-toluidine hydrochloride solution (2) is used by the other student for prilocaine hydrochloride synthesis. The synthesis of prilocaine hydrochloride (5) from 2 (part B in Scheme 1) is accomplished through modifications of the described methodologies11 to suit the laboratory conditions and small scale. The o-toluidine hydrochloride solution (2) is buffered to pH = 5 6 (NaOH and AcONa) and cooled to 5 °C before adding 2-chloropropionyl chloride to afford chloroamide 4 (mp 112 113 °C). Overall yields of 4 from toluene (no isolated intermediates) are typically in the range from 12 to 26%. Synthesis of prilocaine hydrochloride (5) was simplified by allowing a solution of 4 in n-propylamine to stand for two days at room temperature, followed by treatment of the isolated base product with gaseous HCl, either lab generated12 or by the use of a commercially available hydrogen chloride/2-propanol solution. Typical yields of 5 from 4 are around 60 80 %. By omitting several isolation and purification steps, this set of operations may be considered a telescoping synthesis. This is a green chemistry strategy, where one reactant goes through multiple transformations without isolation of intermediates, and is aimed to reduce the number of unit operations, in this way saving time, reducing environmental burden (solvents, energy, etc.), reducing the need to manipulate toxic materials, and increasing yield.13 Benzocaine (8) is prepared from p-methylacetanilide (3) (part C in Scheme 1) in a three-step reaction sequence, involving procedures described in this Journal14 and in several laboratory instruction manuals (overall yields of about 12 to 22 %).
LABORATORY EXPERIMENT
’ ASSOCIATED CONTENT
bS
Supporting Information Background information, experimental procedure with notes for instructors, safety hazards, list of chemicals, gas and HPLC chromatograms, NMR spectra. The lab manual (in Spanish) used in our course is included in a separate compressed file. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful to Santiago Capella Vizcaíno, from Facultad de ngeles Pe~na, from Instituto de Química, UNAM, and Ma. de los A Química, UNAM, for chromatographic and 1H NMR analysis, respectively. We also thank the students David Arias, Araceli Guevara, Ramon Vazquez, Claudia Almazan, Sarahí Barragan, Neftalí Rivera, and Manuel Lopez-Ortiz, who enthusiastically participated in this project. ’ REFERENCES (1) Horowitz, G. J. Chem. Educ. 2007, 84 (2), 346–353. (2) Reid, N.; Shah, I. Chem Educ. Res. Pract. 2007, 8 (2), 172–185. (3) Ruttledge, T. R. J. Chem. Educ. 1998, 75 (12), 1575–1577. (4) Ostercamp, D. L.; Brunsvold, R. J. Chem. Educ. 2006, 83 (12), 1816–1820. (5) Nakamura, K.; Kido, H.; Morimoto, Y.; Morimoto, H.; Kobayashi, S.; Morikawa, M.; Haneji, T. Can. J. Anesth. 1999, 46 (5), 476–482. (6) Akerman, B.; Ross, S. Pharmacol. Toxicol. 1970, 28 (6), 445–453. (7) Russell, R. A.; Switzer, R. W.; Longmore, R. W. J. Chem. Educ. 1990, 67 (1), 68–69. (8) Blankespoor, R. L.; Hogendoorn, S.; Pearson, A. J. Chem. Educ. 2007, 84 (4), 697–698. (9) Zinnen, H. A., U.S. Pat. 4,620,047, October 28, 1986. (10) Hanson, R. W. J. Chem. Educ. 1997, 74 (4), 430–431. (11) (a) Lofgren, N.; Tegner, C. Acta Chem. Scand. 1960, 14, 486–490. (b) Lofgren, N. Tegner, C. P. U.S. Pat. 3,160,662, 1964. (c) Brown, C. L., U.S. Pat. 3,646,137, 1972. (d) Reilly, T. J. J. Chem. Educ. 1999, 76 (11), 1557. (12) Arnaiz, F. J. J. Chem. Educ. 1995, 72 (12), 1139. (13) Clark, J. H. Nature Chem. 2009, 1, 12–13. (14) Kremer, C. B. J. Chem. Educ. 1956, 33 (2), 71–72.
’ HAZARDS All reactants, products, and solvents must be handled in a manner consistent with the information available on their Material Safety Data Sheets (MSDS). Eye protection and gloves must be worn at all times and procedures must be conducted in a fume hood. Nitrotoluene and toluidine isomers are skin irritants and suspected carcinogens. Acetic anhydride is a lachrymator, corrosive, and flammable. 2-Chloropropionyl chloride should be handled with particular care as it is extremely corrosive, lachrymator, water-reactive (generating HCl), and flammable. Toluene, methanol, diethyl ether, hexane, ethyl acetate, and isopropyl alcohol are all volatile, toxic, and flammable liquids; particularly, diethyl ether should be kept away from sparks or fire. Propylamine is highly volatile, toxic, flammable, and irritating to the skin and mucous membranes. Ammonium formate and sodium acetate trihydrate are irritant to skin and eyes. Sodium hydroxide is very caustic. Sulfuric, nitric, and hydrochloric acids are corrosive to eyes, skin, and mucous membranes. Palladium on carbon is pyrophoric when dry; it can cause fire in contact with combustible materials, such as organic solvents or filter paper. Hydrogen chloride is extremely corrosive and irritating to eyes, skin, and mucous membranes. ’ CONCLUSIONS This project was developed and refined with the work of students from several generations in our course. It requires the design of a telescoping sequence for several of the synthetic steps, as well as adaptations to previously reported syntheses, to suit laboratory conditions. Students have described this project as challenging and useful, as it has allowed them to practice a variety of experimental techniques and reaction types, in addition to exposing them to real scientific practice. 149
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