Electrophilic Aromatic Substitution, Promoted by Bentonitic Clay Enrique Angeles, Alberto Ramirez, lgnacio Martinez, and Enrique Moreno
C a m ~ o1. Facultad de Estudios Su~erioresCuautitlan. Universidad Nacional Aut6noma de Mexico. Cuautitlan Izcalli. Estado de ~exico.CP 54740 Mexico. One classical aspect in courses of organic chemistry is electrophilic aromatic substitution reactions. The student learns about the effect of catalysts in typical reactions such as halogenation and the Friedel-Crafts alkylation of benzene. One of the traditional experiments performed a t universities utilizes the knowledge that benzene reacts with chlorine or bromine in presence of a catalytic amount of a Lewis acid to form the corresponding halobenzene. Reagents like alumina supported copper(I1) chloride ( I ) , poly(N-chloromaleirnide) (21, thalium(II1) acetate (31, iodine tricbloride (41, copper(I1) chloride (51, antimony(V) chloride (61, and trichloroisocyanuric acid (71, has been used. The use of clay in chemical technology is common, its application is wide in refining edible oils, phenol production, and petroleum cracking. Likewise bentonitic clay is a s an efficient catalyst in the transformations of organic chemistry (8,9).In this context we wish to report an experiment that uses bentonitic clay1 as a catalysis instead of the conventional Lewis acid in the chlorination and bromination of benzene and dimerization of toluene, in which electrophilic aromatic substitution was involved . Procedure
Chlorination of Benzene
A mixture of 0.50 g of bentonitic earth and 50 mL of anhydrous benzene was refluxed for 2 h. During the stirring period a slow stream of chlorine gas is bubbled into the mixture. The mixture is filtered and the solvent was removed in vacuo which affords chlorobenzene (yield 80 %). Bromhation of Benzene A mixture of 0.50 g of henrnnitic earth and 50 mL of anhvdrous benzene. 1.0e of bromine (6.9 mmol~i g added droo G s e and the mixture& stirred and was heated with an IR lamp. After 10 min, the mixture was filtered and evaporated under reduced pressure to give bromobenzene (yield 90 %). Synthesis of p-Benzyltoluene
To a suspension of stirred dry toluene (50 mL) and 0.5 g of clay, 1.0 g of bromine (6.3 mmol) was added drop wise . The mixture was heated a t reflux temperature. The reaction started immediately with evolution of hydrobromic acid; the reaction was monitored by tlc. Stirring was con-
tinued until the evolution of hydrobromic acid ceased as a sign of the completion of the reaction. An alternative experiment can be-done usinfin-chlorotolut~nein toluene and hmtonitic clay and without bromine.vieldingp-ben7yltolu-. ene. In all cases the fmal products were purified by distillation under reduced pressure and the products were identified by H1-NMR and mass spectrometry techniques. Discussion
Solid acid catalysis is playing an increasingly important role in replacing industrial processes that may produce harmful wastes. To minimize or eliminate these wastes, it is necessary to find cleaner technologies to replace the use of conventional Lewis acids in several chemical transformations. These facts prompted us to promote the study of bentonitic-clav-catalyzed transformations. Furthermore. bcntonit~cclay is a un~fmmht:terugentwus cutalyst in which active sites a m distributed thn~uahoutits bulk.. and.. hence, it may initiate broad categoriesuof reactions along the lines of Lewis acids. Therefore, the use of bentonitic clay as a catalyst, rather than the more usual Lewis acids, has several distinct advantages 1. it is a much cleaner reaction 2. the experimental conditions are simple 3. the reagents used are cheaper 4. the catalyst is easily removed after filtration Acknowledgement
We wish to thank to DGAPA-UNAMby financial support Project IN300293 and the Gobierno del Estado de Mexim by partial support. Literature Cited 1. Kadornari, M.;Sstoh, H.: Yoshit-, S. J Olg Chem 1988, 53, 2093-2094. 2. Yaroslausky. C . ;Kstehalski, E. l & f m h d r m Lrtf 1972.5173-5174. 3. McKllop, A ; Bmmley, D. J Olg.Chem. 1912,37,8&92. 4. Campslgne, E.;Thompmm, W. J A m . Cham. S a 195% 72.62M31. 5. Nonhebel, D.C . Or& 4 n l h . 1963,43,15-17. 6. Kovadc, P .: Sparks, K. J.Am. Chrm. Soc. 1960,82.57406743. 7. Juenge,E.C.: Bed, D. A,: Duncan, W. P. J. Olg Cham 1970,35,719-722. 8. Cabrers,A.;has,N.;Marquez,C.;Salmon,M.;Angeles,E.:Miranda,R.: Lo.an0.R. G o u Chim. lid. 1991,121,127-130 9. Salmon, M.;Angeles, E.: Mirands. R. J. Chrm. Soc. Chem. Commun. 1990, 118% 1190:Salmon,M.: Angeles, E.; Miranda, R. Cham. B Ind. 1981,2637,
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Volume 71 Number 6 June 1994
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