Published: October 13, 2011

Copyright r 2011 American Chemical Society andDivision of Chemical Education, Inc. 147 dx.doi.org/10.1021/ed100838a | J. Chem. Educ. 2012, 89, 147–149

LABORATORY EXPERIMENT

pubs.acs.org/jchemeduc

Synthesis of Two Local Anesthetics from Toluene: An OrganicMultistep Synthesis in a Project-Oriented Laboratory CoursePatricia Demare and Ignacio Regla*

Facultad de Estudios Superiores-Zaragoza, Universidad Nacional Aut�onoma de M�exico, Batalla del 5 de Mayo esq. Fuerte de Loreto,Ej�ercito de Oriente, 09230 M�exico, D.F., M�exico

bS Supporting Information

Our academic approach is based on the premise that learningis facilitated in a research environment, which involves

independent study, decision making, and problem solvingthrough research projects.1�3 To this end, a nontraditionaladvanced undergraduate organic chemistry laboratory coursewas developed. The project-oriented system consists of half-semester research projects (about eight 4-h lab sessions) thatinvolve the multistep synthesis of a variety of classical pharma-ceutical drugs, with methodologies usually adapted from primaryliterature. One of these projects is described; the multistepsynthesis of two local anesthetic drugs, benzocaine and prilo-caine, from a common three-step sequence starting from toluenethat requires the student to formulate a hypothesis and to designexperiments to test it.

’ LABORATORY DYNAMICS

Students work singly or in pairs, with different projects. Eachinstructor is in charge of up to 10 students. The semester workstarts with three to five simple experiments, presented in anonconventional laboratory manual (more accurately, a labora-tory guide) that includes: (i) an introduction describing the worksystem; (ii) 40 experimental proposals (each featuring a reactionscheme, 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, bibliographicresearch, and laboratory rules (the original version of thismanual, in Spanish, is included in the Supporting Information).

Before conducting each experiment, students search forinformation, discuss it with the instructor, and outline a labora-tory work plan, generating a grade that will be averaged with the

lab work and report. In this preliminary evaluation, the instructorinquires about the theoretical background of the experiment(reaction type, mechanism, stoichiometry, etc.) and the proce-dure, frequently advising students against the idea that they must“follow instructions step-by-step” to achieve success. This inter-action is intended to promote a critical process that allowsstudents to design their own work plan and gives them self-confidence. The dialogue between the students and instructormay be time-consuming, but the instructors must be aware of thework the students are carrying out, making sure that the studentsknow, 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 tothe students, who do a time-constrained bibliographic searchin Chemical Abstracts (CA), either in printed form or throughthe SciFinder database, seeking information on syntheses forthe assigned drug. This search (typically 1950 through 1980)frequently renders several nonrecoverable papers or patents, so itis sometimes necessary to discuss alternatives, and then plan theproject and design experimental work based on nondetailedprocedures outlined in CA, turning the process into authenticlaboratory research. Laboratory limitations (time, materials, andequipment) are taken into account in choosing a syntheticstrategy, as well as in designing each experiment; nonetheless,additional support from a research laboratory is sometimesnecessary for reagents and facilities, which facilitates the teaching�research link. Students learn that, even for the reproductionof methodology from a formal paper, adaptations or modifica-tions may be necessary, so they are encouraged to initially explore

ABSTRACT: This article describes one of the projects in the advanced undergraduateorganic 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 apharmaceutical drug, comprising instructor-guided tasks such as literature search, planning,critical discussion, experimental design, observation, and results interpretation. In thisproject, in addition to searching and using information found in primary and secondarysources, students learn to design the methodology for several of the steps in the reaction sequence, bearing in mind safety andenvironmental concerns.

KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives,Problem Solving/Decision Making, Amines/Ammonium Compounds, Chromatography, Drugs/Pharmaceuticals, NMR Spec-troscopy, Synthesis

148 dx.doi.org/10.1021/ed100838a |J. Chem. Educ. 2012, 89, 147–149

Journal of Chemical Education LABORATORY EXPERIMENT

uncertain procedures on a microscale. From a rough calculationof the expected global yield, the quantity of starting materialis calculated to obtain about 1 g of final product. Studentstypically encounter chemical transformations that have not beencovered in lecture, highlighting the importance of qualifiedlaboratory instructors, as well as good communication withlecture instructors.

More than the successful culmination of the synthesis, thelearning process and the interest the students demonstrate inunderstanding their experiments are valued. For each experi-ment, a grade is assigned on the basis of (i) preliminary oralevaluation (correlation between theory and practice, planning ofthe experiment), (ii) performance in the lab (attention to labguidelines, time management, responsibility, ethics, etc.), and(iii) proper documentation in a lab notebook. A final-term reportis assigned on the synthesis and other features (bibliographicresearch, pharmacology, analysis) of the target drug to encouragethe students to have a comprehensive view of the preparedcompound. Oral reports to the class are given by the studentswho carried out the most interesting or successful projects.

’PROJECT DESCRIPTION

The multistep divergent synthesis of two local anesthetics,prilocaine hydrochloride (5) and benzocaine (8), with a com-mon initial three-step reaction sequence starting from toluene isdescribed as an example project (Scheme 1). Local anesthetics,which reversibly block nerve impulses, can be divided into twomain groups, esters (e.g., cocaine, benzocaine, procaine, tetra-caine) and amides (e.g., lidocaine, prilocaine, bupivacaine), bythe functional group that connects the hydrophobic group (generallyan aromatic ring) and the hydrophilic group (frequently asecondary or tertiary amine group).4 Side effects of local anes-thetics are common; a metabolite of benzocaine is p-aminobenzoic acid, which is associated with allergic reactions, and a break-downproduct of prilocaine, o-toluidine, can producemethemoglo-binemia. Also, prilocaine has been reported to induce apoptosisin osteoblastic cells.5 Prilocaine is used as a racemate, althoughisomers differ in potency and in toxicity.6

This project, undertaken by two students as a team, illustratesseveral classic reaction types and involves many common labora-tory techniques. Most importantly, it requires students to designa methodology for the conversion of the nitrotoluene isomermixture obtained in the first step (part A, Scheme 1) into thestarting material for each local anesthetic. The methodology isbased on a hypothesis, which can easily be verified, concerningthe different reactivity of the corresponding amines.

Nitration of toluene is a classical example of an electrophilicaromatic substitution on an activated benzene ring, which affords amixture of nitrotoluene isomers in varying ratios depending on thespecific conditions. In this project, students have employed theclassical sulfuric/nitric method by adapting a laboratory procedurepublished in this Journal,7 leading to a mixture of o-, m-, andp-nitrotoluene (MNT) isomers (1) in a typical ratio of about 59:4:36,respectively, with yields of 62�80%. An alternative nitrationmethod, which students have reproduced, employs in situ gen-erated acetyl nitrate.8 Because the separation of theMNT isomersis too difficult to accomplish in the laboratory,9 the isomeric ratio isestablished by GC or HPLC analysis and the mixture is used as thestarting 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 oftoluidine isomers, from which the p-toluidine may be selectivelyacetylated, owing to its reduced steric demand. After discussingthe steric properties of the toluidines with the instructor, thestudents in charge of the project must propose a hypothesisregarding which of the isomers will be the most reactive towardan acylating agent, develop a work plan to verify it, and a strategythat allows them to isolate both main products, o-toluidine andp-methylacetanilide. To achieve this, students treat the driedsolution of the toluidine isomer mixture, at 0 �C, with one molarequivalent (as to the amount of p-isomer theoretically present inthe mixture) of acetic anhydride. TLC analysis of the reactionmixture allows a clear differentiation of all compounds involved(see the Supporting Information). Following dilute HCl extrac-tion, p-methylacetanilide (3) is isolated and purified by one ofthe students as the starting material for benzocaine synthesis.

Scheme 1. Divergent Synthesis of Prilocaine Hydrochloride (5) and Benzocaine (8)

149 dx.doi.org/10.1021/ed100838a |J. Chem. Educ. 2012, 89, 147–149

Journal of Chemical Education LABORATORY EXPERIMENT

The o-toluidine hydrochloride solution (2) is used by the otherstudent for prilocaine hydrochloride synthesis.

The synthesis of prilocaine hydrochloride (5) from 2 (part Bin Scheme 1) is accomplished through modifications of thedescribed methodologies11 to suit the laboratory conditionsand small scale. The o-toluidine hydrochloride solution (2) isbuffered to pH = 5�6 (NaOH and AcONa) and cooled to 5 �Cbefore adding 2-chloropropionyl chloride to afford chloro-amide 4 (mp 112�113 �C). Overall yields of 4 from toluene(no isolated intermediates) are typically in the range from 12 to26%. Synthesis of prilocaine hydrochloride (5) was simplified byallowing a solution of 4 in n-propylamine to stand for two days atroom temperature, followed by treatment of the isolated baseproduct with gaseous HCl, either lab generated12 or by the use of acommercially 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 ofoperations may be considered a telescoping synthesis. This is agreen chemistry strategy, where one reactant goes throughmultiple transformations without isolation of intermediates,and is aimed to reduce the number of unit operations, in thisway 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) (partC in Scheme 1) in a three-step reaction sequence, involvingprocedures described in this Journal14 and in several laboratoryinstruction manuals (overall yields of about 12 to 22 %).

’HAZARDS

All reactants, products, and solvents must be handled in amanner consistent with the information available on theirMaterial Safety Data Sheets (MSDS). Eye protection and glovesmust be worn at all times and procedures must be conducted in afume hood. Nitrotoluene and toluidine isomers are skin irritantsand suspected carcinogens. Acetic anhydride is a lachrymator,corrosive, and flammable. 2-Chloropropionyl chloride shouldbe handled with particular care as it is extremely corrosive,lachrymator, water-reactive (generating HCl), and flammable.Toluene, methanol, diethyl ether, hexane, ethyl acetate, andisopropyl 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 tothe skin and mucous membranes. Ammonium formate andsodium acetate trihydrate are irritant to skin and eyes. Sodiumhydroxide is very caustic. Sulfuric, nitric, and hydrochloric acidsare corrosive to eyes, skin, andmucousmembranes. Palladium oncarbon is pyrophoric when dry; it can cause fire in contact withcombustible 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 ofstudents from several generations in our course. It requires thedesign of a telescoping sequence for several of the synthetic steps,as well as adaptations to previously reported syntheses, to suitlaboratory conditions. Students have described this project aschallenging and useful, as it has allowed them to practice a varietyof experimental techniques and reaction types, in addition toexposing them to real scientific practice.

’ASSOCIATED CONTENT

bS Supporting InformationBackground information, experimental procedure with notes

for instructors, safety hazards, list of chemicals, gas and HPLCchromatograms, NMR spectra. The lab manual (in Spanish)used in our course is included in a separate compressed file. Thismaterial is available via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: regla@unam.mx.

’ACKNOWLEDGMENT

We are grateful to Santiago Capella Vizcaíno, from Facultad deQuímica, UNAM, andMa. de los �Angeles Pe~na, from Instituto deQuímica, UNAM, for chromatographic and 1H NMR analysis,respectively. We also thank the students David Arias, AraceliGuevara, Ram�on V�azquez, Claudia Almaz�an, Sarahí Barrag�an,Neftalí Rivera, and Manuel L�opez-Ortiz, who enthusiasticallyparticipated 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) Arn�aiz, 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.